Sorting map by key returns sorted Map object with the first key in last index

Question

The following function creates a Map object from two arrays. I am trying to sort the map by key into a new Map object by using the sort method, as shown on Mozilla Docs. The mapSort object is correctly sorting all of the keys, besides the fact that it is putting the first key at the last index.

const sortMapByKey = () => {
  const keyArray = [4, 6, 16, 18, 2]
  const valueArray = [103, 123, 4444, 99, 2000]

  const buildMap = (keys, values) => {
      const map = new Map();
      for(let i = 0; i < keys.length; i++){
        map.set(keys[i], values[i]);
      };
      const mapSort = new Map([...map.entries()].sort(function(a, b) {
        return a - b;
      }));
      return mapSort
    };  
    
    return buildMap(keyArray, valueArray)
}

The current output:

Map { 4 => '103', 6 => '123', 16 => '4444', 18 => '99', 2 => '2000' }

The desired output:

Map {  2 => '2000', 4 => '103', 6 => '123', 16 => '4444', 18 => '99' }

Any clue on what I'm doing wrong with my sort?

The values come from a much longer function which wasn't quite relevant, so I extracted them into variables for easier testing. — ltcrawsh
– ltcrawsh, Commented Nov 29, 2021 at 4:24

CertainPerformance · Accepted Answer · 2021-11-29 04:20:20Z

The entries are a key-value pair, eg [1, 103]. Doing return a - b tries to subtract two such arrays from each other, which doesn't make sense - you need to extract the key from the entry pair first.

const mapSort = new Map([...map.entries()].sort(function(entryA, entryB) {
  return entryA[0] - entryB[0];
}));

const keyArray = [4, 6, 16, 18, 2]
const valueArray = [103, 123, 4444, 2000]

const buildMap = (keys, values) => {
  const map = new Map();
  let objectArray = []
  for (let i = 0; i < keys.length; i++) {
    map.set(keys[i], values[i]);
  };
  const mapSort = new Map([...map.entries()].sort(function(entryA, entryB) {
    return entryA[0] - entryB[0];
  }));
  return mapSort
};

console.log(buildMap(keyArray, valueArray));

or destructure:

const mapSort = new Map(
  [...map.entries()]
    .sort(([a], [b]) => a - b)
);

const keyArray = [4, 6, 16, 18, 2]
const valueArray = [103, 123, 4444, 2000]

const buildMap = (keys, values) => {
  const map = new Map();
  let objectArray = []
  for (let i = 0; i < keys.length; i++) {
    map.set(keys[i], values[i]);
  };
  const mapSort = new Map(
    [...map.entries()]
      .sort(([a], [b]) => a - b)
  );
  return mapSort
};

console.log(buildMap(keyArray, valueArray));

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