Develop Reference

Render functional component on variable change

I can not get my component to re-render when an internal variable changes. The system I working on uses functional components. The case is like this:
export const myComponent = (props: compPropsType) => {
const myClassNames ....
let objectList= getObjectList(window.location.hash, props.pageTree);
window.addEventListener('hashchange', () => {
console.log('hello');
objectList = getObjectList(window.location.hash, props.pageTree);
});
return (
<>
<header className={headerClassNames}>
<Block className={...}>
...
<myChildComp objList={objectList}>
...
)
};
The problem is to render <myCildComp> when the hash updates. (objectListis an array of strings, used to create navigation toolbar.)
When I click a link on the page, hello is written to console, so the listener is working, but it does not re-render the child component.

a function component in react is equivalent to the render() function in a class component.
so in your case you're adding an eventListener every time the component is re-rendered which creates a memory leak, you should use useEffect hook instead to add it once and remove it when the component/hook is destroyed.
your component doesn't re-render because nothing is telling it to.. components in react only re-render when either state or props changes.. so in your case I think you'd need to add objectList to state combined with useEffect hook like so
export const myComponent = (props: compPropsType) => {
const myClassNames ....
const [objectList, setObjectList] = useState([])
const onHashChanged = () => {
console.log('hello')
let newObjectList = getObjectList(window.location.hash, props.pageTree)
setObjectList(newObjectList)
}
useEffect(() => {
window.addEventListener('hashchange', onHashChanged)
return () => window.removeEventListener('hashchange', onHashChanged)
}, [])
return (
<>
<header className={headerClassNames}>
<Block className={...}>
...
<myChildComp objList={objectList}>
...
)
};
when using an empty array as a second argument in useEffect it will only be called once equivalent to componentDidMount and I'm returning a callback function to unsubscribe/remove the listener which works similar to componentWillUnmount

As suggested in a comment, I rewrote the function to be a class:
export class myComponent extends React.PureComponent<PropsTypes> {
constructor(props) {
super(props)
this.state = {objList = getObjectList(window.location.hash, this.props.pageTree)};
}
componentDidMount(): void {
window.addEventListener('hashchange', () => {
this.setState({
breadcrumbList: getObjList(window.location.hash, this.props.pageTree),
});
});
}
render() {
return (....
<myChildComp objList={this.state.objList}>
...
This works, but it is the best way?

Close a dropdown when an element within it is clicked

I'm working on a Notification feature in my app (pretty much like Facebook notifications).
When I click a button in the header navigation, the dropdown opens and shows the notification list. The notification has a Link (from react-router) in it.
What I need to do is to close the dropdown whenever a Link is clicked.
Here's roughly the hierarchy I currently have:
Header > Navigation > Button > Dropdown > List > Notification > Link
Since the dropdown functionality is used more that once, I've abstracted its behavior away into a HOC that uses render prop:
export default function withDropDown(ClickableElement) {
return class ClickableDropdown extends PureComponent {
static propTypes = {
children: PropTypes.func.isRequired,
showOnInit: PropTypes.bool,
};
static defaultProps = {
showOnInit: false,
};
state = {
show: !!this.props.showOnInit,
};
domRef = createRef();
componentDidMount() {
document.addEventListener('mousedown', this.handleGlobalClick);
}
toggle = show => {
this.setState({ show });
};
handleClick = () => this.toggle(true);
handleGlobalClick = event => {
if (this.domRef.current && !this.domRef.current.contains(event.target)) {
this.toggle(false);
}
};
render() {
const { children, ...props } = this.props;
return (
<Fragment>
<ClickableElement {...props} onClick={this.handleClick} />
{this.state.show && children(this.domRef)}
</Fragment>
);
}
};
}
The HOC above encloses the Button component, so I have:
const ButtonWithDropdown = withDropdown(Button);
class NotificationsHeaderDropdown extends PureComponent {
static propTypes = {
data: PropTypes.arrayOf(notification),
load: PropTypes.func,
};
static defaultProps = {
data: [],
load: () => {},
};
componentDidMount() {
this.props.load();
}
renderDropdown = ref => (
<Dropdown ref={ref}>
{data.length > 0 && <List items={this.props.data} />}
{data.length === 0 && <EmptyList />}
</Dropdown>
);
render() {
return (
<ButtonWithDropdown count={this.props.data.length}>
{this.renderDropdown}
</ButtonWithDropdown>
);
}
}
List and Notification are both dumb functional components, so I'm not posting their code here. Dropdown is pretty much the same, with the difference it uses ref forwarding.
What I really need is to call that .toggle() method from ClickableDropdown created by the HOC to be called whenever I click on a Link on the list.
Is there any way of doing this without passing that .toggle() method down the Button > Dropdown > List > Notification > Link subtree?
I'm using redux, but I'm not sure this is the kind of thing I'd put on the store.
Or should I handle this imperatively using the DOM API, by changing the implementation of handleGlobalClick from ClickableDropdown?
Edit:
I'm trying with the imperative approach, so I've changed the handleGlobalClick method:
const DISMISS_KEY = 'dropdown';
function contains(current, element) {
if (!current) {
return false;
}
return current.contains(element);
}
function isDismisser(dismissKey, current, element) {
if (!element || !contains(current, element)) {
return false;
}
const shouldDismiss = element.dataset.dismiss === dismissKey;
return shouldDismiss || isDismisser(dismissKey, current, element.parentNode);
}
// Then...
handleGlobalClick = event => {
const containsEventTarget = contains(this.domRef.current, event.target);
const shouldDismiss = isDismisser(
DISMISS_KEY,
this.domRef.current,
event.target
);
if (!containsEventTarget || shouldDismiss) {
this.toggle(false);
}
return true;
};
Then I changed the Link to include a data-dismiss property:
<Link
to={url}
data-dismiss="dropdown"
>
...
</Link>
Now the dropdown is closed, but I'm not redirected to the provided url anymore.
I tried to defer the execution of this.toggle(false) using requestAnimationFrame and setTimeout, but it didn't work either.
Solution:
Based on the answer by #streletss bellow, I came up with the following solution:
In order to be as generic as possible, I created a shouldHideOnUpdate prop in the ClickableDropdown dropdown component, whose Hindley-Milner-ish signature is:
shouldHideOnUpdate :: Props curr, Props prev => (curr, prev) -> Boolean
Here's the componentDidUpdate implementation:
componentDidUpdate(prevProps) {
if (this.props.shouldHideOnUpdate(this.props, prevProps)) {
this.toggle(false);
}
}
This way, I didn't need to use the withRouter HOC directly in my withDropdown HOC.
So, I lifted the responsibility of defining the condition for hiding the dropdown to the caller, which is my case is the Navigation component, where I did something like this:
const container = compose(withRouter, withDropdown);
const ButtonWithDropdown = container(Button);
function routeStateHasChanged(currentProps, prevProps) {
return currentProps.location.state !== prevProps.location.state;
}
// ... then
render() {
<ButtonWithDropdown shouldHideOnUpdate={routeStateHasChanged}>
{this.renderDropdown}
</ButtonWithDropdown>
}

