score:113

Accepted answer

When you fire a Promise it might take a few seconds before it resolves and by that time user might have navigated to another place in your app. So when Promise resolves setState is executed on unmounted component and you get an error - just like in your case. This may also cause memory leaks.

That's why it is best to move some of your asynchronous logic out of components.

Otherwise, you will need to somehow cancel your Promise. Alternatively - as a last resort technique (it's an antipattern) - you can keep a variable to check whether the component is still mounted:

componentDidMount(){
  this.mounted = true;

  this.props.fetchData().then((response) => {
    if(this.mounted) {
      this.setState({ data: response })
    }
  })
}

componentWillUnmount(){
  this.mounted = false;
}

I will stress that again - this is an antipattern but may be sufficient in your case (just like they did with Formik implementation).

A similar discussion on GitHub

EDIT:

This is probably how would I solve the same problem (having nothing but React) with Hooks:

OPTION A:

import React, { useState, useEffect } from "react";

export default function Page() {
  const value = usePromise("https://something.com/api/");
  return (
    <p>{value ? value : "fetching data..."}</p>
  );
}

function usePromise(url) {
  const [value, setState] = useState(null);

  useEffect(() => {
    let isMounted = true; // track whether component is mounted

    request.get(url)
      .then(result => {
        if (isMounted) {
          setState(result);
        }
      });

    return () => {
      // clean up
      isMounted = false;
    };
  }, []); // only on "didMount"

  return value;
}

OPTION B: Alternatively with useRef which behaves like a static property of a class which means it doesn't make component rerender when it's value changes:

function usePromise2(url) {
  const isMounted = React.useRef(true)
  const [value, setState] = useState(null);


  useEffect(() => {
    return () => {
      isMounted.current = false;
    };
  }, []);

  useEffect(() => {
    request.get(url)
      .then(result => {
        if (isMounted.current) {
          setState(result);
        }
      });
  }, []);

  return value;
}

// or extract it to custom hook:
function useIsMounted() {
  const isMounted = React.useRef(true)

  useEffect(() => {
    return () => {
      isMounted.current = false;
    };
  }, []);

  return isMounted; // returning "isMounted.current" wouldn't work because we would return unmutable primitive
}

Example: https://codesandbox.io/s/86n1wq2z8

score:-3

I think I figured a way around it. The problem is not as much the fetching itself but the setState after the component is dismissed. So the solution was to set this.state.isMounted as false and then on componentWillMount change it to true, and in componentWillUnmount set to false again. Then just if(this.state.isMounted) the setState inside the fetch. Like so:

  constructor(props){
    super(props);
    this.state = {
      isMounted: false,
      isLoading: true,
      dataSource: [{
        name: 'loading...',
        id: 'loading',
      }]
    }
  }

  componentDidMount(){
    this.setState({
      isMounted: true,
    })

    return fetch('LINK HERE')
      .then((response) => response.json())
      .then((responseJson) => {
        if(this.state.isMounted){
          this.setState({
            isLoading: false,
            dataSource: responseJson,
          }, function(){
          });
        }
      })
      .catch((error) =>{
        console.error(error);
      });
  }

  componentWillUnmount() {
    this.setState({
      isMounted: false,
    })
  }

score:0

In addition to the cancellable promise hooks examples in the accepted solution, it can be handy to have a useAsyncCallback hook wrapping a request callback and returning a cancellable promise. The idea is the same, but with a hook working just like a regular useCallback. Here is an example of implementation:

function useAsyncCallback<T, U extends (...args: any[]) => Promise<T>>(callback: U, dependencies: any[]) {
  const isMounted = useRef(true)

  useEffect(() => {
    return () => {
      isMounted.current = false
    }
  }, [])

  const cb = useCallback(callback, dependencies)

  const cancellableCallback = useCallback(
    (...args: any[]) =>
      new Promise<T>((resolve, reject) => {
        cb(...args).then(
          value => (isMounted.current ? resolve(value) : reject({ isCanceled: true })),
          error => (isMounted.current ? reject(error) : reject({ isCanceled: true }))
        )
      }),
    [cb]
  )

  return cancellableCallback
}

score:0

Using CPromise package, you can cancel your promise chains, including nested ones. It supports AbortController and generators as a replacement for ECMA async functions. Using CPromise decorators, you can easily manage your async tasks, making them cancellable.

