Why do we use `Number.prototype.valueOf` inside of a `map()` function

Question

The following code:

let resultsArray = Array.apply(null, Array(10)).map(Number.prototype.valueOf,0);

creates the following array

Answer 1

In your code, 0 is passed to Number.prototype.valueOf which just returns the primitive (0).

Which is essentially the same thing as

Number.prototype.valueOf.call(0)

If your code was:

let resultsArray = Array.apply(null, Array(10)).map(Number.prototype.valueOf,10);

The map() function would essentially be calling Number.prototype.valueOf like this:

Number.prototype.valueOf.call(10)

Here's the documentation - which provides a nice Polyfill that you can read and see exactly what map() is doing.

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Strings would look like this:

Array.apply(null, Array(10)).map(String.prototype.valueOf,"10")

Which would output:

["10", "10", "10", "10", "10", "10", "10", "10", "10", "10"]

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But surely you can do it in a more tricky way ;)

var resultsArray = '0'.repeat(10).split('').map(Number);
console.log(resultsArray);

And in case it's not necessary to get the numbers (but strings), it's even shorter:

var resultsArray = '0'.repeat(10).split('');
console.log(resultsArray);