It seems you could simply make use of withRouter HOC and check if this.props.location.pathname has changed when componentDidUpdate:
export default function withDropDown(ClickableElement) {
class ClickableDropdown extends Component {
// ...
componentDidUpdate(prevProps) {
if (this.props.location.pathname !== prevProps.location.pathname) {
this.toggle(false);
}
}
// ...
};
return withRouter(ClickableDropdown)
}

Is there any way of doing this without passing that .toggle() method down the Button > Dropdown > List > Notification > Link subtree?
In the question, you mention that you are using redux.So I assume that you store showOnInit in redux.We don't usually store a function in redux.In toggle function,I think you should dispatch an CHANGE_SHOW action to change the showOnInit in redux, then pass the show data not the function to the children component.Then after reducer dispatch,the react will change “show” automatically.
switch (action.type) {
case CHANGE_SHOW:
return Object.assign({}, state, {
showOnInit: action.text
})
...
default:
return state
}
Link element and data pass
Use the property in Link-to,not data-...Like this:
<Link
to={{
pathname: url,
state:{dismiss:"dropdown"}
}}
/>
And the state property will be found in this.props.location.
give context a little try(not recommend)
It may lead your project to instable and some other problems.(https://reactjs.org/docs/context.html#classcontexttype)
First,define context
const MyContext = React.createContext(defaultValue);
Second,define pass value
<MyContext.Provider value={this.toggle}>
Then,get the value in the nested component
<div value={this.context} />

React Classes: Referencing class as "this", within an object's function property

I have finally gotten into using react and ES6 and it's going well but I am finally stumped and could use some direction.
I have got my head around binding this to a method to reference the class, but I am trying to go a bit deeper. Take this for example...which works as expected:
class App extends Component {
state = {
myFirstState: false,
};
handleMyFirstState = () => {
this.setState( { myFirstState : true } );
};
render() {
return (
<MyComponent handleMySate={ this.handleMyState } />
);
}
}
export default App;
As the amount of methods increased I decided NOT to pass each method individually as props and to group them in an object first, and to just pass the object as a whole, as a prop. Like So...
class App extends Component {
state = {
myFirstState: false,
mySecondState: false
};
handleMyFirstState = () => {
this.setState( { myFirstState : true } );
};
handleMySecondSate = () => {
this.setState( { mySecondState : true } );
};
render() {
const handleStates = {
first : this.handleMyFirstState,
second : this.handleMySecondState
}
return (
<MyComponent handleStates={ handleStates } />
);
}
}
export default App;
Now, I am trying to avoid redundant code and just build the methods as one object with functions as properties before the render begins. Pretty much like this...
class App extends Component {
state = {
myFirstState: false,
mySecondState: false
};
handleStates = {
// Here is where 'this' does not reference the App class
// I get results from the console log but setstate doesn't pass correctly
first : () => { console.log("First Triggered"); this.setState( { myFirstState : true } ); },
second : () => { console.log("Second Triggered"); this.setState( { mySecondState : true } ); }
};
render() {
return (
<MyComponent handleStates={this.handleStates} />
);
}
}
export default App;
// I trigger the function like this within MyComponent and I get the console log, but `this.setState` breaks.
<Button onClick={ this.props.handleState.first } >Handle First</button>
I have successfully triggered the functions from the child component ,<MyComponent/>, using the latter code, but this no longer refers to the class and I can't figure out how to bind this to handleStates since it's not a function.
Is this just not possible or is there another way to handle what I am trying to achieve?
Thank you in advance!
ADDITIONAL
If I move the handleStates into the render() it works just fine...how could that be?
class App extends Component {
state = {
myFirstState: false,
mySecondState: false
};
render() {
const handleStates = {
first : () => { this.setState( { myFirstState : true } ); },
second : () => { this.setState( { mySecondState : true } ); }
};
return (
<MyComponent handleStates={this.handleStates} />
);
}
}
export default App;

