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mutate a copy of data without changing the original source

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Mutate a copy of data without changing the original source

Setup via NPM

npm install immutability-helper --save

This is a drop-in replacement for

: ```js // import update from 'react-addons-update'; import update from 'immutability-helper';

const state1 = ['x']; const state2 = update(state1, {$push: ['y']}); // ['x', 'y'] ```

Note that this module has nothing to do with React. However, since this module is most commonly used with React, the docs will focus on how it can be used with React.


React lets you use whatever style of data management you want, including mutation. However, if you can use immutable data in performance-critical parts of your application it's easy to implement a fast

method to significantly speed up your app.

Dealing with immutable data in JavaScript is more difficult than in languages designed for it, like Clojure. However, we've provided a simple immutability helper,

, that makes dealing with this type of data much easier, without fundamentally changing how your data is represented. You can also take a look at Facebook's Immutable.js and React’s Using Immutable Data Structures section for more detail on Immutable.js.

The Main Idea

If you mutate data like this:

myData.x.y.z = 7;
// or...

You have no way of determining which data has changed since the previous copy has been overwritten. Instead, you need to create a new copy of

and change only the parts of it that need to be changed. Then you can compare the old copy of
with the new one in
using triple-equals:
const newData = deepCopy(myData);
newData.x.y.z = 7;

Unfortunately, deep copies are expensive, and sometimes impossible. You can alleviate this by only copying objects that need to be changed and by reusing the objects that haven't changed. Unfortunately, in today's JavaScript this can be cumbersome:

const newData = Object.assign({}, myData, {
  x: Object.assign({}, myData.x, {
    y: Object.assign({}, myData.x.y, {z: 7}),
  a: Object.assign({}, myData.a, {b: myData.a.b.concat(9)})

While this is fairly performant (since it only makes a shallow copy of

log n
objects and reuses the rest), it's a big pain to write. Look at all the repetition! This is not only annoying, but also provides a large surface area for bugs.


provides simple syntactic sugar around this pattern to make writing this code easier. This code becomes:
import update from 'immutability-helper';

const newData = update(myData, { x: {y: {z: {$set: 7}}}, a: {b: {$push: [9]}} });

While the syntax takes a little getting used to (though it's inspired by MongoDB's query language) there's no redundancy, it's statically analyzable and it's not much more typing than the mutative version.


-prefixed keys are called commands. The data structure they are "mutating" is called the target.

Available Commands

  • {$push: array}
    all the items in
    on the target.
  • {$unshift: array}
    all the items in
    on the target.
  • {$splice: array of arrays}
    for each item in
    on the target with the parameters provided by the item. **Note:* The items in the array are applied sequentially, so the order matters. The indices of the target may change during the operation.*
  • {$set: any}
    replace the target entirely.
  • {$toggle: array of strings}
    toggles a list of boolean fields from the target object.
  • {$unset: array of strings}
    remove the list of keys in
    from the target object.
  • {$merge: object}
    merge the keys of
    with the target.
  • {$apply: function}
    passes in the current value to the function and updates it with the new returned value.
  • {$add: array of objects}
    add a value to a
    . When adding to a
    you pass in an array of objects to add, when adding to a Map, you pass in
    [key, value]
    arrays like so:
    update(myMap, {$add: [['foo', 'bar'], ['baz', 'boo']]})
  • {$remove: array of strings}
    remove the list of keys in array from a


Additionally, instead of a command object, you can pass a function, and it will be treated as if it was a command object with the

update({a: 1}, {a: function})
. That example would be equivalent to
update({a: 1}, {a: {$apply: function}})



only works for data properties, not for accessor properties defined with
. It just does not see the latter, and therefore might create shadowing data properties which could break application logic depending on setter side effects. Therefore
should only be used on plain data objects that only contain data properties as descendants.


