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Usage: reaction(() => data, data => { sideEffect }, options?).

A variation on autorun that gives more fine grained control on which observables will be tracked. It takes two functions, the first one (the data function) is tracked and returns data that is used as input for the second one, the effect function. Unlike autorun the side effect won't be run directly when created, but only after the data expression returns a new value for the first time. Any observables that are accessed while executing the side effect will not be tracked.

The side effect can be debounced, just like autorunAsync. reaction returns a disposer function. The functions passed to reaction will receive one argument when invoked, the current reaction, which can be used to dispose the when during execution.

It is important to notice that the side effect will only react to data that was accessed in the data expression, which might be less then the data that is actually used in the effect. Also, the side effect will only be triggered when the data returned by the expression has changed. In other words: reaction requires you to produce the things you need in your side effect.


Reaction accepts as third argument an options object with the following optional options:

  • context: The this to be used in the functions passed to reaction. By default undefined (use arrow functions instead!)
  • fireImmediately: Boolean that indicates that the effect function should immediately be triggered after the first run of the data function. false by default. If a boolean is passed as third argument to reaction, it will be interpreted as the fireImmediately option.
  • delay: Number in milliseconds that can be used to debounce the effect function. If zero (the default), no debouncing will happen.
  • compareStructural: false by default. If true, the return value of the data function is structurally compared to its previous return value, and the effect function will only be invoked if there is a structural change in the output. This can also be specified by setting equals to comparer.structural.
  • equals: comparer.default by default. If specified, this comparer function will be used to compare the previous and next values produced by the data function. The effect function will only be invoked if this function returns true. If specified, this will override compareStructural.
  • name: String that is used as name for this reaction in for example spy events.


In the following example both reaction1, reaction2 and autorun1 will react to the addition, removal or replacement of todo's in the todos array. But only reaction2 and autorun will react to the change of a title in one of the todo items, because title is used in the data expression of reaction 2, while it isn't in the data expression of reaction 1. autorun tracks the complete side effect, hence it will always trigger correctly, but is also more suspectible to accidentally accessing unrelevant data. See also what will MobX React to?.

const todos = observable([
        title: "Make coffee",
        done: true,
        title: "Find biscuit",
        done: false

// wrong use of reaction: reacts to length changes, but not to title changes!
const reaction1 = reaction(
    () => todos.length,
    length => console.log("reaction 1:", todos.map(todo => todo.title).join(", "))

// correct use of reaction: reacts to length and title changes
const reaction2 = reaction(
    () => todos.map(todo => todo.title),
    titles => console.log("reaction 2:", titles.join(", "))

// autorun reacts to just everything that is used in its function
const autorun1 = autorun(
    () => console.log("autorun 1:", todos.map(todo => todo.title).join(", "))

todos.push({ title: "explain reactions", done: false });
// prints:
// reaction 1: Make coffee, find biscuit, explain reactions
// reaction 2: Make coffee, find biscuit, explain reactions
// autorun 1: Make coffee, find biscuit, explain reactions

todos[0].title = "Make tea"
// prints:
// reaction 2: Make tea, find biscuit, explain reactions
// autorun 1: Make tea, find biscuit, explain reactions

Reaction is roughly speaking sugar for: computed(expression).observe(action(sideEffect)) or autorun(() => action(sideEffect)(expression)