First, in the second example, you pass this.handleStates as the value for the prop handleStates, but it's undefined. You built handleStates as a local variable, and thus you want your props to reference that local variable:
<MyComponent handleStates={handleStates} />
For your third (last) example, your issue is even simpler: you defined handleStates as an attribute on this which is assigned an object, itself with two attributes, first and second, each of which have a function as their value.
When you ultimately pass this.handleStates to MyComponent, you're passing an object, not a function. If you want to call one of first or second from MyComponent, you can do so like this:
this.props.handleStates.first()
Which has the desired result of updating the state in App.
For what it's worth, there's a more common pattern for this: simply pass a single updater function as the prop, named according to what it does:
class Sandwich extends React.Component {
this.state = {
bread: "",
meat: "",
veggie: "",
}
updateSandwich = (component, selection) => {
this.setState({ [component]: selection })
}
render() {
return(<IngredientSelector updateSandwich={this.updateSandwich} />)
}
}
class IngredientSelector extends React.Component {
return(){
<button value="Rye" onClick={() => this.updateSandwich("bread", "rye")} />
<button value="Wheat" onClick={() => this.updateSandwich("bread", "wheat")} />
<button value="Ham" onClick={() => this.updateSandwich("meat", "ham")} />
<button value="Turkey" onClick={() => this.updateSandwich("meat", "turkey")} />
}
}

Call child method from parent

I have two components:
Parent component
Child component
I was trying to call Child's method from Parent, I tried this way but couldn't get a result:
class Parent extends Component {
render() {
return (
<Child>
<button onClick={Child.getAlert()}>Click</button>
</Child>
);
}
}
class Child extends Component {
getAlert() {
alert('clicked');
}
render() {
return (
<h1 ref="hello">Hello</h1>
);
}
}
Is there a way to call Child's method from Parent?
Note: Child and Parent components are in two different files.

First off, let me express that this is generally not the way to go about things in React land. Usually what you want to do is pass down functionality to children in props, and pass up notifications from children in events (or better yet: dispatch).
But if you must expose an imperative method on a child component, you can use refs. Remember this is an escape hatch and usually indicates a better design is available.
Previously, refs were only supported for Class-based components.
With the advent of React Hooks, that's no longer the case
Modern React with Hooks (v16.8+)
const { forwardRef, useRef, useImperativeHandle } = React;
// We need to wrap component in `forwardRef` in order to gain
// access to the ref object that is assigned using the `ref` prop.
// This ref is passed as the second parameter to the function component.
const Child = forwardRef((props, ref) => {
// The component instance will be extended
// with whatever you return from the callback passed
// as the second argument
useImperativeHandle(ref, () => ({
getAlert() {
alert("getAlert from Child");
}
}));
return <h1>Hi</h1>;
});
const Parent = () => {
// In order to gain access to the child component instance,
// you need to assign it to a `ref`, so we call `useRef()` to get one
const childRef = useRef();
return (
<div>
<Child ref={childRef} />
<button onClick={() => childRef.current.getAlert()}>Click</button>
</div>
);
};
ReactDOM.render(
<Parent />,
document.getElementById('root')
);
<script src="https://unpkg.com/react#16/umd/react.development.js" crossorigin></script>
<script src="https://unpkg.com/react-dom#16/umd/react-dom.development.js" crossorigin></script>
<div id="root"></div>
Documentation for useImperativeHandle() is here:
useImperativeHandle customizes the instance value that is exposed to parent components when using ref.
Legacy API using Class Components (>= react#16.4)
const { Component } = React;
class Parent extends Component {
constructor(props) {
super(props);
this.child = React.createRef();
}
onClick = () => {
this.child.current.getAlert();
};
render() {
return (
<div>
<Child ref={this.child} />
<button onClick={this.onClick}>Click</button>
</div>
);
}
}
class Child extends Component {
getAlert() {
alert('getAlert from Child');
}
render() {
return <h1>Hello</h1>;
}
}
ReactDOM.render(<Parent />, document.getElementById('root'));
<script src="https://unpkg.com/react#16/umd/react.development.js" crossorigin></script>
<script src="https://unpkg.com/react-dom#16/umd/react-dom.development.js" crossorigin></script>
<div id="root"></div>
Callback Ref API
Callback-style refs are another approach to achieving this, although not quite as common in modern React:
const { Component } = React;
const { render } = ReactDOM;
class Parent extends Component {
render() {
return (
<div>
<Child ref={instance => { this.child = instance; }} />
<button onClick={() => { this.child.getAlert(); }}>Click</button>
</div>
);
}
}
class Child extends Component {
getAlert() {
alert('clicked');
}
render() {
return (
<h1>Hello</h1>
);
}
}
render(
<Parent />,
document.getElementById('app')
);
<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="app"></div>

You can use another pattern here:
class Parent extends Component {
render() {
return (
<div>
<Child setClick={click => this.clickChild = click}/>
<button onClick={() => this.clickChild()}>Click</button>
</div>
);
}
}
class Child extends Component {
constructor(props) {
super(props);
this.getAlert = this.getAlert.bind(this);
}
componentDidMount() {
this.props.setClick(this.getAlert);
}
getAlert() {
alert('clicked');
}
render() {
return (
<h1 ref="hello">Hello</h1>
);
}
}
What it does is to set the parent's clickChild method when child is mounted. In this way when you click the button in parent it will call clickChild which calls child's getAlert.
This also works if your child is wrapped with connect() so you don't need the getWrappedInstance() hack.
Note you can't use onClick={this.clickChild} in parent because when parent is rendered child is not mounted so this.clickChild is not assigned yet. Using onClick={() => this.clickChild()} is fine because when you click the button this.clickChild should already be assigned.