Decorators usage Live Demo :

import React from "react";
import { ReactComponent, timeout } from "c-promise2";
import cpFetch from "cp-fetch";

@ReactComponent
class TestComponent extends React.Component {
  state = {
    text: "fetching..."
  };

  @timeout(5000)
  *componentDidMount() {
    console.log("mounted");
    const response = yield cpFetch(this.props.url);
    this.setState({ text: `json: ${yield response.text()}` });
  }

  render() {
    return <div>{this.state.text}</div>;
  }

  componentWillUnmount() {
    console.log("unmounted");
  }
}

All stages there are completely cancelable/abortable. Here is an example of using it with React Live Demo

import React, { Component } from "react";
import {
  CPromise,
  CanceledError,
  ReactComponent,
  E_REASON_UNMOUNTED,
  listen,
  cancel
} from "c-promise2";
import cpAxios from "cp-axios";

@ReactComponent
class TestComponent extends Component {
  state = {
    text: ""
  };

  *componentDidMount(scope) {
    console.log("mount");
    scope.onCancel((err) => console.log(`Cancel: ${err}`));
    yield CPromise.delay(3000);
  }

  @listen
  *fetch() {
    this.setState({ text: "fetching..." });
    try {
      const response = yield cpAxios(this.props.url).timeout(
        this.props.timeout
      );
      this.setState({ text: JSON.stringify(response.data, null, 2) });
    } catch (err) {
      CanceledError.rethrow(err, E_REASON_UNMOUNTED);
      this.setState({ text: err.toString() });
    }
  }

  *componentWillUnmount() {
    console.log("unmount");
  }

  render() {
    return (
      <div className="component">
        <div className="caption">useAsyncEffect demo:</div>
        <div>{this.state.text}</div>
        <button
          className="btn btn-success"
          type="submit"
          onClick={() => this.fetch(Math.round(Math.random() * 200))}
        >
          Fetch random character info
        </button>
        <button
          className="btn btn-warning"
          onClick={() => cancel.call(this, "oops!")}
        >
          Cancel request
        </button>
      </div>
    );
  }
}

Using Hooks and cancel method

import React, { useState } from "react";
import {
  useAsyncEffect,
  E_REASON_UNMOUNTED,
  CanceledError
} from "use-async-effect2";
import cpAxios from "cp-axios";

export default function TestComponent(props) {
  const [text, setText] = useState("");
  const [id, setId] = useState(1);

  const cancel = useAsyncEffect(
    function* () {
      setText("fetching...");
      try {
        const response = yield cpAxios(
          `https://rickandmortyapi.com/api/character/${id}`
        ).timeout(props.timeout);
        setText(JSON.stringify(response.data, null, 2));
      } catch (err) {
        CanceledError.rethrow(err, E_REASON_UNMOUNTED);
        setText(err.toString());
      }
    },
    [id]
  );

  return (
    <div className="component">
      <div className="caption">useAsyncEffect demo:</div>
      <div>{text}</div>
      <button
        className="btn btn-success"
        type="submit"
        onClick={() => setId(Math.round(Math.random() * 200))}
      >
        Fetch random character info
      </button>
      <button className="btn btn-warning" onClick={cancel}>
        Cancel request
      </button>
    </div>
  );
}

score:0

one more alternative way is to wrap your async function in a wrapper that will handle the use case when the component unmounts

as we know function are also object in js so we can use them to update the closure values

const promesifiedFunction1 = (func) => {
  return function promesify(...agrs){
    let cancel = false;
    promesify.abort = ()=>{
      cancel = true;
    }
    return new Promise((resolve, reject)=>{
       function callback(error, value){
          if(cancel){
              reject({cancel:true})
          }
          error ? reject(error) : resolve(value);
       }
       agrs.push(callback);
       func.apply(this,agrs)
    })
  }
}

//here param func pass as callback should return a promise object
//example fetch browser API
//const fetchWithAbort = promesifiedFunction2(fetch)
//use it as fetchWithAbort('http://example.com/movies.json',{...options})
//later in componentWillUnmount fetchWithAbort.abort()
const promesifiedFunction2 = (func)=>{
  return async function promesify(...agrs){
    let cancel = false;
    promesify.abort = ()=>{
      cancel = true;
    }

    try {
      const fulfilledValue = await func.apply(this,agrs);
      if(cancel){
        throw 'component un mounted'
      }else{
        return fulfilledValue;
      }
    }
    catch (rejectedValue) {
      return rejectedValue
    }
  }
}

then inside componentWillUnmount() simply call promesifiedFunction.abort() this will update the cancel flag and run the reject function

score:0

Just four steps:

