Confused as to why this works

[gtrman97]

I came across this code in the blog, but don’t the variables “nextToLast” and “last” need to be declared or defined first?

const fibonacci = (num = 2, array = [0, 1]) => {
    while (num > 2) {
        const [nextToLast, last] = array.slice(-2);
        array.push(nextToLast + last);
        num -= 1;
    }
    return array;
}

console.log(fibonacci(10));

[caryaharper]

This line const [nextToLast, last] = array.slice(-2); uses array destructuring and is a way of declaring, and assigning multiple elements at once.

An equal piece of code using the same values as the example would be:

const nextToLast = array[array.length - 2];
const last = array[array.length - 1];

Destructuring assignment - JavaScript | MDN

[gtrman97]

So we are declaring individual variables with that line and not necessarily a new array with the square brackets correct?

[caryaharper]

That is correct. The “array” on the left is just a list of the individual variable names, the array on the right is a real array and the variables will be assigned to the values based on what index they share.