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123-JavaScript-Interview-Question
1. Difference between undefined and not defined in JavaScript
In JavaScript if you try to use a variable that doesn’t exist and has not been declared, then JavaScript will throw an error
var name is not definedand the script will stop execute thereafter. But If you usetypeof undeclared_variablethen it will returnundefined.
Before starting further discussion let’s understand the difference between declaration and definition.
var x is a declaration because you are not defining what value it holds yet, but you are declaring its existence and the need of memory allocation.
var x; // declaring x
console.log(x); //output: undefined
var x = 1 is both declaration and definition (also we can say we are doing initialisation), Here declaration and assignment of value happen inline for variable x, In JavaScript every variable declaration and function declaration brings to the top of its current scope in which it’s declared then assignment happen in order this term is called hoisting.
A variable that is declared but not define and when we try to access it, It will result
undefined.
var x; // Declaration
if(typeof x === 'undefined') // Will return true
A variable that neither declared nor defined when we try to reference such variable then It result
not defined.
console.log(y); // Output: ReferenceError: y is not defined
Ref Link:
http://stackoverflow.com/questions/20822022/javascript-variable-definition-declaration
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2. What will be the output of the following code?
var y = 1;
if (function f() {}) {
y += typeof f;
}
console.log(y);
Above code would give output
1undefined. If condition statement evaluate usingevalsoeval(function f() {})which returnfunction f() {}which is true so inside if statement code execute.typeof freturn undefined because if statement code execute at run time, so statement insideifcondition evaluated at run time.
var k = 1;
if (1) {
eval(function foo() {});
k += typeof foo;
}
console.log(k);
Above code will also output 1undefined.
var k = 1;
if (1) {
function foo() {};
k += typeof foo;
}
console.log(k); // output 1function
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3. What is the drawback of creating true private in JavaScript?
One of the drawback of creating a true private method in JavaScript is that they are very memory inefficient because a new copy of the method would be created for each instance.
var Employee = function (name, company, salary) {
this.name = name || ""; //Public attribute default value is null
this.company = company || ""; //Public attribute default value is null
this.salary = salary || 5000; //Public attribute default value is null
// Private method
var increaseSlary = function () {
this.salary = this.salary + 1000;
};
// Public method
this.dispalyIncreasedSalary = function() {
increaseSlary();
console.log(this.salary);
};
};
// Create Employee class object
var emp1 = new Employee("John","Pluto",3000);
// Create Employee class object
var emp2 = new Employee("Merry","Pluto",2000);
// Create Employee class object
var emp3 = new Employee("Ren","Pluto",2500);
Here each instance variable emp1, emp2, emp3 has own copy of increaseSalary private method.
So as recommendation don’t go for a private method unless it’s necessary.
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4.What is “closure” in javascript? Provide an example?
A closure is a function defined inside another function (called parent function) and has access to the variable which is declared and defined in parent function scope.
The closure has access to variable in three scopes:
– Variable declared in his own scope
– Variable declared in parent function scope
– Variable declared in global namespace
var globalVar = "abc";
// Parent self invoking function
(function outerFunction (outerArg) { // begin of scope outerFunction
// Variable declared in outerFunction function scope
var outerFuncVar = 'x';
// Closure self-invoking function
(function innerFunction (innerArg) { // begin of scope innerFunction
// variable declared in innerFunction function scope
var innerFuncVar = "y";
console.log(
"outerArg = " + outerArg + "\n" +
"outerFuncVar = " + outerFuncVar + "\n" +
"innerArg = " + innerArg + "\n" +
"innerFuncVar = " + innerFuncVar + "\n" +
"globalVar = " + globalVar);
} // end of scope innerFunction)(5); // Pass 5 as parameter
} // end of scope outerFunction )(7); // Pass 7 as parameter
innerFunction is closure which is defined inside outerFunction and has access to all variable which is declared and defined in outerFunction scope. In addition to this function defined inside function as closure has access to variable which is declared in global namespace.
Output of above code would be:
outerArg = 7
outerFuncVar = x
innerArg = 5
innerFuncVar = y
globalVar = abc
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5. Write a mul function which will properly when invoked as below syntax.
console.log(mul(2)(3)(4)); // output : 24
console.log(mul(4)(3)(4)); // output : 48
Below is code followed by an explanation how it works:
function mul (x) {
return function (y) { // anonymous function
return function (z) { // anonymous function
return x * y * z;
};
};
}
Here mul function accept the first argument and return anonymous function which take the second parameter and return anonymous function which take the third parameter and return multiplication of arguments which is being passed in successive
In Javascript function defined inside has access to outer function variable and function is the first class object so it can be returned by function as well and passed as argument in another function.
