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Observable Arrays

Similar to objects, arrays can be made observable using observable.array(values?) or by passing an array to observable. This works recursively as well, so all (future) values of the array will also be observable.

import {observable, autorun} from "mobx";

var todos = observable([
    { title: "Spoil tea", completed: true },
    { title: "Make coffee", completed: false }

autorun(() => {
    console.log("Remaining:", todos
        .filter(todo => !todo.completed)
        .map(todo => todo.title)
        .join(", ")
// Prints: 'Remaining: Make coffee'

todos[0].completed = false;
// Prints: 'Remaining: Spoil tea, Make coffee'

todos[2] = { title: 'Take a nap', completed: false };
// Prints: 'Remaining: Spoil tea, Make coffee, Take a nap'

// Prints: 'Remaining: Make coffee, Take a nap'

Due to limitations of native arrays in ES5 observable.array will create a faux-array (array-like object) instead of a real array. In practice, these arrays work just as fine as native arrays and all native methods are supported, including index assignments, up-to and including the length of the array.

Bear in mind however that Array.isArray(observable([])) will yield false, so whenever you need to pass an observable array to an external library, it is a good idea to create a shallow copy before passing it to other libraries or built-in functions (which is good practice anyway) by using array.slice(). In other words, Array.isArray(observable([]).slice()) will yield true.

Unlike the built-in implementation of the functions sort and reverse, observableArray.sort and reverse will not change the array in-place, but only will return a sorted / reversed copy.

Besides all built-in functions, the following goodies are available as well on observable arrays:

  • intercept(interceptor). Can be used to intercept any change before it is applied to the array. See observe & intercept
  • observe(listener, fireImmediately? = false) Listen to changes in this array. The callback will receive arguments that express an array splice or array change, conforming to ES7 proposal. It returns a disposer function to stop the listener.
  • clear() Remove all current entries from the array.
  • replace(newItems) Replaces all existing entries in the array with new ones.
  • find(predicate: (item, index, array) => boolean, thisArg?, fromIndex?) Basically the same as the ES7 Array.find proposal, except for the additional fromIndex parameter.
  • findIndex(predicate: (item, index, array) => boolean, thisArg?, fromIndex?) Basically the same as the ES7 Array.findIndex proposal, except for the additional fromIndex parameter.
  • remove(value) Remove a single item by value from the array. Returns true if the item was found and removed.
  • peek() Returns an array with all the values which can safely be passed to other libraries, similar to slice().

In contrast to slice, peek doesn't create a defensive copy. Use this in performance critical applications if you know for sure that you use the array in a read-only manner. In performance critical sections it is recommended to use a flat observable array as well.


Any values assigned to an observable array will be default passed through observable to make them observable. Create a shallow array to disable this behavior and store are values as-is. See also modifiers for more details on this mechanism.

Name argument

Both observable.array and observable.shallowArray take a second parameter which is used as debug name in for example spy or the MobX dev tools.