let nums = [2, 7, 11, 15];
let target = 9;

var twoSum = function(nums, target) {
    let res = {};
    for(let i = 0; i < nums.length; i++) {
        let num = nums[i];
        if(num in res) {
            return [res[num], i];
        } else {
            res[target - num] = i;
        }
    }
}
const res = twoSum(nums, target)
console.log(res)