Javascript reduce function error

Question

I am trying to play with the Reduce function in JavaScript. What I am trying to achieve can be easily done via the filter function but I am trying to do it via Reduce.

I am trying to retrieve all the people where the age is greater or equal to 18 years and store the result in an array

var people = [
{ name: "John", age: 16 },
{ name: "Thomas", age: 20 },
{ name: "Smith", age: 18 },
{ name: "Jessy", age: 17 },
];
var arr = [];
var output = people.reduce(function(arr,ppl){
if(ppl.age >= 18)
    return(arr.push(ppl));
},arr);
console.log(output);

However when I run this snippet, I get an error that says "TypeError: Cannot read property 'push' of undefined". I am not sure where do I need to define the arr (array where I need to store the output)

`arr` is going to be the function’s last return value. Are you always returning a value? — Ry-, Feb 17 '18 at 10:09
`Array.prototype.push` returns the new length of the `Array`. You need to `push` then return `arr`. If the age is < 18 you return `undefined` ... — Vivick, Feb 17 '18 at 10:10
One of the beauties of `reduce()` is that you don't need an outer var. Just start with `} []);` — Roamer-1888, Feb 17 '18 at 10:22

xianshenglu · Answer 1 · 2018-02-17T10:18:07.947

1

try this:

you should return arr,not return arr.push(ppl) because [].push(3) return 1(new length of [] ) not [3].And reduce works with accumulator which in this case the accumulator is arr.So,you should return arr not return (arr.push(ppl));

var people = [{
    name: "John",
    age: 16
  },
  {
    name: "Thomas",
    age: 20
  },
  {
    name: "Smith",
    age: 18
  },
  {
    name: "Jessy",
    age: 17
  },
];
var arr = [];
var output = people.reduce(function(arr, ppl) {
  if (ppl.age >= 18)
    (arr.push(ppl));
  return arr;

}, arr);
console.log(output);

edited Feb 17 '18 at 10:18

answered Feb 17 '18 at 10:09

xianshenglu

4,943
3
17
34

Thanks a lot. This definitely works. Please help me understand whats wrong with my code. Appreciate your help – Manish Feb 17 '18 at 10:13
@Manish,because you return(arr.push(ppl)); you should return arr – xianshenglu Feb 17 '18 at 10:14
_"because `[].push(1)` return `1` not `[1]`"_ Because `.push()` returns the new length of the array (and not the pushed element) – Andreas Feb 17 '18 at 10:17

score 0 · Answer 2 · answered Feb 17 '18 at 10:16

0

 return(arr.push(ppl));

As arr.push returns the new length of the array, it will return a number. So at the next iteration arr will be a number, and you cant push to that. So you need to pass on the array:

 const output = people.reduce((arr, person) => person.age > 17 ? arr.concat(person): arr, []);

That works as arr.concat returns an array.

answered Feb 17 '18 at 10:16

Jonas Wilms

132,000
20
149
151

…and more importantly, in the `else` case nothing was returned at all – Bergi Feb 17 '18 at 11:08

score 0 · Answer 3 · answered May 21 '20 at 17:24

You can use array.concat to push to an array and return it, and put that in a ternary operator:

let people=[{name:"John",age:16},{name:"Thomas",age:20},{name:"Smith",age:18},{name:"Jessy",age:17}]

let output = people.reduce((arr,ppl) => {
    return ppl.age >= 18 ? arr.concat(ppl) : arr
},[])

console.log(output)

Javascript reduce function error

3 Answers3