Alternative method with useEffect:
Parent:
const [refresh, doRefresh] = useState(0);
<Button onClick={() => doRefresh(prev => prev + 1)} />
<Children refresh={refresh} />
Children:
useEffect(() => {
performRefresh(); //children function of interest
}, [props.refresh]);

Here I will give you the four possible combinations that can happen:
Class Parent | Hook Child
Hook Parent | Class Child
Hook Parent | Hook Child
Class Parent | Class Child
1. Class Parent | Hook Child
class Parent extends React.Component {
constructor(props) {
super(props)
this.myRef = React.createRef()
}
render() {
return (<View>
<Child ref={this.myRef}/>
<Button title={'call me'}
onPress={() => this.myRef.current.childMethod()}/>
</View>)
}
}
const Child = React.forwardRef((props, ref) => {
useImperativeHandle(ref, () => ({
childMethod() {
childMethod()
}
}))
function childMethod() {
console.log('call me')
}
return (<View><Text> I am a child</Text></View>)
})
2. Hook Parent | Class Child
function Parent(props) {
const myRef = useRef()
return (<View>
<Child ref={myRef}/>
<Button title={'call me'}
onPress={() => myRef.current.childMethod()}/>
</View>)
}
class Child extends React.Component {
childMethod() {
console.log('call me')
}
render() {
return (<View><Text> I am a child</Text></View>)
}
}
3. Hook Parent | Hook Child
function Parent(props) {
const myRef = useRef()
return (<View>
<Child ref={myRef}/>
<Button title={'call me'}
onPress={() => myRef.current.childMethod()}/>
</View>)
}
const Child = React.forwardRef((props, ref) => {
useImperativeHandle(ref, () => ({
childMethod() {
childMethod()
}
}))
function childMethod() {
console.log('call me')
}
return (<View><Text> I am a child</Text></View>)
})
4. Class Parent | Class Child
class Parent extends React.Component {
constructor(props) {
super(props)
this.myRef = React.createRef()
}
render() {
return (<View>
<Child ref={this.myRef}/>
<Button title={'call me'}
onPress={() => this.myRef.current.childMethod()}/>
</View>)
}
}
class Child extends React.Component {
childMethod() {
console.log('call me')
}
render() {
return (<View><Text> I am a child</Text></View>)
}
}

https://facebook.github.io/react/tips/expose-component-functions.html
for more answers ref here Call methods on React children components
By looking into the refs of the "reason" component, you're breaking encapsulation and making it impossible to refactor that component without carefully examining all the places it's used. Because of this, we strongly recommend treating refs as private to a component, much like state.
In general, data should be passed down the tree via props. There are a few exceptions to this (such as calling .focus() or triggering a one-time animation that doesn't really "change" the state) but any time you're exposing a method called "set", props are usually a better choice. Try to make it so that the inner input component worries about its size and appearance so that none of its ancestors do.

I wasn't satisfied with any of the solutions presented here. There is actually a very simple solution that can be done using pure Javascript without relying upon some React functionality other than the basic props object - and it gives you the benefit of communicating in either direction (parent -> child, child -> parent). You need to pass an object from the parent component to the child component. This object is what I refer to as a "bi-directional reference" or biRef for short. Basically, the object contains a reference to methods in the parent that the parent wants to expose. And the child component attaches methods to the object that the parent can call. Something like this:
// Parent component.
function MyParentComponent(props) {
function someParentFunction() {
// The child component can call this function.
}
function onButtonClick() {
// Call the function inside the child component.
biRef.someChildFunction();
}
// Add all the functions here that the child can call.
var biRef = {
someParentFunction: someParentFunction
}
return <div>
<MyChildComponent biRef={biRef} />
<Button onClick={onButtonClick} />
</div>;
}
// Child component
function MyChildComponent(props) {
function someChildFunction() {
// The parent component can call this function.
}
function onButtonClick() {
// Call the parent function.
props.biRef.someParentFunction();
}
// Add all the child functions to props.biRef that you want the parent
// to be able to call.
props.biRef.someChildFunction = someChildFunction;
return <div>
<Button onClick={onButtonClick} />
</div>;
}
The other advantage to this solution is that you can add a lot more functions in the parent and child while passing them from the parent to the child using only a single property.
An improvement over the code above is to not add the parent and child functions directly to the biRef object but rather to sub members. Parent functions should be added to a member called "parent" while the child functions should be added to a member called "child".
// Parent component.
function MyParentComponent(props) {
function someParentFunction() {
// The child component can call this function.
}
function onButtonClick() {
// Call the function inside the child component.
biRef.child.someChildFunction();
}
// Add all the functions here that the child can call.
var biRef = {
parent: {
someParentFunction: someParentFunction
}
}
return <div>
<MyChildComponent biRef={biRef} />
<Button onClick={onButtonClick} />
</div>;
}
// Child component
function MyChildComponent(props) {
function someChildFunction() {
// The parent component can call this function.
}
function onButtonClick() {
// Call the parent function.
props.biRef.parent.someParentFunction();
}
// Add all the child functions to props.biRef that you want the parent
// to be able to call.
props.biRef {
child: {
someChildFunction: someChildFunction
}
}
return <div>
<Button onClick={onButtonClick} />
</div>;
}
By placing parent and child functions into separate members of the biRef object, you 'll have a clean separation between the two and easily see which ones belong to parent or child. It also helps to prevent a child component from accidentally overwriting a parent function if the same function appears in both.
One last thing is that if you note, the parent component creates the biRef object with var whereas the child component accesses it through the props object. It might be tempting to not define the biRef object in the parent and access it from its parent through its own props parameter (which might be the case in a hierarchy of UI elements). This is risky because the child may think a function it is calling on the parent belongs to the parent when it might actually belong to a grandparent. There's nothing wrong with this as long as you are aware of it. Unless you have a reason for supporting some hierarchy beyond a parent/child relationship, it's best to create the biRef in your parent component.

