array.splice() 返回我想要消除的项目而不是数组减去项目

Question

I'm trying to remove an item from an array using the indexOf() with splice() technique suggested. This is what's happening

let someArray: string[] = [first, second, third, fourth, fifth, sixth];
let newArray: string[] = someArray.splice(3, 1);

console.log(newArray);

//deisred result = [first, second, third, fifth, sixth]

//result I'm getting = [fourth]

That's not what virtually every article I've come across says should happen. Can someone shed light on this?

UPDATE I discovered this problem in my code when I was only ghetting 1 result where I was expecting more and tracked it back to this point.

Answer 1

Because when you splice an array you are mutating it, which means you are changing the original array. You're storing the result (the element you're splicing from the array) within the "newArray" variable that you have created here. So this:

var arr = [1, 2, 3, 4];

var mine = arr.splice(1, 1);
console.log(mine);
console.log(arr);

would return the original ray minus index one if we print arr to the console, and will return [2] if we print mine to the console. To get the output you're expecting, you would have to perform a different operation such as iterating through the array and utilizing splice differently. Here is an example:

var arr = [1, 2, 3, 4];

var mine = [];

for(var i = 0; i < arr.length; i++) {
    if(i !== 3) {
        mine.push(arr[i]);
    }
}

Now I am not mutating the original array, and I am simply pushing the elements to a new array.

But if you want to simply mutate the original array and not store the new array in some sort of variable you can simply splice the original array:

var arr = [1, 2, 3, 4];

arr.splice(3, 1);
console.log(arr);

However, if you are passing it to a function, i'd probably not mutate an array outside of the function, and i'd simply return a value and store that value in a new variable:

var arr = [1, 2, 3, 4];


function deleteIndex(ar, i) {
    var a = [];
    ar.forEach(function(elt, index) {
        if(index === i) {

        }
        else {
            a.push(elt);
        }
    });
    return a;
}

var newArr = deleteIndex(arr, 3);
console.log(newArr);

This way you can delete any index, or pass a function and criteria that you would want to use to determine if an index should be deleted, without changing to top-level structure of your original array by utilizing functional programming. There are also some function in the underscore module that can help you if that's the case.