When you use Array(arrayLength)
to create an array, you will have:
a new JavaScript array with its length property set to that number (Note: this implies an array of arrayLength empty slots, not slots with actual undefined values).
The array does not actually contain any values, not even undefined
values – it simply has a length
property.
When you spread an iterable object with a length
property into an array, spread syntax accesses each index and sets the value at that index in the new array. For example:
const arr1 = [];
arr1.length = 4;
// arr1 does not actually have any index properties:
console.log('1' in arr1);
const arr2 = [...arr1];
console.log(arr2);
console.log('2' in arr2);
And .map
only maps properties/values for which the property actually exists in the array you’re mapping over.
Using the array constructor is confusing. I would suggest using Array.from
instead, when creating arrays from scratch – you can pass it an object with a length
property, as well as a mapping function:
const arr = Array.from(
{ length: 2 },
() => 'foo'
);
console.log(arr);