I hope I'm not repeating anything from above but what about passing a callback prop that sets the function in the parent? This works and is pretty easy. (Added code is between the ////'s)
class Parent extends Component {
/////
getAlert = () => {} // initial value for getAlert
setGetAlertMethod = (newMethod) => {
this.getAlert = newMethod;
}
/////
render() {
return (
<Child setGetAlertMethod={this.setGetAlertMethod}>
<button onClick={this.getAlert}>Click</button>
</Child>
);
}
}
class Child extends Component {
/////
componentDidMount() {
this.props.setGetAlertMethod(this.getAlert);
}
/////
getAlert() => {
alert('clicked');
}
render() {
return (
<h1 ref="hello">Hello</h1>
);
}
}

you can use ref to call the function of the child component from the parent
Functional Component Solution
in functional component, you have to use useImperativeHandle for getting ref into a child like below
import React, { forwardRef, useRef, useImperativeHandle } from 'react';
export default function ParentFunction() {
const childRef = useRef();
return (
<div className="container">
<div>
Parent Component
</div>
<button
onClick={() => { childRef.current.showAlert() }}
>
Call Function
</button>
<Child ref={childRef}/>
</div>
)
}
const Child = forwardRef((props, ref) => {
useImperativeHandle(
ref,
() => ({
showAlert() {
alert("Child Function Called")
}
}),
)
return (
<div>Child Component</div>
)
})
Class Component Solution
Child.js
import s from './Child.css';
class Child extends Component {
getAlert() {
alert('clicked');
}
render() {
return (
<h1>Hello</h1>
);
}
}
export default Child;
Parent.js
class Parent extends Component {
render() {
onClick() {
this.refs.child.getAlert();
}
return (
<div>
<Child ref="child" />
<button onClick={this.onClick}>Click</button>
</div>
);
}
}

I'm using useEffect hook to overcome the headache of doing all this so now I pass a variable down to child like this:
import { useEffect, useState } from "react";
export const ParentComponent = () => {
const [trigger, setTrigger] = useState(false);
return (
<div onClick={() => { setTrigger(trigger => !trigger); }}>
<ChildComponent trigger={trigger}></ChildComponent>
</div>
);
};
export const ChildComponent = (props) => {
const triggerInvokedFromParent = () => {
console.log('TriggerInvokedFromParent');
};
useEffect(() => {
triggerInvokedFromParent();
}, [props.trigger]);
return <span>ChildComponent</span>;
};

We can use refs in another way as-
We are going to create a Parent element, it will render a <Child/> component. As you can see, the component that will be rendered, you need to add the ref attribute and provide a name for it.
Then, the triggerChildAlert function, located in the parent class will access the refs property of the this context (when the triggerChildAlert function is triggered will access the child reference and it will has all the functions of the child element).
class Parent extends React.Component {
triggerChildAlert(){
this.refs.child.callChildMethod();
// to get child parent returned value-
// this.value = this.refs.child.callChildMethod();
// alert('Returned value- '+this.value);
}
render() {
return (
<div>
{/* Note that you need to give a value to the ref parameter, in this case child*/}
<Child ref="child" />
<button onClick={this.triggerChildAlert}>Click</button>
</div>
);
}
}
Now, the child component, as theoretically designed previously, will look like:
class Child extends React.Component {
callChildMethod() {
alert('Hello World');
// to return some value
// return this.state.someValue;
}
render() {
return (
<h1>Hello</h1>
);
}
}
Here is the source code-
Hope will help you !

If you are doing this simply because you want the Child to provide a re-usable trait to its parents, then you might consider doing that using render-props instead.
That technique actually turns the structure upside down. The Child now wraps the parent, so I have renamed it to AlertTrait below. I kept the name Parent for continuity, although it is not really a parent now.
// Use it like this:
<AlertTrait renderComponent={Parent}/>
class AlertTrait extends Component {
// You will need to bind this function, if it uses 'this'
doAlert() {
alert('clicked');
}
render() {
return this.props.renderComponent({ doAlert: this.doAlert });
}
}
class Parent extends Component {
render() {
return (
<button onClick={this.props.doAlert}>Click</button>
);
}
}
In this case, the AlertTrait provides one or more traits which it passes down as props to whatever component it was given in its renderComponent prop.
The Parent receives doAlert as a prop, and can call it when needed.
(For clarity, I called the prop renderComponent in the above example. But in the React docs linked above, they just call it render.)
The Trait component can render stuff surrounding the Parent, in its render function, but it does not render anything inside the parent. Actually it could render things inside the Parent, if it passed another prop (e.g. renderChild) to the parent, which the parent could then use during its render method.
This is somewhat different from what the OP asked for, but some people might end up here (like we did) because they wanted to create a reusable trait, and thought that a child component was a good way to do that.