– A function is an instance of the Object type
– A function can have properties and has a link back to its constructor method
– Function can be stored as variable
– Function can be pass as a parameter to another function
– Function can be returned from function
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6. How to empty an array in JavaScript?
For instance:
var arrayList = ['a', 'b', 'c', 'd', 'e', 'f'];
How can we empty above array?
There are a couple of ways by which we can empty an array, So let’s discuss all the possible way by which we can empty an array.
Method 1
arrayList = [];
Above code will set the variable arrayList to a new empty array. This is recommended if you don’t have references to the original array arrayList anywhere else because It will actually create a new empty array. You should be careful with this way of empty the array, because if you have referenced this array from another variable, then the original reference array will remain unchanged, Only use this way if you have only referenced the array by its original variable arrayList.
For Instance:
var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
var anotherArrayList = arrayList; // Referenced arrayList by another variable
arrayList = []; // Empty the array
console.log(anotherArrayList); // Output ['a', 'b', 'c', 'd', 'e', 'f']
Method 2
arrayList.length = 0;
Above code will clear the existing array by setting its length to 0. This way of empty the array also update all the reference variable which pointing to the original array. This way of empty the array is useful when you want to update all the another reference variable which pointing to arrayList.
For Instance:
var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
var anotherArrayList = arrayList; // Referenced arrayList by another variable
arrayList.length = 0; // Empty the array by setting length to 0
console.log(anotherArrayList); // Output []
Method 3
arrayList.splice(0, arrayList.length);
Above implementation will also work perfectly. This way of empty the array will also update all the references of the original array.
var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
var anotherArrayList = arrayList; // Referenced arrayList by another variable
arrayList.splice(0, arrayList.length); // Empty the array by setting length to 0
console.log(anotherArrayList); // Output []
Method 4
while(arrayList.length) {
arrayList.pop();
}
Above implementation can also empty the array. But not recommended to use often.
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7. How to check if an object is an array or not?
The best way to find whether an object is instance of a particular class or not using
toStringmethod fromObject.prototype
var arrayList = [1 , 2, 3];
One of the best use cases of type checking of an object is when we do method overloading in JavaScript. For understanding this let say we have a method called greet which take one single string and also a list of string, so making our greet method workable in both situation we need to know what kind of parameter is being passed, is it single value or list of value?
function greet(param) {
if() {
// here have to check whether param is array or not
}
else {
}
}
However, in above implementation it might not necessary to check type for array, we can check for single value string and put array logic code in else block, let see below code for the same.
function greet(param) {
if(typeof param === 'string') {
}
else {
// If param is of type array then this block of code would execute
}
}
Now it’s fine we can go with above two implementations, but when we have a situation like a parameter can be single value, array, and object type then we will be in trouble.
Coming back to checking type of object, As we mentioned that we can use Object.prototype.toString
if(Object.prototype.toString.call(arrayList) === '[object Array]') {
console.log('Array!');
}
If you are using jQuery then you can also used jQuery isArray method:
if($.isArray(arrayList)) {
console.log('Array');
} else {
console.log('Not an array');
}
FYI jQuery uses Object.prototype.toString.call internally to check whether an object is an array or not.
In modern browser, you can also use:
Array.isArray(arrayList);
Array.isArray is supported by Chrome 5, Firefox 4.0, IE 9, Opera 10.5 and Safari 5
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8. What will be the output of the following code?
var output = (function(x) {
delete x;
return x;
})(0);
console.log(output);
Above code will output
0as output.deleteoperator is used to delete a property from an object. Herexis not an object it’s local variable.deleteoperator doesn’t affect local variable.
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9. What will be the output of the following code?
var x = 1;
var output = (function() {
delete x;
return x;
})();
console.log(output);
Above code will output
1as output.deleteoperator is used to delete property from object. Herexis not an object it’s global variable of typenumber.
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10. What will be the output of the following code?
var x = { foo : 1};
var output = (function() {
delete x.foo;
return x.foo;
})();
console.log(output);
Above code will output
undefinedas output.deleteoperator is used to delete a property from an object. Herexis an object which has foo as a property and from self-invoking function we are deleting foo property of objectxand after deletion we are trying to reference deleted propertyfoowhich resultundefined.
