Effect is a container for async function.

It can be safely used in place of the original async function.

Methods

use(handler)

Provides a function, which will be called when the effect is triggered.

Formulae

effect.use(fn);

• Set handler fn for effect
• If effect already had an implementation at the time of the call, it will be replaced by a new one

Hint: current handler can be extracted with effect.use.getCurrent().

You must provide a handler either through .use method or handler property in createEffect, otherwise effect will throw with no handler used in _%effect name%_ error when effect will be called.

Arguments

1. handler (Function): Function, that receives the first argument passed to an effect call.

Returns

(Effect): The same effect

Example

const fetchUserReposFx = createEffect();

fetchUserReposFx.use(async (params) => {
  console.log("fetchUserReposFx called with", params);

  const url = `https://api.github.com/users/${params.name}/repos`;
  const req = await fetch(url);
  return req.json();
});

fetchUserReposFx({ name: "zerobias" });
// => fetchUserRepos called with {name: 'zerobias'}

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use.getCurrent()

Returns current handler of effect. Useful for testing.

Formulae

fn = effect.use.getCurrent();

• Returns current handler for effect
• If no handler was assigned to effect, default handler will be returned (that throws an error)
• Hint: to set a new handler use effect.use(handler)

Returns

(Function): Current handler, defined by handler property or via use call.

Example

const handlerA = () => "A";
const handlerB = () => "B";

const fx = createEffect(handlerA);

console.log(fx.use.getCurrent() === handlerA);
// => true

fx.use(handlerB);
console.log(fx.use.getCurrent() === handlerB);
// => true

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watch(watcher)

Subscribe to effect calls.

Formulae

const unwatch = effect.watch(fn);

• Call fn on each effect call, pass payload of effect as argument to fn
• When unwatch is called, stop calling fn

Arguments

1. watcher (Watcher): A function that receives payload.

Returns

Subscription: Unsubscribe function.

Example

import { createEffect } from "effector";

const fx = createEffect((params) => params);

fx.watch((params) => {
  console.log("effect called with value", params);
});

await fx(10);
// => effect called with value 10

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prepend(fn)

Creates an event, upon trigger it sends transformed data into the source event. Works kind of like reverse .map. In case of .prepend data transforms before the original event occurs and in the case of .map, data transforms after original event occurred.

Formulae

const event = effect.prepend(fn);

• When event is triggered, call fn with payload from event, then trigger effect with the result of fn()

Arguments

1. fn (Function): A function that receives payload, should be pure.

Returns

Event: New event.

map(fn)

Creates a new event, which will be called after the original effect is called, applying the result of a fn as a payload. It is a special function which allows you to decompose dataflow, extract or transform data.

Formulae

const second = first.map(fn);

• When first is triggered, pass payload from first to fn
• Trigger second with the result of the fn() call as payload

Arguments

1. fn (Function): A function that receives payload, should be pure.

Returns

Event: New event.

Example

import { createEffect } from "effector";

const userUpdate = createEffect(({ name, role }) => {
  console.log(name, role);
});
const userNameUpdated = userUpdate.map(({ name }) => name); // you may decompose dataflow with .map() method
const userRoleUpdated = userUpdate.map(({ role }) => role.toUpperCase()); // either way you can transform data

userNameUpdated.watch((name) => console.log(`User's name is [${name}] now`));
userRoleUpdated.watch((role) => console.log(`User's role is [${role}] now`));

await userUpdate({ name: "john", role: "admin" });
// => User's name is [john] now
// => User's role is [ADMIN] now
// => john admin

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Properties

You are not supposed to use parts of effect (like .done and .pending) as a target in sample or forward (even though they are events and stores), since effect is a complete entity on its own. This behavior will not be supported.

done

Event, which is triggered when handler is resolved.

Properties

Event triggered with an object of params and result:

1. params (Params): An argument passed to the effect call
2. result (Done): A result of the resolved handler

Example

import { createEffect } from "effector";

const fx = createEffect((value) => value + 1);

fx.done.watch(({ params, result }) => {
  console.log("Call with params", params, "resolved with value", result);
});

await fx(2);
// => Call with params 2 resolved with value 3

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doneData

Event, which is triggered by the result of the effect execution:

Formulae

event = effect.doneData;

• doneData is an event, that triggered when effect is successfully resolved with result from .done

Event triggered when handler is resolved.

