Javascript Interview Question: Why does [9,8,7,6][1,2] = 7 ?

JavaScript Puzzles Unveiled: A Closer Look at Unexpected Array Operations in Interviews

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To cut it short, here is the question. What will be logged in the console when we run this code snippet?

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.

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When we run this code snippet the value res will be 7 . And 7 will be logged in the console.

Let’s Break this down.

Step 1: [1,2] This will be converted into [2].

Why?

The subsequent element [1,2] cannot function as an array; hence, it operates as an array subscript. In the context of a subscript operation, the contents do not constitute a delimited list of operands but rather a singular expression.

In javascript, we can write expressions separated by a comma (,). and the result of the last expression will be returned.

// Example 1
const z = (1,2,3,4,5);
console.log(z); // outputs 5

// Example 2
function a() {
    return 'a';
}

function b() {
    return 'b';
}

function c() {
    return 'c';
}

const d = (a(), b(), c());
console.log(d); // Outputs 'c'

Step 2: After the above code becomes [9,8,7,6][2] which is straightforward, we are trying to access the element at 2 index, which is 7.

Here are more variations:

[9,8,7,6][1,2,3] // outputs 6
[9,8,7,6][8,2,3,1] // outputs 8