For functional components easiest way is
Parent Component
parent.tsx
import React, { useEffect, useState, useRef } from "react";
import child from "../../child"
const parent: React.FunctionComponent = () => {
const childRef: any = useRef();
}
const onDropDownChange: any = (event): void => {
const target = event.target;
childRef.current.onFilterChange(target.value);
};
return <child ref={childRef} />
export default parent;
Child Component
child.tsx
import React, { useState, useEffect, forwardRef, useRef, useImperativeHandle, } from "react";
const Child = forwardRef((props, ref) => {
useImperativeHandle(ref, () => ({
onFilterChange(id) {
console.log("Value from parent", id)
},
}));
})
Child.displayName = "Child";
export default Child;

The logic is simple.
Create a function in parent using child or use ref.
I prefer the creating function in parent using child. There are multiple ways to do it.
When using functional components
In Parent
function Parent(){
const [functionToCall, createFunctionToCall] = useState(()=>()=>{})
return (
<Child createFunctionToCall={createFunctionToCall} />
)
}
In Child
function Child({createFunctionToCall}){
useEffect(()=>{
function theFunctionToCall(){
// do something like setting something
// don't forget to set dependancies properly.
}
createFunctionToCall(()=>theFunctionToCall)
},[createFunctionToCall])
}

This pattern is similar to #brickingup answer. But in this version you can set as many child actions you want.
import { useEffect } from "react";
export const Parent = () => {
const childEvents = { click: () => {} };
return (
<div onClick={() => childEvents.click()}>
<Child events={childEvents}></Child>
</div>
);
};
export const Child = (props) => {
const click = () => {
alert("click from child");
};
useEffect(() => {
if (props.events) {
props.events.click = click;
}
}, []);
return <span>Child Component</span>;
};

We're happy with a custom hook we call useCounterKey. It just sets up a counterKey, or a key that counts up from zero. The function it returns resets the key (i.e. increment). (I believe this is the most idiomatic way in React to reset a component - just bump the key.)
However this hook also works in any situation where you want to send a one-time message to the client to do something. E.g. we use it to focus a control in the child on a certain parent event - it just autofocuses anytime the key is updated. (If more props are needed they could be set prior to resetting the key so they're available when the event happens.)
This method has a bit of a learning curve b/c it's not as straightforward as a typical event handler, but it seems the most idiomatic way to handle this in React that we've found (since keys already function this way). Def open to feedback on this method but it is working well!
// Main helper hook:
export function useCounterKey() {
const [key, setKey] = useState(0);
return [key, () => setKey(prev => prev + 1)] as const;
}
Sample usages:
// Sample 1 - normal React, just reset a control by changing Key on demand
function Sample1() {
const [inputLineCounterKey, resetInputLine] = useCounterKey();
return <>
<InputLine key={inputLineCounterKey} />
<button onClick={() => resetInputLine()} />
<>;
}
// Second sample - anytime the counterKey is incremented, child calls focus() on the input
function Sample2() {
const [amountFocusCounterKey, focusAmountInput] = useCounterKey();
// ... call focusAmountInput in some hook or event handler as needed
return <WorkoutAmountInput focusCounterKey={amountFocusCounterKey} />
}
function WorkoutAmountInput(props) {
useEffect(() => {
if (counterKey > 0) {
// Don't focus initially
focusAmount();
}
}, [counterKey]);
// ...
}
(Credit to Kent Dodds for the counterKey concept.)

Parent component
import Child from './Child'
export default function Parent(props) {
const [childRefreshFunction, setChildRefreshFunction] = useState(null);
return (
<div>
<button type="button" onClick={() => {
childRefreshFunction();
}}>Refresh child</button>
<Child setRefreshFunction={(f) => {
setChildRefreshFunction(f);
}} />
</div>
)
}
Child component
export default function Child(props) {
useEffect(() => {
props.setRefreshFunction(() => refreshMe);
}, []);
function refreshMe() {
fetch('http://example.com/data.json')....
};
return (
<div>
child
</div>
)
}

You can achieve this easily in this way
Steps-
Create a boolean variable in the state in the parent class. Update this when you want to call a function.
Create a prop variable and assign the boolean variable.
From the child component access that variable using props and execute the method you want by having an if condition.
class Child extends Component {
Method=()=>{
--Your method body--
}
render() {
return (
//check whether the variable has been updated or not
if(this.props.updateMethod){
this.Method();
}
)
}
}
class Parent extends Component {
constructor(){
this.state={
callMethod:false
}
}
render() {
return (
//update state according to your requirement
this.setState({
callMethod:true
}}
<Child updateMethod={this.state.callMethod}></Child>
);
}
}

