How can I perform an inner join with two object arrays in JavaScript?

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I don’t know how reduce would help here, but you could use a Map to
accomplish the same task in O(n):

const a = [
  {id: 4, name: 'Greg'},
  {id: 1, name: 'David'},
  {id: 2, name: 'John'},
  {id: 3, name: 'Matt'}];

const b = [
  {id: 5, name: 'Mathew', position: '1'},
  {id: 6, name: 'Gracia', position: '2'},
  {id: 2, name: 'John', position: '2'},
  {id: 3, name: 'Matt', position: '2'}];

var m = new Map();
// Insert all entries keyed by ID into the Map, filling in placeholder
// 'position' since the Array 'a' lacks 'position' entirely:
a.forEach(function(x) { x.position = null; m.set(x.id, x); });

// For values in 'b', insert them if missing, otherwise, update existing values:
b.forEach(function(x) {
    var existing = m.get(x.id);
    if (existing === undefined)
        m.set(x.id, x);
    else
        Object.assign(existing, x);
});

// Extract resulting combined objects from the Map as an Array
var result = Array.from(m.values());

console.log(JSON.stringify(result));
.as-console-wrapper { max-height: 100% !important; top: 0; }

Because Map accesses and updates are O(1) (on average – because of hash
collisions and rehashing, it can be longer), this makes O(n+m) (where n
and m are the lengths of a and b respectively; the naive solution you
gave would be O(n*m) using the same meaning for n and m).

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