LeetCode Question - 2634. Filter Elements from Array

Question

When I try to run this code without the Number(), it doesn't work.

ar filter = (arr, fn) => {
 let filteredArr = [];

   for(const i in arr){

      if(fn(arr[i], Number(i)))  filteredArr.push(arr[i])
      
   }

return filteredArr;
};

I don't understand why this code doesn't work without the Number(), if the index is some number, at least 0, it's a integer, then it should work... Could you help me? I would be really grateful.

Answer 1

The for...in loop iterates over enumerable string properties. All the indexes used to access elements of an array are such properties, so you need to convert each index to a number when iterating with for (const i in arr).

Note that it is generally a very bad idea to use this form of loop when you want to access indexes. If you cannot use Array#filter, you could use Array#forEach which passes the index to its callback, or a standard index-based for loop.

arr.forEach((x, i) => {
    if (fn(x, i)) filteredArr.push(x);
});

Answer 2

Because you use for...in:

var filter = (arr, fn) => {
    let filteredArr = [];
    for(const i in arr){
        console.log(typeof i)
        if(fn(arr[i], Number(i)))  filteredArr.push(arr[i])
    }
    return filteredArr;
};

This loop is used for objects and is not recommended for arrays.

You can try to use the loop for that consists of three optional expressions:

var filter = (arr, fn) => {
    let filteredArr = [];
    for(let i=0; i<arr.length; i++){
        if(fn(arr[i], i))  filteredArr.push(arr[i])
    }
    return filteredArr;
};