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How to call the function and pass the parameter from import in React js? [duplicate]

I have a seemingly trivial question about props and function components. Basically, I have a container component which renders a Modal component upon state change which is triggered by user click on a button. The modal is a stateless function component that houses some input fields which need to connect to functions living inside the container component.
My question: How can I use the functions living inside the parent component to change state while the user is interacting with form fields inside the stateless Modal component? Am I passing down props incorrectly?
Container
export default class LookupForm extends Component {
constructor(props) {
super(props);
this.state = {
showModal: false
};
}
render() {
let close = () => this.setState({ showModal: false });
return (
... // other JSX syntax
<CreateProfile fields={this.props} show={this.state.showModal} onHide={close} />
);
}
firstNameChange(e) {
Actions.firstNameChange(e.target.value);
}
};
Function (Modal) Component
const CreateProfile = ({ fields }) => {
console.log(fields);
return (
... // other JSX syntax
<Modal.Body>
<Panel>
<div className="entry-form">
<FormGroup>
<ControlLabel>First Name</ControlLabel>
<FormControl type="text"
onChange={fields.firstNameChange} placeholder="Jane"
/>
</FormGroup>
);
};
Example: say I want to call this.firstNameChange from within the Modal component. I guess the "destructuring" syntax of passing props to a function component has got me a bit confused. i.e:
const SomeComponent = ({ someProps }) = > { // ... };

You would need to pass down each prop individually for each function that you needed to call
<CreateProfile
onFirstNameChange={this.firstNameChange}
onHide={close}
show={this.state.showModal}
/>
and then in the CreateProfile component you can either do
const CreateProfile = ({onFirstNameChange, onHide, show }) => {...}
with destructuring it will assign the matching property names/values to the passed in variables. The names just have to match with the properties
or just do
const CreateProfile = (props) => {...}
and in each place call props.onHide or whatever prop you are trying to access.

I'm using react function component
In parent component first pass the props like below shown
import React, { useState } from 'react';
import './App.css';
import Todo from './components/Todo'
function App() {
const [todos, setTodos] = useState([
{
id: 1,
title: 'This is first list'
},
{
id: 2,
title: 'This is second list'
},
{
id: 3,
title: 'This is third list'
},
]);
return (
<div className="App">
<h1></h1>
<Todo todos={todos}/> //This is how i'm passing props in parent component
</div>
);
}
export default App;
Then use the props in child component like below shown
function Todo(props) {
return (
<div>
{props.todos.map(todo => { // using props in child component and looping
return (
<h1>{todo.title}</h1>
)
})}
</div>
);
}

An addition to the above answer.
If React complains about any of your passed props being undefined, then you will need to destructure those props with default values (common if passing functions, arrays or object literals) e.g.
const CreateProfile = ({
// defined as a default function
onFirstNameChange = f => f,
onHide,
// set default as `false` since it's the passed value
show = false
}) => {...}

just do this on source component
<MyDocument selectedQuestionData = {this.state.selectedQuestionAnswer} />
then do this on destination component
const MyDocument = (props) => (
console.log(props.selectedQuestionData)
);

A variation of finalfreq's answer
You can pass some props individually and all parent props if you really want (not recommended, but sometimes convenient)
<CreateProfile
{...this.props}
show={this.state.showModal}
/>
and then in the CreateProfile component you can just do
const CreateProfile = (props) => {
and destruct props individually
const {onFirstNameChange, onHide, show }=props;

Can we pass props from Parent to Child component in React, and set the props as state of child component?

const Parent=()=>{
return(
)
}
const Child=({data})=>{
return (
{data}
)
}
Can I set the props in Child component to its state?

Yes you can. Something like:
class Child extends Component {
constructor(props){
const {myProp} = props;
this.state = {myNumber: myProp}
render(){
const {myNumber} = this.state;
return <div>{myNumber}</div>
}
class Parent extends Component {
render () {
return <Child myProp={5} />
}
}
BUT: if your "parent" component refreshes, so does the child and the state is reverted back to the value set by the parent, and all the state changes you did on the child are lost.

