Using a for loop to push items onto a Map in JavaScript reverses the order of the array

Question

Why is the order in the below code reversed in the array that is created?

let conns =  [
    {"jobId":"BPOM","id":{"makeupUID":"4"}},
    {"jobId":"BPOM","id":{"makeupUID":"3"}},
    {"jobId":"BPOM","id":{"makeupUID":"2"}},
    {"jobId":"BPOM","id":{"makeupUID":"1"}}];


console.log(`Before: ${JSON.stringify(conns)}`)
    let unique = new Map();
    for (let item of conns) {
      unique[item.id.makeupUID] = item;
    }
    
    this._allConnections = Object.values(unique);
    console.log(`After: ${JSON.stringify(this._allConnections)}`)

question from:https://stackoverflow.com/questions/65617181/using-a-for-loop-to-push-items-onto-a-map-in-javascript-reverses-the-order-of-th

Answer 1

As people by now pointed out in the comments, you need to use .set() to insert elements into the map, and .values to get them out again. Otherwise you are actually butchering your Map object with additional fields that don't belong there, but certainly don't do want you want, namely use the properties of Map.

let conns =  [
  {"jobId":"BPOM","id":{"makeupUID":"4"}},
  {"jobId":"BPOM","id":{"makeupUID":"3"}},
  {"jobId":"BPOM","id":{"makeupUID":"2"}},
  {"jobId":"BPOM","id":{"makeupUID":"1"}}];
  
  
console.log(`Before: ${JSON.stringify(conns)}`)
let unique = new Map();
for (let item of conns) {
  unique.set(item.id.makeupUID, item);
}

this._allConnections = Array.from(unique.values());
console.log(`After: ${JSON.stringify(this._allConnections)}`)