When defining arrays in JavaScript using Array(n), it creates an empty array. If I write the following code:
Array(2).map((val) => console.log(val))Nothing happens when running this code, but if I spread out the elements of the array into another array like this:
[...Array(2)].map((val) => console.log(val))I end up getting undefined console logs. Can someone help me understand what is going on here?
When an array is not spread, it remains as an object with only a length property and no populated items:
{ length: 2, proto: Array(0) }
However, when you use spread operator (...) on an array, it will populate the array with items based on the original object’s properties starting from obj[0], obj[1], etc. If the object does not have these numeric properties, the result of spreading will create an array filled with undefineds:
console.log([...Array(2)]);An array of undefineds can still be iterated over – even though the values are not assigned to anything:
const arr = [undefined, undefined];
console.log(1 in arr);Therefore, the distinction lies between an array like
{ length: 2 } // no elements to iterate over
and
{ length: 2, 0: undefined, 1: undefined }
It's important to choose the correct array method for your task - if you simply need to loop through each item without transforming them, consider using forEach instead of map.
The reason for this behavior is due to the usage of the spread operator, which first determines the length of the iterable and then iterates over the provided value (refer to MDN documentation).
In the case of iterating over Array(2), all returned values are undefined. Consequently, the second line of code uses map on [undefined, undefined], while the first line maps over [empty x 2]. It's important to note that "empty" does not represent actual values.
When using map, it skips over "empty" entries like those generated by Array(2) but does iterate over undefined values:
When using Array(2), the array is not populated with any elements, but its length property is set to 2. This causes an issue for the map function because there are no elements to iterate over initially. However, destructuring helps by creating an array of 2 elements filled with undefined, making it possible for map to successfully execute since undefined is a valid primitive.
Array.map is a powerful method in JavaScript - make sure to check out the official documentation here: Array.prototype.map()
The callback function specified in Array.map is only called for indexes with assigned values, even if they are set as undefined. It does not run for missing elements in the array that have never been initialized or deleted.
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