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11. What will be the output of the following code?
var Employee = {
company: 'xyz'
}
var emp1 = Object.create(Employee);
delete emp1.company
console.log(emp1.company);
Above code will output
xyzas output. Hereemp1object got company as prototype property. delete operator doesn’t delete prototype property.
emp1 object doesn’t have company as its own property. you can test it console.log(emp1.hasOwnProperty('company')); //output : false However, we can delete company property directly from Employee object using delete Employee.company or we can also delete from emp1 object using __proto__ property delete emp1.__proto__.company.
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12. What is undefined x 1 in JavaScript
var trees = ["redwood", "bay", "cedar", "oak", "maple"];
delete trees[3];
when you run above code and do console.log(trees); in chrome developer console then you will get ["redwood", "bay", "cedar", undefined × 1, "maple"] and when you run above code in Firefox browser console then you will get ["redwood", "bay", "cedar", undefined, "maple"] so from these it’s cleared that chrome has its own way of displaying uninitialized index in array. But when you check trees[3] === undefined in both of the browser you will get similar output as true.
Note: Please remember you need not check for uninitialized index of array in trees[3] === 'undefined × 1' it will give an error, Because 'undefined × 1' this is just way of displaying uninitialized index of array in chrome.
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13. What will be the output of the following code?
var trees = ["xyz", "xxxx", "test", "ryan", "apple"];
delete trees[3];
console.log(trees.length);
Above code will output
5as output. When we useddeleteoperator for deleting an array element then, the array length is not affected from this. This holds even if you deleted all the element of array usingdeleteoperator.
So when delete operator removes an array element that deleted element is not longer present in array. In place of value at deleted index undefined x 1 in chrome and undefined is placed at the index. If you do console.log(trees) output ["xyz", "xxxx", "test", undefined × 1, "apple"] in Chrome and in Firefox ["xyz", "xxxx", "test", undefined, "apple"].
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14. What will be the output of the following code?
var bar = true;
console.log(bar + 0);
console.log(bar + "xyz");
console.log(bar + true);
console.log(bar + false);
Above code will output
1, "truexyz", 2, 1as output. General guideline for addition of operator:
– Number + Number -> Addition
– Boolean + Number -> Addition
– Number + String -> Concatenation
– String + Boolean -> Concatenation
– String + String -> Concatenation
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15. What will be the output of the following code?
var z = 1, y = z = typeof y;
console.log(y);
Above code will output
undefinedas output. According toassociativityrule operator with the same precedence are processed based on their associativity property of operator. Here associativity of the assignment operator isRight to Leftso firsttypeof ywill evaluate first which isundefinedand assigned tozand thenywould be assigned the value of z and thenzwould be assign value1.
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16. What will be the output of the following code?
// NFE (Named Function Expression
var foo = function bar() { return 12; };
typeof bar();
Above code will output
Reference Erroras output. For making above code work you can re-write above code as follow:
Sample 1
var bar = function() { return 12; };
typeof bar();
or
Sample 2
function bar() { return 12; };
typeof bar();
The function definition can have only one reference variable as a function name, In above code sample 1 bar is reference variable which is pointing to anonymous function and in sample 2 function definition is name function.
var foo = function bar() {
// foo is visible here
// bar is visible here
console.log(typeof bar()); // Work here :)
};
// foo is visible here
// bar is undefined here
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17. What is the difference between declaring a function in below format?
var foo = function() {
// Some code
};
function bar() {
// Some code
};
The main difference is function foo is defined at run-time whereas function bar is defined at parse time. For understanding It in better way let see below code :
// Run-Time function declaration
foo(); // Call foo function here, It will give an error
var foo = function() {
console.log("Hi I am inside Foo");
};
// Parse-Time function declaration
bar(); // Call bar function here, It will not give an Error
function bar() {
console.log("Hi I am inside Foo");
};
The another advantage of first-one way of declaration that you can declare function based on certain condition for example:
if(testCondition) {// If testCondition is true then
var foo = function() {
console.log("inside Foo with testCondition True value");
};
} else {
var foo = function() {
console.log("inside Foo with testCondition false value");
};
}
But If you try to run similar code in the following format, it would give an error
if(testCondition) { // If testCondition is true then
function foo() {
console.log("inside Foo with testCondition True value");
};
} else {
function foo() {
console.log("inside Foo with testCondition false value");
};
}
18. what is function hoisting in JavaScript?