Yeah here made a code sandbox example showing how to do it: https://codesandbox.io/s/stack-pass-props-fgq67n
Child component looks like this
const Input = ({ string }) => {
const [value, setValue] = useState(string);
return <input value={value} onChange={(e) => setValue(e.target.value)} />;
};
Parent Component:
export default function App() {
return (
<div className="App">
<Input string="default" />
</div>
);
}
Sometimes if the parent reloads the passed props may reset to defaults

There is a concept in React known as controlled and uncontrolled components, When your component only have props and no state inside itself then it a controlled component, that is controlled by the parent component.
Also, it is not advisable to convert the props to the state of the child component, if you want to change the value of the props, just pass an additional method as a props which will be called by the Child to update the value of the props, let show you with an example
const Parent = () => {
const [number, setNumber] = useState(0);
const updateNumberHandler = (numberToUpdate) => setNumber(numberToUpdate)
return <Child number={number} onUpdateNumber={updateNumberHandler} />
}
const Child = (props) => {
const { number, onUpdateNumber } = props;
return <button onClick={() => onUpdateNumber(number + 1)}>Updated Number: {number}</button>
}
Here you can see I am passing two props one value and one method to update that value, when the onUpdateNumber method is called the value on the parent is updated so it gets re-render and also the child gets re-render with the updated number value.

How to get element height and width from ReactNode?

I have a dynamic component in which I pass in children as prop.
So the props look something like:
interface Props {
...some props
children: React.ReactNode
}
export default Layout({...some props, children}: Props) {...}
I need to access the size of the children elements (height and width), in the Layout component. Note that the children are from completely different components and are non-related.
I can use the Layout component as follow:
<Layout ...some props>
<Child1 /> // I need to know the height and width of this child
<Child2 /> // as well as this child
<Child3 /> // and this child.
</Layout>
How can I do so dynamically? Do I somehow have to convert ReactNode to HTMLDivElement? Note that there is no way I can pass in an array of refs as a prop into Layout. Because that the pages which use Layout are dynamically generated.
Since many doesn't really understand what I meant by dynamically generated. It means that the pages which are using the Layout component can pass in x amount of children. The amount of children is unknown but never 0.

You can achieve this by using React.Children to dynamically build up a list of references before rendering the children. If you have access to the children element references, you can follow the below approach. If you don't then you can follow the bit at the bottom.
You have access to the children element references
If the children components pass up their element reference, you can use React.Children to loop through each child and get each element reference. Then use this to perform calculations before the children components are rendered.
i.e. This is a very simple example on how to retrieve the references and use them.
interface LayoutWrapperProps {
onMount: () => void;
}
const LayoutWrapper: React.FC<LayoutWrapperProps> = ({ onMount, children }) => {
React.useEffect(() => {
onMount();
}, [onMount]);
return <>{children}</>;
};
const Layout: React.FC = ({ children }) => {
const references = React.useRef<HTMLElement[]>([]);
React.useEffect(() => {
references.current = [];
});
function getReference(ref: HTMLElement) {
references.current = references.current.filter(Boolean).concat(ref);
}
function getHeights() {
const heights = references.current.map((ref) =>
ref?.getBoundingClientRect()
);
console.log(heights);
}
const clonedChildren = React.Children.map(children, (child) => {
return React.cloneElement(child as any, {
ref: getReference
});
});
return <LayoutWrapper onMount={getHeights}>{clonedChildren}</LayoutWrapper>;
};
If you don't have access to the children element references
If the children components aren't passing up an element as the reference, you'll have to wrap the dynamic children components in a component so we can get an element reference. i.e.
const WrappedComponent = React.forwardRef((props, ref) => {
return (
<div ref={ref}>
{props.children}
</div>
)
});
When rendering the children components, then the code above that gets the references will work:
<Layout>
<WrappedComponent>
<Child1 />
</WrappedComponent>
</Layout>

Since we don't know how your children is built, here is what I can propose you :
import React from 'react';
import { render } from 'react-dom';
const App = () => {
const el1Ref = React.useRef();
const el2Ref = React.useRef();
const [childrenValues, setChildrenValues] = React.useState([]);
React.useEffect(() => {
setChildrenValues([
el1Ref.current.getBoundingClientRect(),
el2Ref.current.getBoundingClientRect()
]);
}, []);
return (
<Parent childrenVals={childrenValues}>
<span ref={el1Ref}>
<Child value="Hello" />
</span>
<span ref={el2Ref}>
<Child value="<div>Hello<br />World</div>" />
</span>
</Parent>
);
};
const Parent = ({ children, childrenVals }) => {
React.useEffect(() => {
console.log('children values from parent = ', childrenVals);
});
return <>{children}</>;
};
const Child = ({ value }) => {
return <div dangerouslySetInnerHTML={{ __html: value }} />;
};
render(<App />, document.getElementById('root'));
And here is the repro on Stackblitz.
The idea is to manipulate how your children is built.

how to pass an event from a child component to parent component back down to another child in React

