how is undefined treated while looping

Question

[1, 3, 4, 5, 5, undefined, 4, 3, 4].map((item) => {
    if (item) {
        return item
    }
})

The output of the above code is

Why is undefined also returned, shouldn't it be stopped by the if loop?

You're probably looking for `filter` because `map` return the same size like the original array — Alon Eitan, Dec 26 '20 at 09:26
Does this answer your question? [Removing undefined values from Array](https://stackoverflow.com/questions/28607451/removing-undefined-values-from-array) (or [How to remove undefined and create a new array Javascript?](https://stackoverflow.com/questions/44749528/how-to-remove-undefined-and-create-a-new-array-javascript), or [Why does javascript map function return undefined?](https://stackoverflow.com/questions/16037049/why-does-javascript-map-function-return-undefined) — pilchard, Dec 26 '20 at 11:12

ASDFGerte · Accepted Answer · 2020-12-26T09:40:54.530

Your map function doesn't return anything explicitly if the item is falsy, hence it returns undefined (per default).

As another comment has pointed out, you are probably looking for Array.prototype.filter:

console.log([1,3,4,5,5,undefined,4,3,4].filter((item)=>{
  if(item){
    return item;
  }
}));

Note that the above is slightly verbose, .filter(x => x) would suffice, filter calls ToBoolean on the return anyways. It will still filter out any falsy values though, e.g. 0.

how is undefined treated while looping

1 Answers1