function twoSum(nums, target) {
  const seen = new Map(); // value -> index
  for (let i = 0; i < nums.length; i++) {
    const need = target - nums[i];
    if (seen.has(need)) return [seen.get(need), i];
    seen.set(nums[i], i);
  }
  return [];
}
twoSum([2, 7, 11, 15], 9); // [0, 1]

Nested loops are O(n²); a Map of already-seen values brings it to O(n). Return the indices in ascending order.

sandbox.js

function twoSum(nums, target) {
  // TODO
}

Output

Hit “Run tests” to check your solution.

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JavaScriptMedium12/15

Write twoSum(nums, target) returning the indices of the two numbers adding up to target (empty array if none)

By FrontendPro Editorial Team Updated 8/7/2026

Explore the JavaScript interview guide

Examples

Write twoSum so that the following cases hold.

InputtwoSum([2,7,11,15], 9)
Output[0,1]
InputtwoSum([3,2,4], 6)
Output[1,2]
InputtwoSum([3,3], 6)
Output[0,1]

1 more hidden test cases - 4 in total will be graded.

Answer

function twoSum(nums, target) {
  const seen = new Map(); // value -> index
  for (let i = 0; i < nums.length; i++) {
    const need = target - nums[i];
    if (seen.has(need)) return [seen.get(need), i];
    seen.set(nums[i], i);
  }
  return [];
}
twoSum([2, 7, 11, 15], 9); // [0, 1]