Function Expression
var foo = function foo() {
return 12;
};
In JavaScript variable and functions are
hoisted. Let’s take functionhoistingfirst. Basically, the JavaScript interpreter looks ahead to find all the variable declaration and hoists them to the top of the function where it’s declared. For Example:
foo(); // Here foo is still undefined
var foo = function foo() {
return 12;
};
Above code behind the scene look something like below code:
var foo = undefined;
foo(); // Here foo is undefined
foo = function foo() {
// Some code stuff
}
var foo = undefined;
foo = function foo() {
// Some code stuff
}
foo(); // Now foo is defined here
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19. What will be the output of the following code?
var salary = "1000$";
(function () {
console.log("Original salary was " + salary);
var salary = "5000$";
console.log("My New Salary " + salary);
})();
Above code will output:
undefined, 5000$. JavaScript has hoisting concept where newbie gets tricked. In above code, you might be expectingsalaryto retain it values from outer scope until the point thatsalarywas re-declared in the inner scope. But due tohoistingsalary value wasundefinedinstead. To understand it better have a look of the following code, heresalaryvariable is hoisted and declared at the top in function scope and while doing console.log it’s resultundefinedand after that it’s been redeclared and assigned5000$.
var salary = "1000$";
(function () {
var salary = undefined;
console.log("Original salary was " + salary);
salary = "5000$";
console.log("My New Salary " + salary);
})();
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20. What is the instanceof operator in JavaScript? what would be the output of the following code?
function foo() {
return foo;
}
new foo() instanceof foo;
instanceofoperator checks the current object and return true if the object is of the specified type.
For Example:
var dog = new Animal();
dog instanceof Animal; // Output : true
Here dog instanceof Animal is true since dog inherits from Animal.prototype
var name = new String("xyz");
name instanceof String; // Output : true
Here name instanceof String is true since name inherits from String.prototype. Now let’s understand the working of the following code
function foo() {
return foo;
}
new foo() instanceof foo;
Here function foo is returning foo which is again pointer to function foo
function foo() {
return foo;
}
var bar = new foo();
// here bar is pointer to function foo() {return foo}.
So the new foo() instanceof foo return false;
Ref Link: http://stackoverflow.com/questions/2449254/what-is-the-instanceof-operator-in-javascript
21. If we have JavaScript associative array as below code:
var counterArray = {
A : 3,
B : 4
};
counterArray["C"] = 1;
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How we will calculate length of the above associative array counterArray
There are no built-in function and property available to calculate length of associative array object. However, there are ways by which we can calculate the length of associative array object, In addition to this we can also extend
Objectby adding method or property on prototype for calculating length but extending object might break enumeration in various libraries or might create cross-browser issue, so it’s not recommended unless it’s necessary. There is various ways by which we can calculate length.
Object has keys method which can we used to calculate the length of object.
Object.keys(counterArray).length; // Output 2
We can also calculate length of object by iterating through the object and by doing a count of own property of object.
function getSize(object) {
var count = 0;
for(key in object) {
// hasOwnProperty method check own property of object
if(object.hasOwnProperty(key)) count++;
}
return count;
}
We can also add
lengthmethod directly onObjectsee below code.
Object.length = function() {
var count = 0;
for(key in object) {
// hasOwnProperty method check own property of object
if(object.hasOwnProperty(key)) count++;
}
return count;
}
//Get the size of any object using
console.log(Object.length(counterArray));
Bonus: We can also use Underscore (recommended, as it’s lightweight) to calculate object length.
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Question 22
Difference between Function, Method and Constructor calls in JavaScript.
If your are familiar with Object-oriented programming, More likely familiar to thinking of functions, methods, and class constructors as three separate things. But In JavaScript, these are just three different usage patterns of one single construct.
functions : The simplest usages of function call:
function helloWorld(name) {
return "hello world, " + name;
}
helloWorld("JS Geeks"); // "hello world JS Geeks"
Methods in JavaScript are nothing more than object properties that reference to a function.
var obj = {
helloWorld : function() {
return "hello world, " + this.name;
},
name: 'John Carter'
}
obj.helloWorld(); // // "hello world John Carter"
Notice how helloWorld refer to this properties of obj. Here it’s clear or you might have already understood that this gets bound to obj. But the interesting point that we can copy a reference to the same function helloWorld in another object and get a difference answer. Let see:
var obj2 = {
helloWorld : obj.helloWorld,
name: 'John Doe'
}
obj2.helloWorld(); // "hello world John Doe"
You might be wonder what exactly happens in a method call here. Here we call the expression itself determine the binding of this this, The expression obj2.helloWorld() looks up the helloWorld property of obj and calls it with receiver object obj2.
