I am trying to iterate through an array of objects to create a new Object that fits my plan ahead better. Everything works fine until I put a if sentence in the map and I have no clue why?

const TestArray = [{
    value: 0.2,
    Time: '1980',
    Region: 'Oklahoma',
    RegionNumber: '1620',
    validTo: 2017
  },
  {
    value: 0.3,
    Time: '1983',
    Region: 'Oklahoma',
    RegionNumber: '1620',
    validTo: 2017
  },
  {
    value: 0.2,
    Time: '1986',
    Region: 'Oklahoma',
    RegionNumber: '1620',
    validTo: 2017
  },
  {
    value: 0.2,
    Time: '1988',
    Region: 'Oklahoma',
    RegionNumber: '1620',
    validTo: 2017
  },
  {
    value: 0.2,
    Time: '2018',
    Region: 'Oklahoma',
    RegionNumber: '1620',
    validTo: 2017
  }
]
console.log("This works fine:")
console.log(Object.fromEntries(TestArray.map((item) => [item["Time"], item["value"]])))
console.log("but when adding filter it crashes:")
console.log(Object.fromEntries(TestArray.map((item) => {
  if (!(parseInt(item["validTo"]) > item["Time"])) {
    [item["Time"], item["value"]]
  }
})))

I am trying to iterate through an array of objects to create a new Object that fits my plan ahead better. Everything works fine until I put a if sentence in the map and I have no clue why?

const TestArray = [{
    value: 0.2,
    Time: '1980',
    Region: 'Oklahoma',
    RegionNumber: '1620',
    validTo: 2017
  },
  {
    value: 0.3,
    Time: '1983',
    Region: 'Oklahoma',
    RegionNumber: '1620',
    validTo: 2017
  },
  {
    value: 0.2,
    Time: '1986',
    Region: 'Oklahoma',
    RegionNumber: '1620',
    validTo: 2017
  },
  {
    value: 0.2,
    Time: '1988',
    Region: 'Oklahoma',
    RegionNumber: '1620',
    validTo: 2017
  },
  {
    value: 0.2,
    Time: '2018',
    Region: 'Oklahoma',
    RegionNumber: '1620',
    validTo: 2017
  }
]
console.log("This works fine:")
console.log(Object.fromEntries(TestArray.map((item) => [item["Time"], item["value"]])))
console.log("but when adding filter it crashes:")
console.log(Object.fromEntries(TestArray.map((item) => {
  if (!(parseInt(item["validTo"]) > item["Time"])) {
    [item["Time"], item["value"]]
  }
})))

• javascript
• node.js
• arrays
• object

• 2 You are not returning from your .map() callback. You should use .filter() followed by .map() if you want to remove elements and then map them. – Nick Parsons Commented Apr 20, 2022 at 1:08
• @NickParsons Ah that worked thanks, Could you maybe link me to a site explaining why the if sentence makes the error happen as I don't really understand why still? But what you suggested fixed my problem. – matieva Commented Apr 20, 2022 at 1:10
• 2 The .map() method won't ever remove elements, it will always return an array of the same length. So you must map each element from TestArray to another element by returning that from your callback. If you don't return anything, then you get undefined (so you end up with [undefined, undefined, ...] as the mapped array) that you then pass to Object.fromEntries(). This method doesn't expect to see undefined, it instead expects to see a [key, value] pair array, so your code crashes – Nick Parsons Commented Apr 20, 2022 at 1:23

2 Answers 2

If anyone is ending up on this Question while working with the JavaScript Map object like me:

const map = new Map(["a", 1], ["b", 2]);    // Iterator value is not an entry object
const map = new Map([["a", 1], ["b", 2]]);  // Correct, just forgot the []

Please find the corrected code below

Object.fromEntries(TestArray.map((item) => {
  if (!(parseInt(item["validTo"]) > item["Time"])) {
    return [item["Time"], item["value"]]
  }
}).filter(item => item))

I have seen that you have already understood why your code did not work with the comments above.