1.create instance of AbortController::const controller = new AbortController()

2.get signal:: const signal = controller.signal

3.pass signal to fetch parameter

4.controller abort anytime:: controller.abort();

const controller = new AbortController()
const signal = controller.signal

function beginFetching() {
    var urlToFetch = "https://xyxabc.com/api/tt";

    fetch(urlToFetch, {
            method: 'get',
            signal: signal,
        })
        .then(function(response) {
            console.log('Fetch complete');
        }).catch(function(err) {
            console.error(` Err: ${err}`);
        });
}


function abortFetching() {
    controller.abort()
}

score:0

If you have a timeout clear them when component unmount.

useEffect(() => {
    getReusableFlows(dispatch, selectedProject);
    dispatch(fetchActionEvents());

    const timer = setInterval(() => {
      setRemaining(getRemainingTime());
    }, 1000);

    return () => {
      clearInterval(timer);
    };
  }, []);

score:0

There are many great answers here and i decided to throw some in too. Creating your own version of useEffect to remove repetition is fairly simple:

import { useEffect } from 'react';

function useSafeEffect(fn, deps = null) {
  useEffect(() => {
    const state = { safe: true };
    const cleanup = fn(state);
    return () => {
      state.safe = false;
      cleanup?.();
    };
  }, deps);
}

Use it as a normal useEffect with state.safe being available for you in the callback that you pass:

useSafeEffect(({ safe }) => {
  // some code
  apiCall(args).then(result => {
    if (!safe) return;
    // updating the state
  })
}, [dep1, dep2]);

score:0

This is a more general solution for async/await and promises. I did this because my React callbacks were in between important async calls, so I couldn't cancel all the promises.

// TemporalFns.js
let storedFns = {};
const nothing = () => {};
export const temporalThen = (id, fn) => {
    if(!storedFns[id]) 
        storedFns[id] = {total:0}
    let pos = storedFns[id].total++;
    storedFns[id][pos] = fn;
    
    return data => { const res = storedFns[id][pos](data); delete storedFns[id][pos]; return res; }
}
export const cleanTemporals = (id) => {
    for(let i = 0; i<storedFns[id].total; i++) storedFns[id][i] = nothing;
}

Usage: (Obviously each instance should have different id)

const Test = ({id}) => {
    const [data,setData] = useState('');
    useEffect(() => {
        someAsyncFunction().then(temporalThen(id, data => setData(data))
            .then(otherImportantAsyncFunction).catch(...);
        return () => { cleanTemporals(id); }
    }, [])
    return (<p id={id}>{data}</p>);
}

score:0

we can create a custom hook to wrap the fetch function like this:

//my-custom-fetch-hook.js
import {useEffect, useRef} from 'react'
function useFetch(){
  const isMounted = useRef(true)

  useEffect(() => {
    isMounted.current = true   //must set this in useEffect or your will get a error when the debugger refresh the page
    return () => {isMounted.current = false}
  }, [])


  return (url, config) => {
    return fetch(url, config).then((res) => {
      if(!isMounted.current)
        throw('component unmounted')
      return res
    })
  }
}

export default useFetch

Then in our functional component:

import useFetch from './my-custom-fetch-hook.js'

function MyComponent(){
  const fetch = useFetch()
  ...  

  fetch(<url>, <config>) 
    .then(res => res.json())
    .then(json => { ...set your local state here})
    .catch(err => {...do something})
}

score:2

When I need to "cancel all subscriptions and asynchronous" I usually dispatch something to redux in componentWillUnmount to inform all other subscribers and send one more request about cancellation to server if necessary

score:2

The crux of this warning is that your component has a reference to it that is held by some outstanding callback/promise.

To avoid the antipattern of keeping your isMounted state around (which keeps your component alive) as was done in the second pattern, the react website suggests using an optional promise; however that code also appears to keep your object alive.

Instead, I've done it by using a closure with a nested bound function to setState.

