Javascript ES6, how to add value to array inside a newly copied object?

Question

in ES6 there is a new way to copy objects so you can have a nice way to handle inmutable states:

let oldObj = { foo: 1}; // { foo: 1 }

let newObj = { ...oldObj, bar: 2 }; // { foo: 1, bar: 2}

However what I want to achieve is:

let oldObj = { foo: [1] }; // { foo: [1] }

let newObj = ??? // { foo: [1, 2] }

Any idea on how to achieve this?

{ ...oldObj, foo: [...oldObj.foo, 2] } --- no easy way here. PS: the syntax you're referring to is not "ES6" and is not even standardised yet. — zerkms
– zerkms, Commented Jul 21, 2016 at 2:08
github.com/sebmarkbage/ecmascript-rest-spread --- it is a stage-2 proposal — zerkms
– zerkms, Commented Jul 21, 2016 at 2:14
just to clarify, spread operator is part of ES6 for Arrays, just not for Objects - developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Hugo Silva
– Hugo Silva, Commented Jul 21, 2016 at 2:27

Hugo Silva · Accepted Answer · 2016-07-21 02:15:51Z

4

let newObj = { foo: [...oldObj.foo, 2] }

You could also have a look at Immutable js - http://facebook.github.io/immutable-js

answered Jul 21, 2016 at 2:15

Hugo Silva

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Oscar Franco · Accepted Answer · 2016-07-21 15:19:15Z

1

I like Zerkms answer a bit better because it is more general, if your object contains more keys other than the array:

{ ...oldObj, foo: [...oldObj.foo, 2] }

Credit to @Zerkms.

answered Jul 21, 2016 at 15:19

Oscar Franco

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ironCat · Accepted Answer · 2020-03-31 23:52:59Z

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you can use dynamic value [...oldObj.foo, bar]

let oldObj = { foo: [1] }; // { foo: [1] }
let newObj = { ...oldObj, foo: [1, 2] } // { foo: [1, 2] }
console.log(newObj)

let oldObj2 = { foo: [1] }; // { foo: [1] }
let bar = 2
let newObj2 = { foo:  [...oldObj.foo, bar] } // { foo: [1, 2] }
console.log(newObj2)

answered Mar 31, 2020 at 23:52

ironCat

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