Javascript create array of objects of length n [duplicate]

Question

Is there a one-liner to create an array of objects of length n?

const arr = [
    { first_name: '', last_name: '' },
    { first_name: '', last_name: '' },
    { first_name: '', last_name: '' },
]

I've tried below but am getting SyntaxError: unexpected token: ':'

const arr = [...Array(3).map(x => { first_name: '', last_name: '' })]

not to be smart, but you could minify your code to one line. for loops work good too. — sao
– sao, Commented Sep 26, 2019 at 16:19
Map not working with empty array, b/c Array(3) set only length, but no elements there. To make map work you should fill your array first const arr = [...Array(3).fill(1).map(... — Julia Shestakova
– Julia Shestakova, Commented Sep 26, 2019 at 16:24
Try this Array.from({length:3}).map(x => ({first_name: '', last_name: ''})) — Faisal Umair
– Faisal Umair, Commented Sep 26, 2019 at 16:31
@JuliaShestakova yes, but then the spread syntax is pointless. Typically it's one or the other, e.g., [...Array(n)].map(() => ({})) or Array(n).fill().map(() => ({})). I don't recommend fill though--more typing and prone to aliasing bugs. — ggorlen
– ggorlen, Commented Sep 21, 2022 at 19:05

Code Maniac · Accepted Answer · 2019-09-27 06:03:09Z

27

You need to wrap return value by () whenever you want to return object from arrow function you need to wrap () else it will treat it as start of function body,

console.log([...Array(3).map(x => ({ first_name: '', last_name: '' }))])

Now you see error is gone but why the values are undefined ? because map doesn't loop over the empty slots you need to use fill

console.log([...Array(3).fill(0).map(x => ({ first_name: '', last_name: '' }))])

Or you can use Array.from

let arr = Array.from({ length: 3 } , () => ({ first_name: '', last_name: '' }))

console.log(arr)

edited Sep 27, 2019 at 6:03

answered Sep 26, 2019 at 16:22

Code Maniac

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1 Comment

ggorlen Over a year ago

In version 2, what's the point of the spread syntax? I'd use either fill or spread, but not both.

Nenad Vracar · Accepted Answer · 2019-09-26 16:18:41Z

2

You just need to wrap returned object in () but you can also use Array.from method instad.

const array = Array.from(Array(3), () => ({ first_name: '', last_name: '' }))
console.log(array)

answered Sep 26, 2019 at 16:18

Nenad Vracar

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Comments

epascarello · Accepted Answer · 2019-09-26 16:24:15Z

1

It is seen not as an object, you need to wrap the object in () and you need to use fill() for map to work.

const arr = [...Array(3).fill().map(x => ({ first_name: '', last_name: '' }))]

edited Sep 26, 2019 at 16:24

answered Sep 26, 2019 at 16:18

epascarello

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1 Comment

Barmar Over a year ago

You should explain why you need to use fill().

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