The third use of functions is as constructors. Like function and method, constructors are defined with function.
function Employee(name, age) {
this.name = name;
this.age = age;
}
var emp1 = new Employee('John Doe', 28);
emp1.name; // "John Doe"
emp1.age; // 28
Unlike function calls and method calls, a constructor call new Employee('John Doe', 28) create a brand new object and passes it as the value of this, and implicitly returns the new object as its result.
The primary role of the constructor function is to initialize the object.
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Question 23
What would be the output of the following code?
function User(name) {
this.name = name || "JsGeeks";
}
var person = new User("xyz")["location"] = "USA";
console.log(person);
The output of above code would be USA. Here new User("xyz") creates a brand new object and created property location on that and USA has been assigned to object property location and that has been referenced by the person.
Let say new User("xyz") crated a object called foo and returned now foo["location"] would be assigned value as USA and person is referencing to foo["location"].
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Question 24
Write a mul function which will output when invoked as below syntax.
console.log(mul(2)(3)(4)); // output : 24
console.log(mul(4)(3)(4)); // output : 48
Below is code followed by an explanation how it works:
function mul (x) {
return function (y) { // anonymous function
return function (z) { // anonymous function
return x * y * z;
};
};
}
Here mul function accept the first argument and return anonymous function which take the second parameter and return anonymous function which take the third parameter and return multiplication of arguments which is being passed in successive.
In Javascript function defined inside has access to outer function variable and function is a first class object so it can be returned by a function as well and passed as the argument in another function.
– A function is an instance of the Object type
– A function can have properties and has a link back to its constructor method
– Function can be stored as variable
– Function can be pass as a parameter to another function
– Function can be returned from function
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Question 25
What is the difference between a method and a function in javascript?
A function is a piece of code that is called by name and function itself not associated with any object and not defined inside any object. It can be passed data to operate on (i.e. parameter) and can optionally return data (the return value).
// Function definition
function myFunc() {
// Do some stuff;
}
// Calling function
myFunc();
Here myFunc() function call is not associated with object hence not invoked through any object.
A function can be self-invoking anonymous function or named self-invoking function
// Named Self-invoking Function
(function myFunc() {
// Do some stuff;
})();
// Anonymous Self-invoking Function
(function() {
// Do some stuff;
})();
In a case of named self-invoking anonymous function or anonymous self-invoking function, there is no need of call function explicitly.
A method is a piece of code that is called by name and that is associated with the object. In most respects it is identical to function call except for some key difference:
– It is implicitly passed for the object for which it was called.
– It is able to operate on data that is contained within the class (remembering that an object is an instance of a class- the class is the definition, the object is an instance of that)
– Method call is always associated with object
var methodObject = {
attribute: "xyz",
display: function () { // Method
console.log(this.attribute);
}
};
// Call method
methodObject.display();
Here methodObject is an object and display is a method which is associated with methodObject.
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Question 26
What is JavaScript Self-Invoking anonymous function or Self-Executing anonymous function.
A self-invoking anonymous function also called self-executing anonymous function runs immediately or automatically when we define it and self-invoking anonymous function doesn’t have any name at all. Self-Invoking anonymous function syntax:
(function() {
console.log("Self-Invoking function code logic ");
})();
Output: Self-Invoking function code logic
We must have to know the fact that JavaScript functions run immediately when we put () after their names.
function display() {
console.log("Display me");
}
display(); // This will run immediately
Output: "Display me"
/*
This will not run immediately as we haven't put () after function
name, function name is use full when we want to pass it as
callback to another function or method.
*/
display;
function testCallBack(callback) {
callback (); // This will be call display method immediately if callback parameter is being set method display
}
testCallBack(display); // Here display function is being passed as callback
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Question 27
Describe Singleton Pattern In JavaScript?
The singleton pattern is the most commonly used design pattern and one that you will probably is more than any others. It provides a great way to wrap the code into a logical unit that can be accessed through a single variable. The Singleton design pattern is used when only one instance of an object is needed throughout the lifetime of an application.
