gistfile1.js

'use strict'

const a = [{ id: 1 }, { id: 2 }, { id: 1 }, { id: 3 }]; // guaranteed to have a length less than 25

function unique1(a) {
  const r = [];

  o: for (let o of a) {
    for (let i of r)
      if (o.id === i.id)
        continue o;

    r.push(o);
  }

  return r;
}

function unique2(a) {
  const m = new Map();
  const r = [];

  for (let e of a)
    m.set(e.id, e);

  for (let v of m.values())
    r.push(v);

  return r;
}