create array and object using push and forEach

Question

What's wrong with my code below? grandParent is undefined, what I want to achieve is each grandparent have a property of parent, and each parent have child (arrays of object too)

like so

[{
  name: 'a',
  parent: [{
    name: 'b',
    child: [{
      name: 'c'
    }]
  }]
}]

below code doesn't work

let grandParent = []

_.times(5, () => grandParent.push({
  name: faker.name.findName(),
  parent: []
}))

grandParent = grandParent.forEach(function(o) {
  return o.parent.push({ name: faker.name.findName() })
})

console.log(grandParent) //why here is undefined?

Click `<>` and create a [mcve] - include lowdash or whatever framework you use — mplungjan, Mar 13 '18 at 06:08
it is `undefined` because `Array.prototype.forEach` returns `undefined`. You don't need to reassign `grandParent`. Also you don't need to return from `forEach` callback. — Yury Tarabanko, Mar 13 '18 at 06:11

Shubham Khatri · Answer 1 · 2018-03-13T06:35:09.377

0

forEach doesn't return anything, you must use map instead. Also push returns the new size of the array, to return a new array use concat

grandParent = grandParent.map(function(o) {
  return o.parent.concat([{ name: faker.name.findName() }])
})

console.log(grandParent)

Other way to do it is to not return anything from forEach and just update the original array.

grandParent = grandParent.forEach(function(o) {
    o.parent.push({ name: faker.name.findName()})
})

console.log(grandParent)

edited Mar 13 '18 at 06:35

answered Mar 13 '18 at 06:11

Shubham Khatri

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the second example won't work. I think you need to remove `grandParent =` – Sharon Chai Mar 13 '18 at 06:34
Sorry for the error there, it should have been push instead of concat in the second example. It was a copy paste error – Shubham Khatri Mar 13 '18 at 06:35
your first example have problem too, look at this https://jsbin.com/xoqecuvaha/edit?html,js,console. The result that I expect is like wscourge's answer. – Sharon Chai Mar 13 '18 at 06:37

score 0 · Accepted Answer · answered Mar 13 '18 at 06:12

From MDN docs of Array.prototype.forEach()

Return value: undefined

let grandParent = [
{ name: 'grandname0',  parent: [] },
{ name: 'grandname1',  parent: [] },
{ name: 'grandname2',  parent: [] },
{ name: 'grandname3',  parent: [] },
{ name: 'grandname4',  parent: [] }
];


grandParent.forEach(function(o) {
  
  o.parent.push({ name: 'parent1' });
  
});
console.log(grandParent);

score 0 · Answer 3 · answered Mar 13 '18 at 06:12

.forEach does not return anything. using the map function will help. You code should look something like this.

let grandParent = []

_.times(5, () => grandParent.push({
  name: faker.name.findName(),
  parent: []
}))

grandParent = grandParent.map(function(o) {
  return o.parent.concat({ name: faker.name.findName() })
})

console.log(grandParent)

create array and object using push and forEach

3 Answers3