Lets say for instance that I have three components in React, an App (the parent component), a button component and a component that is meant to display something, can be anything doesn't really matter. Lets say in the button component is activated, how would I pass the information (ie that the event actually happened) to the App parent component back down to the other child component to let it know a specific event happened to display some message?

this is how I would go about dong this using hooks :
const Parent=(props)=>{
[eventHappend,setEventHappend]=useState(false)
return (
<div>
<Child1 setEventHappend={setEventHappend} />
<Child2 eventHappend={eventHappend} />
</div>
)
}
const Child =({setEventHappend})=>{
return (
<div>
<button onClick={e=>setEventHappend(true)} > click me 1 </button>
</div>
)
}
const Child2 =({eventHappend})=>{
return (
<div>
<button onClick={e=>{/* some code*/ }} > {eventHappend?'event happend':'event didnt happen yet '} </button>
</div>
)
}

There are various ways you can achieve this pass state as props to the child elements (must know before other methods), context or use redux which has a store.
Generally speaking. React has one way data flow, uni directional. As in the parent will hold the state and will be passed to child elements.
Here App holds the state buttonClick which has the information about the event.
const App = () => {
const [ buttonClick, setButtonClick] = React.useState(false);
const messageToBeDispalyed = "The button has been clicked"
return (
<div>
<CustomButton setEventHappened={setButtonClick} />
<DisplayText value = {buttonClick ? messageToBeDispalyed : ""} />
</div>
)
}
const CustomButton = (props) =>{
return <button onClick={(e)=>props.setEventHappened(true)} > Click Me </button>
}
const DisplayText = (props) => {
return <h1> {props.value} </h1>
}

Similar answers to the others, but you would pass down a method to the child from the parent to update the state. But be aware that by doing this will cause a rerender for all of the parent's children.
const Parent = () => {
const [state, setState] = React.useState(false);
const handleClick = value => {
setState(value);
};
return (
<Child state={state} handleClick={handleClick} />
<OtherChild isTrue={state} /> // this component needs data changed by <Child />
)
};
const Child = props => {
const {state, handleClick} = props;
return (
<button onClick={() => handleClick(!state)} >click me</button>
);
};
This way the parent alone handles the state change and provides that method to the child.

as #Loveen Dyall and #Shruti B mentioned you can use RXJS for a more modular approach ,While RxJS is typically thought of as being used with Angular projects, it's a completely separate library that can be used with other JavaScript frameworks like React and Vue.
When using RxJS with React, the way to communicate between components is to use an Observable and a Subject (which is a type of observable), I won't go too much into the details about how observables work here since it's a big subject, but in a nutshell there are two methods that we're interested in: Observable.subscribe() and Subject.next().
learn more about RXJS and Observables : https://blog.logrocket.com/understanding-rxjs-observables/
Observable.subscribe()
The observable subscribe method is used by React components to subscribe to messages that are sent to an observable.
Subject.next()
The subject next method is used to send messages to an observable which are then sent to all React components that are subscribers (a.k.a. observers) of that observable.
here is how you implement it in this use case :
this is called a service and you would put this file in a services folder
import { Subject } from 'rxjs';
const subject = new Subject();
//here where sending { event: eventTitle } , that way you can listen to diffrent events , for example 'INCREMENTED' you could even send values
export const eventsService= {
sendEvent: eventTitle => subject.next({ title: eventTitle }),
getEventNotification: () => subject.asObservable()
};
in your Child 1 component you would subscribe to the observable in useEffect or compoentDidMount if your using class component:
import { eventsService} from '../services';
const Child1 =()=>{
const [child2EventFired,setChild2EventFired]=useState(false)
useEffect(()=>{
let subscription = eventsService.getEventNotification().subscribe(eventTitle =>
{
if (eventTitle=="CHILD2_BUTTON_CLICK" ) {
setChild2EventFired(true)
}else{
setChild2EventFired(false)
}
});
return ()=>{
subscription.unsubscribe();
}
},[])
return <div>
<button> {child2EventFired? 'child2 button event fired':'event not fired yet'} </button>
</div>
}
in your Child 2 component
import { eventsService} from '../services';
const Child2 =()=>{
Child2Click=(e)=>{
//some code,
//then send messages to the observable observable
eventsService.sendEvent('CHILD2_BUTTON_CLICK');
}
return <div>
<button onClick={Child2Click} >click me</button>
</div>
}

How to access a child's state in React

I have the following structure:
FormEditor - holds multiple instances of FieldEditor
FieldEditor - edits a field of the form and saving various values about it in its state
When a button is clicked within FormEditor, I want to be able to collect information about the fields from all FieldEditor components, information that's in their state, and have it all within FormEditor.
I considered storing the information about the fields outside of FieldEditor's state and put it in FormEditor's state instead. However, that would require FormEditor to listen to each of its FieldEditor components as they change and store their information in its state.
Can't I just access the children's state instead? Is it ideal?