Another way of triggering a child function from parent is to make use of the componentDidUpdate function in child Component. I pass a prop triggerChildFunc from Parent to Child, which initially is null. The value changes to a function when the button is clicked and Child notice that change in componentDidUpdate and calls its own internal function.
Since prop triggerChildFunc changes to a function, we also get a callback to the Parent. If Parent don't need to know when the function is called the value triggerChildFunc could for example change from null to true instead.
const { Component } = React;
const { render } = ReactDOM;
class Parent extends Component {
state = {
triggerFunc: null
}
render() {
return (
<div>
<Child triggerChildFunc={this.state.triggerFunc} />
<button onClick={() => {
this.setState({ triggerFunc: () => alert('Callback in parent')})
}}>Click
</button>
</div>
);
}
}
class Child extends Component {
componentDidUpdate(prevProps) {
if (this.props.triggerChildFunc !== prevProps.triggerChildFunc) {
this.onParentTrigger();
}
}
onParentTrigger() {
alert('parent triggered me');
// Let's call the passed variable from parent if it's a function
if (this.props.triggerChildFunc && {}.toString.call(this.props.triggerChildFunc) === '[object Function]') {
this.props.triggerChildFunc();
}
}
render() {
return (
<h1>Hello</h1>
);
}
}
render(
<Parent />,
document.getElementById('app')
);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>
<div id='app'></div>
CodePen: https://codepen.io/calsal/pen/NWPxbJv?editors=1010

Here my demo: https://stackblitz.com/edit/react-dgz1ee?file=styles.css
I am using useEffect to call the children component's methods. I have tried with Proxy and Setter_Getter but sor far useEffect seems to be the more convenient way to call a child method from parent. To use Proxy and Setter_Getter it seems there is some subtlety to overcome first, because the element firstly rendered is an objectLike's element through the ref.current return => <div/>'s specificity.
Concerning useEffect, you can also leverage on this approach to set the parent's state depending on what you want to do with the children.
In the demo's link I have provided, you will find my full ReactJS' code with my draftwork inside's so you can appreciate the workflow of my solution.
Here I am providing you my ReactJS' snippet with the relevant code only. :
import React, {
Component,
createRef,
forwardRef,
useState,
useEffect
} from "react";
{...}
// Child component
// I am defining here a forwardRef's element to get the Child's methods from the parent
// through the ref's element.
let Child = forwardRef((props, ref) => {
// I am fetching the parent's method here
// that allows me to connect the parent and the child's components
let { validateChildren } = props;
// I am initializing the state of the children
// good if we can even leverage on the functional children's state
let initialState = {
one: "hello world",
two: () => {
console.log("I am accessing child method from parent :].");
return "child method achieve";
}
};
// useState initialization
const [componentState, setComponentState] = useState(initialState);
// useEffect will allow me to communicate with the parent
// through a lifecycle data flow
useEffect(() => {
ref.current = { componentState };
validateChildren(ref.current.componentState.two);
});
{...}
});
{...}
// Parent component
class App extends Component {
// initialize the ref inside the constructor element
constructor(props) {
super(props);
this.childRef = createRef();
}
// I am implementing a parent's method
// in child useEffect's method
validateChildren = childrenMethod => {
// access children method from parent
childrenMethod();
// or signaling children is ready
console.log("children active");
};
{...}
render(){
return (
{
// I am referencing the children
// also I am implementing the parent logic connector's function
// in the child, here => this.validateChildren's function
}
<Child ref={this.childRef} validateChildren={this.validateChildren} />
</div>
)
}

You can apply that logic very easily using your child component as a react custom hook.
How to implement it?
Your child returns a function.
Your child returns a JSON: {function, HTML, or other values} as the example.
In the example doesn't make sense to apply this logic but it is easy to see:
const {useState} = React;
//Parent
const Parent = () => {
//custome hook
const child = useChild();
return (
<div>
{child.display}
<button onClick={child.alert}>
Parent call child
</button>
{child.btn}
</div>
);
};
//Child
const useChild = () => {
const [clickCount, setClick] = React.useState(0);
{/* child button*/}
const btn = (
<button
onClick={() => {
setClick(clickCount + 1);
}}
>
Click me
</button>
);
return {
btn: btn,
//function called from parent
alert: () => {
alert("You clicked " + clickCount + " times");
},
display: <h1>{clickCount}</h1>
};
};
const rootElement = document.getElementById("root");
ReactDOM.render(<Parent />, rootElement);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<div id="root"></div>

I tried using createRef or useRef. Somehow they all return null.
Secondly, this answer proposes to pass a prop that sets a function that seems the most reasonable to me. But if your child component is used in multiple places, you should add that extra prop to other places. Also if you want to call a method in the grandchild, this method might be too verbose or mouthful.
So I made my own function store in a very primitive way.
Below is functionStore.js file
const fns = {};
export function setFn(componentName, fnName, fn) {
if (fns[componentName]) {
fns[componentName][fnName] = fn;
} else {
fns[componentName] = { fnName: fn };
}
}
export function callFn(componentName, fnName) {
fns[componentName][fnName]();
}
I just set the functions that need to be called from any component.
import { setFn } from "./functionStore";
export class AComponent extends React.Component {
componentDidMount() {
setFn("AComponent", "aFunc", this.aFunc);
}
aFunc = () => { console.log("aFunc is called!"); };
}
Then I just call it from some other component
import { callFn } from "./functionStore";
export class BComponent extends React.Component {
// just call the function
bFunc = () => {
callFn("AComponent", "aFunc");
};
}
One disadvantage is the function to be called should be parameterless. But this might be fixed somehow as well. Currently, I don't need to pass parameters.

