Top 10 JavaScript Interview Questions You Must Prepare in 2020

Q1. What are the scopes of a variable in JavaScript?

The scope of a variable is the region of your program in which it is defined. JavaScript variable will have only two scopes.
• Global Variables − A global variable has global scope which means it is visible everywhere in your JavaScript code.
• Local Variables − A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.

Q2. What is JavaScript?

JavaScript is a lightweight, interpreted programming language with object-oriented capabilities that allows you to build interactivity into otherwise static HTML pages. The general-purpose core of the language has been embedded in Netscape, Internet Explorer, and other web browsers.

Q3. What is argument objects in JavaScript & how to get the type of arguments passed to a function?

JavaScript variable arguments represents the arguments that are passed to a function. Using typeof operator, we can get the type of arguments passed to a function. For example

            function func(x){

console.log(typeof x, arguments.length);

}

func(); //==> "undefined", 0

func(7); //==> "number", 7

func("1", "2", "3"); //==> "string", 3

Q4. How to delete a cookie using JavaScript?

If you want to delete a cookie so that subsequent attempts to read the cookie return nothing, you just need to set the expiration date to a time in the past. You should define the cookie path to ensure that you delete the right cookie. Some browsers will not let you delete a cookie if you don’t specify the path.

Now let’s move on to the next section of JavaScript interview questions.

Q5. In how many ways a JavaScript code can be involved in an HTML file?

There are 3 different ways in which a JavaScript code can be involved in an HTML file:

•           Inline

•           Internal

•           External

An inline function is a JavaScript function, which is assigned to a variable created at runtime. You can differentiate between Inline Functions and Anonymous since an inline function is assigned to a variable and can be easily reused. When you need a JavaScript for a function, you can either have the script integrated in the page you are working on, or you can have it placed in a separate file that you call, when needed. This is the difference between an internal script and an externalscript.

Q6. How do JavaScript primitive/object types passed in functions?

One of the differences between the two is that Primitive Data Types are passed By Value and Objects are passed By Reference.

•           By Value means creating a COPY of the original. Picture it like twins: they are born exactly the same, but the first twin doesn’t lose a leg when the second twin loses his in the war.

•           By Reference means creating an ALIAS to the original. When your Mom calls you “Pumpkin Pie” although your name is Margaret, this doesn’t suddenly give birth to a clone of yourself: you are still one, but you can be called by these two very different names.

Q7. What will be the output of the code below?

            var Y = 1;

if (function F(){})

{

y += Typeof F;</span>

}

console.log(y);

The output would be 1undefined. The if condition statement evaluates using eval, so eval(function f(){}) returns function f(){} (which is true). Therefore, inside the if statement, executing typeof f returns undefined because the if statement code executes at run time, and the statement inside the if condition is evaluated during run time.

Q8. How to empty an Array in JavaScript?

There are a number of methods you can use to empty an array:

Method 1 –

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

Method 2 –

1          arrayList.length = 0;

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.

Method 3 –

1          arrayList.splice(0, arrayList.length);

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

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.

Q9. 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);

The output would be undefined. The delete operator is used to delete the property of an object. Here, x is an object which has the property foo, and as it is a self-invoking function, we will delete the foo property from object x. After doing so, when we try to reference a deleted property foo, the result is undefined.

Q10. How does TypeOf Operator work?

The typeof operator is used to get the data type of its operand. The operand can be either a literalor a data structure such as a variable, a function, or an object. It is a unary operator that is placed before its single operand, which can be of any type. Its value is a string indicating the data type of the operand.