In JavaScript, there is a different way to achieve singleton object than any other object oriented supported language (Java, C++). In JavaScript Singleton pattern have many uses, they can be used for NameSpacing, which reduce the number of global variables in your page (prevent from polluting global space), organizing the code in a consistent manner, which increase the readability and maintainability of your pages. There are two important points in the traditional definition of Singleton pattern:
– There should be only one instance allowed for a class and
– We should allow global point of access to that single instance
– Let me define singleton pattern in JavaScript context:
It is an object that is used to create namespace and group together a related set of methods and attributes (encapsulation) and if we allow to initiate then it can be initiated only once.
In JavaScript, we can create singleton though object literal. However, there is some another way but that I will cover in next post.
A singleton object consists of two parts: The object itself, containing the members (Both methods and attributes) within it, and global variable used to access it. The variable is global so that object can be accessed anywhere in the page, this is a key feature of the singleton pattern.
JavaScript: A Singleton as a Namespace
As I have already stated above that singleton can be used to declare Namespace in JavaScript. NameSpacing is a large part of responsible programming in JavaScript. Because everything can be overwritten, and it is very easy to wipe out variable by mistake or a function, or even a class without even knowing it. A common example which happens frequently when you are working with another team member parallel,
function findUserName(id) {
}
/* Later in the page another programmer
added code */
var findUserName = $('#user_list');
/* You are trying to call :( */
console.log(findUserName())
One of the best ways to prevent accidentally overwriting variable is to namespace your code within a singleton object.
/* Using Namespace */
var MyNameSpace = {
findUserName : function(id) {},
// Other methods and attribute go here as well
}
/* Later in the page another programmer
added code */
var findUserName = $('#user_list');
/* You are trying to call and you make this time workable */
console.log(MyNameSpace.findUserName());
Singleton Design Pattern Implementation
/* Lazy Instantiation skeleton for a singleton pattern */
var MyNameSpace = {};
MyNameSpace.Singleton = (function() {
// Private attribute that holds the single instance
var singletonInstance;
// All of the normal code goes here
function constructor() {
// Private members
var privateVar1 = "Nishant";
var privateVar2 = [1,2,3,4,5];
function privateMethod1() {
// code stuff
}
function privateMethod1() {
// code stuff
}
return {
attribute1 : "Nishant",
publicMethod: function() {
alert("Nishant");// some code logic
}
}
}
return {
// public method (Global access point to Singleton object)
getInstance: function() {
//instance already exist then return
if(!singletonInstance) {
singletonInstance = constructor();
}
return singletonInstance;
}
}
})();
// getting access of publicMethod
console.log(MyNamespace.Singleton.getInstance().publicMethod());
The singleton implemented above is easy to understand. The singleton class maintains a static reference to the lone singleton instance and return that reference from the static getInstance() method.
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Question 28
What are the way by which we can create object in JavaScript ?
Method 1: Function Based
If we want to create several similar objects. In below code sample, Employee which is called constructor function. This is similar to classes in object oriented languages.
function Employee(fName, lName, age, salary){
this.firstName = fName;
this.lastName = lName;
this.age = age;
this.salary = salary;
}
// Creating multiple object which have similar property but diff value assigned to object property.
var employee1 = new Employee('John', 'Moto', 24, '5000$');
var employee1 = new Employee('Ryan', 'Jor', 26, '3000$');
var employee1 = new Employee('Andre', 'Salt', 26, '4000$');
Method 2: Object Literal
Object Literal is best way to create an object and this is used frequently. Below is code sample for create employee object which contains property as well as method.
var employee = {
name : 'Nishant',
salary : 245678,
getName : function(){
return this.name;
}
}
Below code sample is Nested Object Literal, Here address is an object inside employee object.
var employee = {
name : 'Nishant',
salary : 245678,
address : {
addressLine1 : 'BITS Pilani',
addressLine2 : 'Vidya Vihar'.
phoneNumber: {
workPhone: 7098889765,
homePhone: 1234567898
}
}
}
Method 3: Using JavaScript new keyword
In below code sample object has been created using Object constructor function.
var employee = new Object(); // Created employee object using new keywords and Object()
employee.name = 'Nishant';
employee.getName = function(){
return this.name;
}
Note: As a best practices object literal way is used to create object over this method.
