Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

Example 2:

Input: nums = [3,2,4], target = 6
Output: [1,2]

Example 3:

Input: nums = [3,3], target = 6
Output: [0,1]

Constraints:

• 2 <= nums.length <= 104
• -109 <= nums[i] <= 109
• -109 <= target <= 109
• Only one valid answer exists.

 The easiest solution for two sum program in JS is as follows:

var twoSum = function(nums, target) {
    
    for(let i=0;i < nums.length;i++){
        for(let j=i+1;j < nums.length;j++){
            if((parseInt(nums[i])+parseInt(nums[j]))===target){
               return [i,j];
               }
        }    
    }
};

Time complexity is O(n2).

Another optimized Solution of the two sum program in js is as follows

1. Create new map variable 
2. Loop array 
3. secondNumber = get second Number by subtracting target-arr[I] 
4. if secondNumber is available in map then
   return [map.get(secondNumber),i]// in short indexes of the both numbers 
5. else
   set map with value and index => map.set(arr[i],i)
   */

const twoSum = (arr, target) => {
    const map = new Map();
    for (let i = 0; i < arr.length; i++) {
        const secondNumber = target - arr[i];
        if (map.has(secondNumber)) {
            return [map.get(secondNumber), i];
        } else {
            map.set(arr[i], i)
        }
    }
    return [];
}

console.log('Output should be [0,1]: ', twoSum([2, 7, 11, 15], 9));
console.log('Output should be [1,2]: ', twoSum([3, 2, 4], 6));
console.log('Output should be [1,3]: ', twoSum([2, 7, 11, 15], 22));
console.log('Output should be [1,2]: ', twoSum([2, 7, 11, 15], 18));
console.log('Output should be [0,2]: ', twoSum([2, 7, 11, 15], 13));
console.log('Output should be [0,4]: ', twoSum([2, 7, 11, 15, 32], 34));
  
  

Time complexity is O(n).