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JavaScript Quiz – what really is the array index value

JavaScript Quiz – what really is the array index value

JavaScript Array Index Value

In javascript, an array is a special variable like an object which can store multiple types of data like number, string, boolean, array, object, etc. As to access array index value, an index number is passed to get the value.

Example of Array Index Value –

In the following example, an array variable is declare and assign an javascript array to it. Then Index is pass to access array value of particular location in array.

const array = [1, "JS", true];

console.log(array[2]); // Here, 2 is an index
// => "JS"

For more detail about javascript array click here.

Array push method

Javascript array push method adds an element at the end of the array and element can be number, string, boolean, array & object etc.

For Example –

const array = [1, 2];
array.push(3); // => [1, 2, 3]

Array unshift method

Javascript array unshift method add element at the start of the array and element can be number, string, boolean, array & object etc.

For example –

const array = [1, 2];
array.unshift(0); // => [0, 1, 2]

JavaScript Question

In this javascript quiz, find out what really is the array index value when value-added to an array at a particular index and push, unshift.

First, we declare a variable array and using the const keyword, assign an javascript array to variable array. An Array only contain a numbers only.

const array = [10, 15, 20];

In the second step, value of 25 assign to array at index 5.

array[5] = 25;

Now, we use the array push method and pass 30 value to it and also use another array method unshift and 35 value pass to it.


Finally, we are checking the value of array at index 5 using console log.


JavaScript Quiz Test

const array = [10, 15, 20];

array[5] = 25;

console.log(array[5]); // output => ?

Any idea what should the output above code snippet ?

Answer – undefined

Click To Run Code


Now, you know the answer is “undefined” But why we haven’t added undefined value to an array and even if add 25 value to an array at index 5. Let me explain

So, as you are correct the value at index is 25. But when we also push the 30 to array. But it didn’t change the position of 25. As 30 added at the bottom of array.

But when we use the unshift method with value 35. This value-added at top of the array. And when we assign 25 to index 5 then automatically at index 3 & 4 undefined is assigned.

So, the value array look process like –

const array = [10, 15, 20];
// => [10, 15, 20]

array[5] = 25;
// => [10, 15, 20, undefined, undefined, 25]

// => [10, 15, 20, undefined, undefined, 25, 30]

// => [30, 10, 15, 20, undefined, undefined, 25, 30]

console.log(array[5]); // output => undefined

At last, That’s why we get an answer “undefined“.

Let me know in the comment section, what you think about it.

I hope you understand the concept and logic behind it.

But wait, i have something more for you –

Check out our other javascript quiz – 

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  4. JavaScript Quiz – can array really be access with an array


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