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Question 29
Write a function called deepClone which takes an object and creates a object copy of it.
var newObject = deepClone(obj);
Solution:
function deepClone(object){
var newObject = {};
for(var key in object){
if(typeof object[key] === 'object'){
newObject[key] = deepClone(object[key]);
}else{
newObject[key] = object[key];
}
}
return newObject;
}
Explanation: We have been asked to do deep copy of object so What’s basically it’s mean ??. Let’s understand in this way you have been given an object personalDetail this object contains some property which again a type of object here as you can see address is an object and phoneNumber in side an address is also an object. In simple term personalDetail is nested object(object inside object). So Here deep copy means we have to copy all the property of personalDetail object including nested object.
var personalDetail = {
name : 'Nishant',
address : {
location: 'xyz',
zip : '123456',
phoneNumber : {
homePhone: 8797912345,
workPhone : 1234509876
}
}
}
So when we do deep clone then we should copy every property (including the nested object).
Question 30
Best way to detect undefined object property in JavaScript.
Suppose we have given an object
person
var person = {
name: 'Nishant',
age : 24
}
Here the person object has a name and age property. Now we are trying to access the salary property which we haven’t declared on the person object so while accessing it will return undefined. So how we will ensure whether property is undefined or not before performing some operation over it?
Explanation:
We can use typeof operator to check undefined
if(typeof someProperty === 'undefined'){
console.log('something is undefined here');
}
Now we are trying to access salary property of person object.
if(typeof person.salary === 'undefined'){
console.log("salary is undefined here because we haven't declared");
}
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Question 31
Write a function called Clone which takes an object and creates a object copy of it but not copy deep property of object.
var objectLit = {foo : 'Bar'};
var cloneObj = Clone(obj); // Clone is the function which you have to write
console.log(cloneObj === Clone(objectLit)); // this should return false
console.log(cloneObj == Clone(objectLit)); // this should return true
solution:
function Clone(object){
var newObject = {};
for(var key in object){
newObject[key] = object[key];
}
return newObject;
}
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Question 32
Best way to detect undefined object property in JavaScript.
Suppose we have given an object
person
var person = {
name: 'Nishant',
age : 24
}
here person object has name and age property. Now we are trying to access salary property which we haven’t declared on person object so while accessing it will return undefined. So how we will ensure whether property is undefined or not before performing some operation over it.
Explanation:
We can use typeof operator to check undefined
if(typeof someProperty === 'undefined'){
console.log('something is undefined here');
}
Now we are trying to access salary property of person object.
if(typeof person.salary === 'undefined'){
console.log("salary is undefined here because we haven't declared");
}
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Question 33
How to check whether a key exist in a JavaScript object or not.
Let say we have
personobject with property name and age
var person = {
name: 'Nishant',
age: 24
}
Now we want to check whether name property exist in person object or not ?
In JavaScript object can have own property, in above example name and age is own property of person object. Object also have some of inherited property of base object like toString is inherited property of person object.
So how we will check whether property is own property or inherited property.
Method 1: We can use in operator on objet to check own property or inherited property.
console.log('name' in person); // checking own property print true
console.log('salary' in person); // checking undefined property print false
in operator also look into inherited property if it doesn’t find property defined as own property. For instance If I check existence of toString property as we know that we haven’t declared this property on person object so in operator look into there base property.
Here
console.log('toString' in person); // Will print true
If we want to test property of object instance not inherited properties then we will use hasOwnProperty method of object instance.
console.log(person.hasOwnProperty('toString')); // print false
console.log(person.hasOwnProperty('name')); // print true
console.log(person.hasOwnProperty('salary')); // print false
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Objective Question
Hoisting
1. console.log(employeeId);
- Some Value
- Undefined
- Type Error
- ReferenceError: employeeId is not defined
Answer: 4) ReferenceError: employeeId is not defined
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2. What would be the output of following code?
console.log(employeeId);
var employeeId = '19000';
- Some Value
- undefined
- Type Error
- ReferenceError: employeeId is not defined
Answer: 2) undefined
3. What would be the output of following code?
var employeeId = '1234abe';
(function(){
console.log(employeeId);
var employeeId = '122345';
})();
- ‘122345’
- undefined
- Type Error
- ReferenceError: employeeId is not defined
Answer: 2) undefined
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Object
1. What would be the output of following code ?
(function() {
'use strict';
var person = {
name: 'Nishant'
};
person.salary = '10000$';
person['country'] = 'USA';
Object.defineProperty(person, 'phoneNo', {
value: '8888888888',
enumerable: true
})
console.log(Object.keys(person));
})();
- Type Error
- undefined
- [“name”, “salary”, “country”, “phoneNo”]
- [“name”, “salary”, “country”]
Answer: 3) [“name”, “salary”, “country”, “phoneNo”]
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2. What would be the output of following code ?