Just before I go into detail about how you can access the state of a child component, please make sure to read Markus-ipse's answer regarding a better solution to handle this particular scenario.
If you do indeed wish to access the state of a component's children, you can assign a property called ref to each child. There are now two ways to implement references: Using React.createRef() and callback refs.
Using React.createRef()
This is currently the recommended way to use references as of React 16.3 (See the documentation for more information). If you're using an earlier version then see below regarding callback references.
You'll need to create a new reference in the constructor of your parent component and then assign it to a child via the ref attribute.
class FormEditor extends React.Component {
constructor(props) {
super(props);
this.FieldEditor1 = React.createRef();
}
render() {
return <FieldEditor ref={this.FieldEditor1} />;
}
}
In order to access this kind of ref, you'll need to use:
const currentFieldEditor1 = this.FieldEditor1.current;
This will return an instance of the mounted component so you can then use currentFieldEditor1.state to access the state.
Just a quick note to say that if you use these references on a DOM node instead of a component (e.g. <div ref={this.divRef} />) then this.divRef.current will return the underlying DOM element instead of a component instance.
Callback Refs
This property takes a callback function that is passed a reference to the attached component. This callback is executed immediately after the component is mounted or unmounted.
For example:
<FieldEditor
ref={(fieldEditor1) => {this.fieldEditor1 = fieldEditor1;}
{...props}
/>
In these examples the reference is stored on the parent component. To call this component in your code, you can use:
this.fieldEditor1
and then use this.fieldEditor1.state to get the state.
One thing to note, make sure your child component has rendered before you try to access it ^_^
As above, if you use these references on a DOM node instead of a component (e.g. <div ref={(divRef) => {this.myDiv = divRef;}} />) then this.divRef will return the underlying DOM element instead of a component instance.
Further Information
If you want to read more about React's ref property, check out this page from Facebook.
Make sure you read the "Don't Overuse Refs" section that says that you shouldn't use the child's state to "make things happen".

If you already have an onChange handler for the individual FieldEditors I don't see why you couldn't just move the state up to the FormEditor component and just pass down a callback from there to the FieldEditors that will update the parent state. That seems like a more React-y way to do it, to me.
Something along the line of this perhaps:
const FieldEditor = ({ value, onChange, id }) => {
const handleChange = event => {
const text = event.target.value;
onChange(id, text);
};
return (
<div className="field-editor">
<input onChange={handleChange} value={value} />
</div>
);
};
const FormEditor = props => {
const [values, setValues] = useState({});
const handleFieldChange = (fieldId, value) => {
setValues({ ...values, [fieldId]: value });
};
const fields = props.fields.map(field => (
<FieldEditor
key={field}
id={field}
onChange={handleFieldChange}
value={values[field]}
/>
));
return (
<div>
{fields}
<pre>{JSON.stringify(values, null, 2)}</pre>
</div>
);
};
// To add the ability to dynamically add/remove fields, keep the list in state
const App = () => {
const fields = ["field1", "field2", "anotherField"];
return <FormEditor fields={fields} />;
};
Original - pre-hooks version:
class FieldEditor extends React.Component {
constructor(props) {
super(props);
this.handleChange = this.handleChange.bind(this);
}
handleChange(event) {
const text = event.target.value;
this.props.onChange(this.props.id, text);
}
render() {
return (
<div className="field-editor">
<input onChange={this.handleChange} value={this.props.value} />
</div>
);
}
}
class FormEditor extends React.Component {
constructor(props) {
super(props);
this.state = {};
this.handleFieldChange = this.handleFieldChange.bind(this);
}
handleFieldChange(fieldId, value) {
this.setState({ [fieldId]: value });
}
render() {
const fields = this.props.fields.map(field => (
<FieldEditor
key={field}
id={field}
onChange={this.handleFieldChange}
value={this.state[field]}
/>
));
return (
<div>
{fields}
<div>{JSON.stringify(this.state)}</div>
</div>
);
}
}
// Convert to a class component and add the ability to dynamically add/remove fields by having it in state
const App = () => {
const fields = ["field1", "field2", "anotherField"];
return <FormEditor fields={fields} />;
};
ReactDOM.render(<App />, document.body);

