7 Tricks with Resting and Spreading JavaScript Objects

  • 2019-03-14 12:09 AM
  • 21

7 Tricks with Resting and Spreading JavaScript Objects

Resting and spreading can be used for more than just resting arguments and spreading arrays.

Here are seven lesser known tricks when using rest and spread with JavaScript objects.

1. Adding Properties

Clone an object while simultaneously adding additional properties to the cloned object.

In this example user is cloned and password is added into userWithPass.

const user = { id: 100, name: 'Howard Moon'}
const userWithPass = { ...user, password: 'Password!' }

user //=> { id: 100, name: 'Howard Moon' }
userWithPass //=> { id: 100, name: 'Howard Moon', password: 'Password!' }

2. Merge Objects

Merge two objects into a new object.

part1 and part2 are merged together into user1.

const part1 = { id: 100, name: 'Howard Moon' }
const part2 = { id: 100, password: 'Password!' }

const user1 = { ...part1, ...part2 }
//=> { id: 100, name: 'Howard Moon', password: 'Password!' }

Objects also be merged with this syntax:

const partial = { id: 100, name: 'Howard Moon' }
const user = { ...partial, ...{ id: 100, password: 'Password!' } }

user //=> { id: 100, name: 'Howard Moon', password: 'Password!' }

3. Exclude Object Properties

Properties can be removed using destructuring in combination with the rest operator. Here password is destructured out (ignored) and the rest of the properties are returned as rest.

const noPassword = ({ password, ...rest }) => rest
const user = {
  id: 100,
  name: 'Howard Moon',
  password: 'Password!'
}

noPassword(user) //=> { id: 100, name: 'Howard moon' }

4. Dynamically Exclude Properties

The removeProperty function takes a prop as an argument. Using computed object property names the prop can be excluded dynamically from the clone.

const user1 = {
  id: 100,
  name: 'Howard Moon',
  password: 'Password!'
}
const removeProperty = prop => ({ [prop]: _, ...rest }) => rest
//                     ----       ------
//                          \   /
//                dynamic destructuring

const removePassword = removeProperty('password')
const removeId = removeProperty('id')

removePassword(user1) //=> { id: 100, name: 'Howard Moon' }
removeId(user1) //=> { name: 'Howard Moon', password: 'Password!' }

5. Organize Properties

Sometimes properties aren’t in the order we need them to be. Using a couple of tricks we can push properties to the top of the list or move them to the bottom.

To move id to the first position, add id: undefined to the new Object before spreading object.

const user3 = {
  password: 'Password!'
  name: 'Naboo',
  id: 300
}

const organize = object => ({ id: undefined, ...object })
//                            -------------
//                          /
//  move id to the first property

organize(user3)
//=> { id: 300, password: 'Password!', name: 'Naboo' }

To move password to the last property, first destructe password out of object. Then set password after spreading object.

const user3 = {
  password: 'Password!'
  name: 'Naboo',
  id: 300
}

const organize = ({ password, ...object }) =>
  ({ ...object, password })
//              --------
//             /
// move password to last property

organize(user3)
//=> { name: 'Naboo', id: 300, password: 'Password!' }

6. Default Properties

Default properties are values that will be set only when they are not included in the original object.

In this example, user2 does not contain quotes. The setDefaults function ensures all objects have quotes set otherwise it will be set to [].

When calling setDefaults(user2), the return value will include quotes: [].

When calling setDefaults(user4), because user4 already has quotes, that property will not be modified.

const user2 = {
  id: 200,
  name: 'Vince Noir'
}

const user4 = {
  id: 400,
  name: 'Bollo',
  quotes: ["I've got a bad feeling about this..."]
}

const setDefaults = ({ quotes = [], ...object}) =>
  ({ ...object, quotes })

setDefaults(user2)
//=> { id: 200, name: 'Vince Noir', quotes: [] }

setDefaults(user4)
//=> {
//=>   id: 400,
//=>   name: 'Bollo',
//=>   quotes: ["I've got a bad feeling about this..."]
//=> }

It can also be written like this if you want the defaults to appear first instead of last:

const setDefaults = ({ ...object}) => ({ quotes: [], ...object })

7: Renaming Properties

By combining the techniques above, a function can be created to rename properties.

Imagine there are some objects with an uppercase ID that should be lowercase. Start by destructuring ID out of the object. Then add it back as id while object is being spread.

const renamed = ({ ID, ...object }) => ({ id: ID, ...object })

const user = {
  ID: 500,
  name: "Bob Fossil"
}

renamed(user) //=> { id: 5000, name: 'Bob Fossil' }

Summary

I tried to list out a handful lesser known spread and rest tricks, if you know any that I haven’t listed here, please let everyone know in the comments! If you learned something new, please share on Twitter and with your friends, it really helps!

Learn more

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Originally published by Joel Thoms at https://blog.bitsrc.io

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