为什么我必须在从我的数组中提取之前从我的参数中提取以从数组中删除一个项目？

Question

Here's the code I'm working with:

const removeFromArray = function(arr) {
    let args = Array.from(arguments);
    for(let x=0; x<arr.length; x++) {
        for(let y=1; y<args.length; y++) {
            if(arr[x]===args[y]) {
                arr.splice(x,1);
            }
        }
    }
    return arr;
}

I can get the code to work if I switch the outer loop with the inner loop.

I just can't figure out logically why it doesn't work if my outer loop pulls from the parameter array first. It works for most things, but it has problems with this for example:

removefromArray([1,2,3,4], 3, 2)

It only removes 2 leaving 3 untouched.

I thought maybe the loop removes the 2 and pushes 3 down the index. But in that case, removeFromArray([1,2,3,4], 1, 2, 3, 4) wouldn't work but it does.

What am I not getting?

Answer 1

splice will mutate the array. If you're iterating over the array at the same time you splice , you'll have to be careful to reset the index back to account for the newly rearranged elements. For example, with [1, 2, 3, 4] , if both 2 and 3 match the condition to remove element from the array:

• On one iteration, x will be 1, matching value 2. 2 will be removed from the array with splice . The array will then become [1, 3, 4] .
• If, at this point, you increment x , it'll then be 2, so the item at index 2 will be checked. But the item at that index is now 4; the 3 is never checked. Or, if you increment y , the index of the argument being checked, then the argument that was just removed won't be checked again.

Either manually decrement the index when a match is found

 // Not recommended; verbose, ugly, and impure const removeFromArray = function(arr) { let args = Array.from(arguments); for(let x=0; x<arr.length; x++) { for(let y=1; y<args.length; y++) { if(arr[x]===args[y]) { arr.splice(x,1); x--; } } } return arr; } console.log(removeFromArray([1,2,3,4], 3, 2));

or use .filter instead:

 const removeFromArray = (arr, ...toRemove) => arr.filter( item => !toRemove.includes(item) ); console.log(removeFromArray([1,2,3,4], 3, 2));

Also make sure to use the same capitalization - if you define the function as removeFromArray , make sure to call it with a capital F . (using removefromArray instead will throw a ReferenceError)