Simple push

const initialArray = [1, 2, 3];
const newArray = update(initialArray, {$push: [4]}); // => [1, 2, 3, 4]

is still
[1, 2, 3]

Nested collections

const collection = [1, 2, {a: [12, 17, 15]}];
const newCollection = update(collection, {2: {a: {$splice: [[1, 1, 13, 14]]}}});
// => [1, 2, {a: [12, 13, 14, 15]}]

This accesses

's index
, key
, and does a splice of one item starting from index
(to remove
) while inserting

Updating a value based on its current one

const obj = {a: 5, b: 3};
const newObj = update(obj, {b: {$apply: function(x) {return x * 2;}}});
// => {a: 5, b: 6}
// This is equivalent, but gets verbose for deeply nested collections:
const newObj2 = update(obj, {b: {$set: obj.b * 2}});

(Shallow) Merge

const newObj = update(obj, {$merge: {b: 6, c: 7}}); // => {a: 5, b: 6, c: 7}

Computed Property Names

Arrays can be indexed into with runtime variables via the ES2015 Computed Property Names feature. An object property name expression may be wrapped in brackets [] which will be evaluated at runtime to form the final property name.

const collection = {children: ['zero', 'one', 'two']};
const index = 1;
const newCollection = update(collection, {children: {[index]: {$set: 1}}});
// => {children: ['zero', 1, 'two']}

Removing an element from an array

// Delete at a specific index, no matter what value is in it
update(state, { items: { $splice: [[index, 1]] } });


Autovivification is the auto creation of new arrays and objects when needed. In the context of javascript that would mean something like this

const state = {}
state.a.b.c = 1; // state would equal { a: { b: { c: 1 } } }

Since javascript doesn't have this "feature", the same applies to

. The reason why this is practically impossible in javascript and by extension
is the following:
var state = {}
state.thing[0] = 'foo' // What type should state.thing have? Should it be an object or array?
state.thing2[1] = 'foo2' // What about thing2? This must be an object!
state.thing3 = ['thing3'] // This is regular js, this works without autovivification
state.thing3[1] = 'foo3' // Hmm, notice that state.thing2 is an object, yet this is an array
state.thing2.slice // should be undefined
state.thing3.slice // should be a function

If you need to set something deeply nested and don't want to have to set each layer down the line, consider using this technique which is shown with a contrived example:

var state = {}
var desiredState = {
  foo: [
      bar: ['x', 'y', 'z']

const state2 = update(state, { foo: foo => update(foo || [], { 0: fooZero => update(fooZero || {}, { bar: bar => update(bar || [], { $push: ["x", "y", "z"] }) }) }) });

console.log(JSON.stringify(state2) === JSON.stringify(desiredState)) // true // note that state could have been declared as any of the following and it would still output true: // var state = { foo: [] } // var state = { foo: [ {} ] } // var state = { foo: [ {bar: []} ] }

You can also choose to use the extend functionality to add an

import update, { extend } from 'immutability-helper';

extend('$auto', function(value, object) { return object ? update(object, value): update({}, value); }); extend('$autoArray', function(value, object) { return object ? update(object, value): update([], value); });

var state = {} var desiredState = { foo: [ { bar: ['x', 'y', 'z'] }, ], }; var state2 = update(state, { foo: {$autoArray: { 0: {$auto: { bar: {$autoArray: {$push: ['x', 'y', 'z']}} }} }} }); console.log(JSON.stringify(state2) === JSON.stringify(desiredState)) // true

Adding your own commands

The main difference this module has with

is that you can extend this to give it more functionality:

extend('$addtax', function(tax, original) { return original + (tax * original); }); const state = { price: 123 }; const withTax = update(state, { price: {$addtax: 0.8}, }); assert(JSON.stringify(withTax) === JSON.stringify({ price: 221.4 }));

Note that

in the function above is the original object, so if you plan making a mutation, you must first shallow clone the object. Another option is to use
to make the change
return update(original, { foo: {$set: 'bar'} })

If you don't want to mess around with the globally exported

function you can make a copy and work with that copy:
import { Context } from 'immutability-helper';

const myContext = new Context();

myContext.extend('$foo', function(value, original) { return 'foo!'; });

myContext.update(/* args */);
