Mapping through an undefined array using apply

Question

Can anyone explain the next weird behavior?

This works:

const defaultSides = 10;
const stats = Array.apply(null, { length: defaultSides }).map(() => 100);
// Array [100, 100, 100, 100, 100, 100, 100, 100, 100, 100]

This doesn't:

const stats2 = new Array(defaultSides);
const res = stats2.map(() => 100);
console.log(res)
//Array [undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined]

I've already solved the problem using either Array.from or Array.of. However, I want to know what causes Javascript to still return me an undefined array after obviously having mapped through it in the second code block.

Nina Scholz · Accepted Answer · 2021-01-08 19:23:17Z

2

new Array(defaultSides) creates a sparse array.

Array#map iterates only non sparse items with the callback.

Due to the algorithm defined in the specification, if the array which map was called upon is sparse, resulting array will also be sparse keeping same indices blank.

answered Jan 8, 2021 at 19:23

Nina Scholz

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MinusFour · Accepted Answer · 2021-01-08 19:26:20Z

0

Array constructor does different things depending on the number of arguments. If you give it a number value as it's sole argument it will set the array length property to that value. E.g.

let arr = [];
arr.length = val;

This doesn't actually sets any value from property 0 to val.

If you feed the Array constructor a list of values however (which is what your apply call is doing) it will create an array with those values.

answered Jan 8, 2021 at 19:26

MinusFour

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