在for-of循环中更改项目的值

Question

I'm learning some of the new tricks in ES6, and I quite like the for-of loop for arrays. One thing I'm having some trouble with is if I want to manipulate the current item's value inside the loop.

For example in ES5:

var myArr = [1,2,3];
for(var i = 0; i < myArr; i++){
    myArr[i] = "moo"
}

When using the for-of loop in ES6 if I change the value of the current item inside the loop it's not reflected in the array. Is there a way to do this?

Answer 1

If you also need the index in an array iteration with a for … of loop, use the .entries() method :

const myArr = [1,2,3];
for (const [i, el] of myArr.entries()) {
    myArr[i] = "moo" + el;
}

If you're only interested in the indices and no values, you could iterate myArr .keys() instead.

Answer 2

Not with for-of , no. You don't have the index of the item, and writing to your variable has no effect on the original array.

You could hack it:

let index = 0;
for (const v of myArr) {
    if (/*...some condition...*/) {
        myArr[index] = "moo";
    }
    ++index;
}

...and that works, because of the definition of the array iterator, but it's pretty verbose. In that case, forEach might be a better choice if you want to update the array in-place, or map if you're happy to create a new array, or a good old-fashioned for loop.

Gratuitous example of the above (also showing that it works with sparse arrays, because of the way the array iterator is defined):

 let myArr = [1,2,3]; myArr[10] = "sparse"; // Just to prove it works console.log("before", myArr); let index = 0; for (const v of myArr) { if (v === "sparse") { myArr[index] = "moo"; } ++index; } console.log("after", myArr);