Here's my constructor(typescript)…

constructor(props: any, context?: any) {
    super(props, context);

    let cancellable = {
        // it's important that this is one level down, so we can drop the
        // reference to the entire object by setting it to undefined.
        setState: this.setState.bind(this)
    };

    this.componentDidMount = async () => {
        let result = await fetch(…);            
        // ideally we'd like optional chaining
        // cancellable.setState?.({ url: result || '' });
        cancellable.setState && cancellable.setState({ url: result || '' });
    }

    this.componentWillUnmount = () => {
        cancellable.setState = undefined; // drop all references.
    }
}

score:2

I think if it is not necessary to inform server about cancellation - best approach is just to use async/await syntax (if it is available).

constructor(props){
  super(props);
  this.state = {
    isLoading: true,
    dataSource: [{
      name: 'loading...',
      id: 'loading',
    }]
  }
}

async componentDidMount() {
  try {
    const responseJson = await fetch('LINK HERE')
      .then((response) => response.json());

    this.setState({
      isLoading: false,
      dataSource: responseJson,
    }
  } catch {
    console.error(error);
  }
}

score:15

Since the post had been opened, an "abortable-fetch" has been added. https://developers.google.com/web/updates/2017/09/abortable-fetch

(from the docs:)

The controller + signal manoeuvre Meet the AbortController and AbortSignal:

const controller = new AbortController();
const signal = controller.signal;

The controller only has one method:

controller.abort(); When you do this, it notifies the signal:

signal.addEventListener('abort', () => {
  // Logs true:
  console.log(signal.aborted);
});

This API is provided by the DOM standard, and that's the entire API. It's deliberately generic so it can be used by other web standards and JavaScript libraries.

for example, here's how you'd make a fetch timeout after 5 seconds:

const controller = new AbortController();
const signal = controller.signal;

setTimeout(() => controller.abort(), 5000);

fetch(url, { signal }).then(response => {
  return response.text();
}).then(text => {
  console.log(text);
});

score:26

You can use AbortController to cancel a fetch request.

See also: https://www.npmjs.com/package/abortcontroller-polyfill

class FetchComponent extends React.Component{
  state = { todos: [] };
  
  controller = new AbortController();
  
  componentDidMount(){
    fetch('https://jsonplaceholder.typicode.com/todos',{
      signal: this.controller.signal
    })
    .then(res => res.json())
    .then(todos => this.setState({ todos }))
    .catch(e => alert(e.message));
  }
  
  componentWillUnmount(){
    this.controller.abort();
  }
  
  render(){
    return null;
  }
}

class App extends React.Component{
  state = { fetch: true };
  
  componentDidMount(){
    this.setState({ fetch: false });
  }
  
  render(){
    return this.state.fetch && <FetchComponent/>
  }
}

ReactDOM.render(<App/>, document.getElementById('root'))
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="root"></div>

score:30

The friendly people at React recommend wrapping your fetch calls/promises in a cancelable promise. While there is no recommendation in that documentation to keep the code separate from the class or function with the fetch, this seems advisable because other classes and functions are likely to need this functionality, code duplication is an anti-pattern, and regardless the lingering code should be disposed of or canceled in componentWillUnmount(). As per React, you can call cancel() on the wrapped promise in componentWillUnmount to avoid setting state on an unmounted component.

The provided code would look something like these code snippets if we use React as a guide:

const makeCancelable = (promise) => {
    let hasCanceled_ = false;

    const wrappedPromise = new Promise((resolve, reject) => {
        promise.then(
            val => hasCanceled_ ? reject({isCanceled: true}) : resolve(val),
            error => hasCanceled_ ? reject({isCanceled: true}) : reject(error)
        );
    });

    return {
        promise: wrappedPromise,
        cancel() {
            hasCanceled_ = true;
        },
    };
};

const cancelablePromise = makeCancelable(fetch('LINK HERE'));

constructor(props){
    super(props);
    this.state = {
        isLoading: true,
        dataSource: [{
            name: 'loading...',
            id: 'loading',
        }]
    }
}

componentDidMount(){
    cancelablePromise.
        .then((response) => response.json())
        .then((responseJson) => {
            this.setState({
                isLoading: false,
                dataSource: responseJson,
            }, () => {

            });
        })
        .catch((error) =>{
            console.error(error);
        });
}

componentWillUnmount() {
    cancelablePromise.cancel();
}

---- EDIT ----

I have found the given answer may not be quite correct by following the issue on GitHub. Here is one version that I use which works for my purposes:

export const makeCancelableFunction = (fn) => {
    let hasCanceled = false;

    return {
        promise: (val) => new Promise((resolve, reject) => {
            if (hasCanceled) {
                fn = null;
            } else {
                fn(val);
                resolve(val);
            }
        }),
        cancel() {
            hasCanceled = true;
        }
    };
};

The idea was to help the garbage collector free up memory by making the function or whatever you use null.


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