I think that the most basic way to call methods is by setting a request on the child component. Then as soon as the child handles the request, it calls a callback method to reset the request.
The reset mechanism is necessary to be able to send the same request multiple times after each other.
In parent component
In the render method of the parent:
const { request } = this.state;
return (<Child request={request} onRequestHandled={()->resetRequest()}/>);
The parent needs 2 methods, to communicate with its child in 2 directions.
sendRequest() {
const request = { param: "value" };
this.setState({ request });
}
resetRequest() {
const request = null;
this.setState({ request });
}
In child component
The child updates its internal state, copying the request from the props.
constructor(props) {
super(props);
const { request } = props;
this.state = { request };
}
static getDerivedStateFromProps(props, state) {
const { request } = props;
if (request !== state.request ) return { request };
return null;
}
Then finally it handles the request, and sends the reset to the parent:
componentDidMount() {
const { request } = this.state;
// todo handle request.
const { onRequestHandled } = this.props;
if (onRequestHandled != null) onRequestHandled();
}

Here's a bug? to look out for:
I concur with rossipedia's solution using forwardRef, useRef, useImperativeHandle
There's some misinformation online that says refs can only be created from React Class components, but you can indeed use Function Components if you use the aforementioned hooks above. A note, the hooks only worked for me after I changed the file to not use withRouter() when exporting the component. I.e. a change from
export default withRouter(TableConfig);
to instead be
export default TableConfig;
In hindsight the withRouter() is not needed for such a component anyway, but usually it doesn't hurt anything having it in. My use case is that I created a component to create a Table to handle the viewing and editing of config values, and I wanted to be able to tell this Child component to reset it's state values whenever the Parent form's Reset button was hit. UseRef() wouldn't properly get the ref or ref.current (kept on getting null) until I removed withRouter() from the file containing my child component TableConfig

How to access component methods from “outside” in ReactJS?

Why can’t I access the component methods from “outside” in ReactJS? Why is it not possible and is there any way to solve it?
Consider the code:
var Parent = React.createClass({
render: function() {
var child = <Child />;
return (
<div>
{child.someMethod()} // expect "bar", got a "not a function" error.
</div>
);
}
});
var Child = React.createClass({
render: function() {
return (
<div>
foo
</div>
);
},
someMethod: function() {
return 'bar';
}
});
React.renderComponent(<Parent />, document.body);

React provides an interface for what you are trying to do via the ref attribute. Assign a component a ref, and its current attribute will be your custom component:
class Parent extends React.Class {
constructor(props) {
this._child = React.createRef();
}
componentDidMount() {
console.log(this._child.current.someMethod()); // Prints 'bar'
}
render() {
return (
<div>
<Child ref={this._child} />
</div>
);
}
}
Note: This will only work if the child component is declared as a class, as per documentation found here: https://facebook.github.io/react/docs/refs-and-the-dom.html#adding-a-ref-to-a-class-component
Update 2019-04-01: Changed example to use a class and createRef per latest React docs.
Update 2016-09-19: Changed example to use ref callback per guidance from the ref String attribute docs.

If you want to call functions on components from outside React, you can call them on the return value of renderComponent:
var Child = React.createClass({…});
var myChild = React.renderComponent(Child);
myChild.someMethod();
The only way to get a handle to a React Component instance outside of React is by storing the return value of React.renderComponent. Source.

Alternatively, if the method on Child is truly static (not a product of current props, state) you can define it on statics and then access it as you would a static class method. For example:
var Child = React.createClass({
statics: {
someMethod: function() {
return 'bar';
}
},
// ...
});
console.log(Child.someMethod()) // bar

As of React 16.3 React.createRef can be used, (use ref.current to access)
var ref = React.createRef()
var parent = (
<div>
<Child ref={ref} />
<button onClick={e=>console.log(ref.current)}
</div>
);
React.renderComponent(parent, document.body)

Since React 0.12 the API is slightly changed. The valid code to initialize myChild would be the following:
var Child = React.createClass({…});
var myChild = React.render(React.createElement(Child, {}), mountNode);
myChild.someMethod();

You could also do it like this, not sure if it's a good plan :D
class Parent extends Component {
handleClick() {
if (this._getAlert !== null) {
this._getAlert()
}
}
render() {
return (
<div>
<Child>
{(getAlert, childScope) => (
<span> {!this._getAlert ? this._getAlert = getAlert.bind(childScope) : null}</span>
)}
</Child>
<button onClick={() => this.handleClick()}> Click me</button>
</div>
);
}
}
class Child extends Component {
constructor() {
super();
this.state = { count: 0 }
}
getAlert() {
alert(`Child function called state: ${this.state.count}`);
this.setState({ count: this.state.count + 1 });
}
render() {
return this.props.children(this.getAlert, this)
}
}

As mentioned in some of the comments, ReactDOM.render no longer returns the component instance. You can pass a ref callback in when rendering the root of the component to get the instance, like so:
// React code (jsx)
function MyWidget(el, refCb) {
ReactDOM.render(<MyComponent ref={refCb} />, el);
}
export default MyWidget;
and:
// vanilla javascript code
var global_widget_instance;
MyApp.MyWidget(document.getElementById('my_container'), function(widget) {
global_widget_instance = widget;
});
global_widget_instance.myCoolMethod();

Another way so easy:
function outside:
function funx(functionEvents, params) {
console.log("events of funx function: ", functionEvents);
console.log("this of component: ", this);
console.log("params: ", params);
thisFunction.persist();
}
Bind it:
constructor(props) {
super(props);
this.state = {};
this.funxBinded = funx.bind(this);
}
}
Please see complete tutorial here: How to use "this" of a React Component from outside?