Example

import { createEffect } from "effector";

const fx = createEffect((value) => value + 1);

fx.doneData.watch((result) => {
  console.log(`Effect was successfully resolved, returning ${result}`);
});

await fx(2);
// => Effect was successfully resolved, returning 3

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fail

Event, which is triggered when handler is rejected or throws error.

Properties

Event triggered with an object of params and error:

1. params (Params): An argument passed to effect call
2. error (Fail): An error caught from the handler

Example

import { createEffect } from "effector";

const fx = createEffect(async (value) => {
  throw Error(value - 1);
});

fx.fail.watch(({ params, error }) => {
  console.log("Call with params", params, "rejected with error", error.message);
});

fx(2);
// => Call with params 2 rejected with error 1

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failData

Event, which is triggered with error thrown by the effect

Formulae

event = effect.failData;

• failData is an event, that triggered when effect is rejected with error from .fail

Event triggered when handler is rejected or throws error.

Example

import { createEffect } from "effector";

const fx = createEffect(async (value) => {
  throw Error(value - 1);
});

fx.failData.watch((error) => {
  console.log(`Execution failed with error ${error.message}`);
});

fx(2);
// => Execution failed with error 1

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finally

Event, which is triggered when handler is resolved, rejected or throws error.

Properties

Event, which is triggered with an object of status, params and error or result:

1. status (string): A status of effect (done or fail)
2. params (Params): An argument passed to effect call
3. error (Fail): An error caught from the handler
4. result (Done): A result of the resolved handler

Example

import { createEffect } from "effector";

const fetchApiFx = createEffect(async ({ time, ok }) => {
  await new Promise((resolve) => setTimeout(resolve, time));
  if (ok) return `${time} ms`;
  throw Error(`${time} ms`);
});

fetchApiFx.finally.watch((value) => {
  switch (value.status) {
    case "done":
      console.log("Call with params", value.params, "resolved with value", value.result);
      break;
    case "fail":
      console.log("Call with params", value.params, "rejected with error", value.error.message);
      break;
  }
});

await fetchApiFx({ time: 100, ok: true });
// => Call with params {time: 100, ok: true}
//    resolved with value 100 ms

fetchApiFx({ time: 100, ok: false });
// => Call with params {time: 100, ok: false}
//    rejected with error 100 ms

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pending

Store contains true when effect is called but not resolved yet. Useful to show loaders.

Formulae

$store = effect.pending;

• $store will update when done or fail are triggered
• $store contains true value until the effect is resolved or rejected

Returns

DerivedStore: Store that represents current state of the effect

Example

import React from "react";
import { createEffect } from "effector";
import { useStore } from "effector-react";

const fetchApiFx = createEffect((ms) => new Promise((resolve) => setTimeout(resolve, ms)));

fetchApiFx.pending.watch(console.log);

const Loading = () => {
  const loading = useStore(fetchApiFx.pending);

  return <div>{loading ? "Loading..." : "Load complete"}</div>;
};

ReactDOM.render(<Loading />, document.getElementById("root"));

fetchApiFx(3000);

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It’s a shorthand for common use case

import { createEffect, createStore } from "effector";

const fetchApiFx = createEffect();

//now you can use fetchApiFx.pending instead
const $isLoading = createStore(false)
  .on(fetchApiFx, () => true)
  .on(fetchApiFx.done, () => false)
  .on(fetchApiFx.fail, () => false);

inFlight

Shows how many effect calls aren’t settled yet. Useful for rate limiting.

Formulae

$count = effect.inFlight;

• Store $count will be 0 if no calls of effect in pending state, its default state
• On each call of effect state in $count store will be increased
• When effect resolves to any state(done or fail) state in $count store will be decreased

Returns

DerivedStore: Store that represents count of the running effects

Example

import { createEffect } from "effector";

const fx = createEffect(() => new Promise((rs) => setTimeout(rs, 500)));

fx.inFlight.watch((amount) => {
  console.log("in-flight requests:", amount);
});
// => 0

const req1 = fx();
// => 1

const req2 = fx();
// => 2

await Promise.all([req1, req2]);

// => 1
// => 0

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