JavaScript question

1. What is the difference between undefined and not defined in JavaScript?

> var x; // declaring x

> console.log(x); //output: undefined 

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> console.log(y);  // Output: ReferenceError: y is not defined

2. What will be the output of the code below?

var y = 1;

  if (function f(){}) {

    y += typeof f;

  }

  console.log(y);

The output would be 1undefined.

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var k = 1;

  if (1) {

    eval(function foo(){});

    k += typeof foo;

  }

  console.log(k); 

The code above will also output 1undefined.

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var k = 1;

  if (1) {

    function foo(){};

    k += typeof foo;

  }

  console.log(k); // output 1function

3.What is the drawback of creating true private methods in JavaScript?

One of the drawbacks of creating true private methods in JavaScript is that they are very memory-inefficient, as a new copy of the method would be created for each instance.

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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 increaseSalary = 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);

emp1.dispalyIncreasedSalary()

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error: Uncaught ReferenceError: increaseSlary is not defined

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Here each instance variable emp1, emp2, emp3 has its own copy of the increaseSalary private method.

So, as a recommendation, don’t use private methods unless it’s necessary.

4. What is a “closure” in JavaScript? Provide an example

A closure is a function defined inside another function (called the parent function), and has access to variables that are declared and defined in the parent function scope.

The closure has access to variables in three scopes:

Variables declared in their own scope

Variables declared in a parent function scope

Variables declared in the global namespace

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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);

 

    })(5)// end of scope innerFunction)(5); // Pass 5 as parameter 

})(7)// end of scope outerFunction )(7); // Pass 7 as parameter 

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outerArg = 7

outerFuncVar = x

innerArg = 5

innerFuncVar = y

globalVar = abc

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5. Write a mul function which will produce the following outputs when invoked:

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function mul (x) {

    return function (y) { // anonymous function 

        return function (z) { // anonymous function

            console.log("x:"+x+",y:"+y+",z:"+z);

            return x * y * z; 

        };

    };

}

console.log(mul(2)(3)(4)); // output : 24 

console.log(mul(4)(3)(4)); // output : 48

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x:2,y:3,z:4

24

x:4,y:3,z:4

48

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6. How to empty an array in JavaScript?

var arrayList =  ['a','b','c','d','e','f'];

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Method 1

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']

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 method of emptying the array, because if you have referenced this array from another variable, then the original reference array will remain unchanged.

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Method 2

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 []

The code above will clear the existing array by setting its length to 0. This way of emptying the array also updates all the reference variables that point to the original array. Therefore, this method is useful when you want to update all reference variables pointing to arrayList.

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Method 3

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 []

The implementation above will also work perfectly. This way of emptying the array will also update all the references to the original array.

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Method 4

while(arrayList.length){

  arrayList.pop();

}

The implementation above can also empty arrays, but it is usually not recommended to use this method often.

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7.How do you check if an object is an array or not?

The best way to find out whether or not an object is an instance of a particular class is to use the toString method from Object.prototype:

 var arrayList = [1,2,3];

Array.isArray(arrayList);

8.What will be the output of the following code?

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var output = (function(x){

    delete x;

    return x;

  })(0);

  

  console.log(output);

The output would be 0. The delete operator is used to delete properties from an object. Here x is not an object but a local variable. delete operators don't affect local variables.

Reference:

https://www.codementor.io/@nihantanu/21-essential-javascript-tech-interview-practice-questions-answers-du107p62z