Why does forEach return undefined?

Question

Here is my code to solve leetcode- Two sum

let nums = [2, 7, 11, 15];
let  target = 9;
for (let i = 0; i < nums.length; i++) {
  let x = target - nums[i];
  if (nums[i + 1] === x) {
    console.log([i, i+1]) // [0, 1]
    return [i, i + 1];
  }
}  //it ok

nums.forEach((item, index) => {
  let x = target - nums[index];
  if (nums[index + 1] === x) {
    console.log([index, index + 1]);// [0, 1]
    return [index,index+1]
  }
}); // undefined

I use forEach instead of for(...) , both console.log are same, but forEach returns undefined . Why?

Answer 1

You can use JS map because the forEach always return undefined

let nums = [2, 7, 11, 15],
  target = 9;
let m = {};
nums.map((value, index) => {
  m[value] = index;
});

for (i = 0; i < nums.length; i++) {
  let diff = target - nums[i];
  if (m.hasOwnProperty(diff)) {
    console.log(i, m[diff]);
  }
}