(function() {
'use strict';
var person = {
name: 'Nishant'
};
person.salary = '10000$';
person['country'] = 'USA';
Object.defineProperty(person, 'phoneNo', {
value: '8888888888',
enumerable: false
})
console.log(Object.keys(person));
})();
- Type Error
- undefined
- [“name”, “salary”, “country”, “phoneNo”]
- [“name”, “salary”, “country”]
Answer: 4) [“name”, “salary”, “country”]
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3. What would be the output of following code ?
(function() {
var objA = {
foo: 'foo',
bar: 'bar'
};
var objB = {
foo: 'foo',
bar: 'bar'
};
console.log(objA == objB);
console.log(objA === objB);
}());
- false true
- false false
- true false
- true true
Answer: 2) false false
4. What would be the output of following code ?
(function() {
var objA = new Object({foo: "foo"});
var objB = new Object({foo: "foo"});
console.log(objA == objB);
console.log(objA === objB);
}());
- false true
- false false
- true false
- true true
Answer: 2) false false
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Array
1. What would be the output of following code?
(function() {
var array = new Array('100');
console.log(array);
console.log(array.length);
}());
- undefined undefined
- [undefined × 100] 100
- [“100”] 1
- ReferenceError: array is not defined
Answer: 3) [“100”] 1
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2. What would be the output of following code?
(function() {
var array1 = [];
var array2 = new Array(100);
var array3 = new Array(['1',2,'3',4,5.6]);
console.log(array1);
console.log(array2);
console.log(array3);
console.log(array3.length);
}());
- [] [] [Array[5]] 1
- [] [undefined × 100] Array[5] 5
- [] [] [‘1′,2,’3’,4,5.6] 5
- [] [] [Array[5]] 5
Answer: 1) [] [] [Array[5]] 1
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3. What would be the output of following code?
(function () {
var array = new Array('a', 'b', 'c', 'd', 'e');
array[10] = 'f';
delete array[10];
console.log(array.length);
}());
- 11
- 5
- 6
- undefined
Answer: 1) 11
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4. What would be the output of following code?
(function(){
var animal = ['cow','horse'];
animal.push('cat');
animal.push('dog','rat','goat');
console.log(animal.length);
})();
- 4
- 5
- 6
- undefined
Answer: 3) 6
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Function:
1. What would be the output of following code ?
function funcA(){
console.log("funcA ", this);
(function innerFuncA1(){
console.log("innerFunc1", this);
(function innerFunA11(){
console.log("innerFunA11", this);
})();
})();
}
console.log(funcA());
- funcA Window {…}
innerFunc1 Window {…}
innerFunA11 Window {…} - undefined
- Type Error
- ReferenceError: this is not defined
Answer: 1)
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2. What would be the output of following code ?
var obj = {
message: "Hello",
innerMessage: !(function() {
console.log(this.message);
})()
};
console.log(obj.innerMessage);
- ReferenceError: this.message is not defined
- undefined
- Type Error
- undefined true
Answer: 4) undefined true
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3. What would the output of following code ?
var obj = {
message: "Hello",
innerMessage: function() {
console.log(this.message);
}
};
console.log(obj.innerMessage());
- Hello
- undefined
- Type Error
- ReferenceError: this.message is not defined
Answer: 1) Hello
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4. What would the output of following code ?
var obj = {
message: 'Hello',
innerMessage: function () {
(function () {
console.log(this.message);
}());
}
};
console.log(obj.innerMessage());
- Type Error
- Hello
- undefined
- ReferenceError: this.message is not defined
Answer: 3) undefined
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Object Oriented
1. What would the output of following code ?
function Person(name, age){
this.name = name || "John";
this.age = age || 24;
this.displayName = function(){
console.log(this.name);
}
}
Person.name = "John";
Person.displayName = function(){
console.log(this.name);
}
var person1 = new Person('Nishant');
person1.displayName();
Person.displayName();
- Nishant Person
- Nishant John
- Nishant undefined
- John John
Answer: 1) Nishant Person
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License
This book is released under a Creative Commons Attribution-Noncommercial- No Derivative Works 3.0 United States License.
What this means it that the project is free to read and use, but the license does not permit commercial use of the material (i.e you can freely print out the questions for your own use, but you can’t sell it). I’m trying to best to publish all of my books in a free + purchased (if you would like to support these projects) form so I would greatly appreciate it if you would respect these terms.
Copyright Nishant Kumar, 2016.
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