As the previous answers said, try to move the state to a top component and modify the state through callbacks passed to its children.
In case that you really need to access to a child state that is declared as a functional component (hooks) you can declare a ref in the parent component, and then pass it as a ref attribute to the child, but you need to use React.forwardRef and then the hook useImperativeHandle to declare a function you can call in the parent component.
Take a look at the following example:
const Parent = () => {
const myRef = useRef();
return <Child ref={myRef} />;
}
const Child = React.forwardRef((props, ref) => {
const [myState, setMyState] = useState('This is my state!');
useImperativeHandle(ref, () => ({getMyState: () => {return myState}}), [myState]);
})
Then you should be able to get myState in the Parent component by calling:
myRef.current.getMyState();

It's 2020 and lots of you will come here looking for a similar solution but with Hooks (they are great!) and with the latest approaches in terms of code cleanliness and syntax.
So as previous answers had stated, the best approach to this kind of problem is to hold the state outside of child component fieldEditor. You could do that in multiple ways.
The most "complex" is with a global context (state) that both parent and children could access and modify. It's a great solution when components are very deep in the tree hierarchy and so it's costly to send props in each level.
In this case I think it's not worth it, and a more simple approach will bring us the results we want, just using the powerful React.useState().
An approach with a React.useState() hook - way simpler than with Class components
As said, we will deal with changes and store the data of our child component fieldEditor in our parent fieldForm. To do that we will send a reference to the function that will deal and apply the changes to the fieldForm state, you could do that with:
function FieldForm({ fields }) {
const [fieldsValues, setFieldsValues] = React.useState({});
const handleChange = (event, fieldId) => {
let newFields = { ...fieldsValues };
newFields[fieldId] = event.target.value;
setFieldsValues(newFields);
};
return (
<div>
{fields.map(field => (
<FieldEditor
key={field}
id={field}
handleChange={handleChange}
value={fieldsValues[field]}
/>
))}
<div>{JSON.stringify(fieldsValues)}</div>
</div>
);
}
Note that React.useState({}) will return an array with position 0 being the value specified on call (Empty object in this case), and position 1 being the reference to the function
that modifies the value.
Now with the child component, FieldEditor, you don't even need to create a function with a return statement. A lean constant with an arrow function will do!
const FieldEditor = ({ id, value, handleChange }) => (
<div className="field-editor">
<input onChange={event => handleChange(event, id)} value={value} />
</div>
);
Aaaaand we are done, nothing more. With just these two slim functional components we have our end goal "access" our child FieldEditor value and show it off in our parent.
You could check the accepted answer from 5 years ago and see how Hooks made React code leaner (by a lot!).
Hope my answer helps you learn and understand more about Hooks, and if you want to check a working example here it is.

Now you can access the InputField's state which is the child of FormEditor.
Basically, whenever there is a change in the state of the input field (child), we are getting the value from the event object and then passing this value to the Parent where in the state in the Parent is set.
On a button click, we are just printing the state of the input fields.
The key point here is that we are using the props to get the input field's id/value and also to call the functions which are set as attributes on the input field while we generate the reusable child input fields.
class InputField extends React.Component{
handleChange = (event)=> {
const val = event.target.value;
this.props.onChange(this.props.id , val);
}
render() {
return(
<div>
<input type="text" onChange={this.handleChange} value={this.props.value}/>
<br/><br/>
</div>
);
}
}
class FormEditorParent extends React.Component {
state = {};
handleFieldChange = (inputFieldId , inputFieldValue) => {
this.setState({[inputFieldId]:inputFieldValue});
}
// On a button click, simply get the state of the input field
handleClick = ()=>{
console.log(JSON.stringify(this.state));
}
render() {
const fields = this.props.fields.map(field => (
<InputField
key={field}
id={field}
onChange={this.handleFieldChange}
value={this.state[field]}
/>
));
return (
<div>
<div>
<button onClick={this.handleClick}>Click Me</button>
</div>
<div>
{fields}
</div>
</div>
);
}
}
const App = () => {
const fields = ["field1", "field2", "anotherField"];
return <FormEditorParent fields={fields} />;
};
ReactDOM.render(<App/>, mountNode);

You may access the child state by passing a callback to the child component.
const Parent = () => {
return (
<Child onSubmit={(arg) => {
console.log('accessing child state from parent callback: ', arg)
}}
/>
)
}
const Child = ({onSubmit}) => {
const [text, setText] = useState('');
return (
<>
<input value={text} onChange={setText}>
<button onClick={() => onSubmit(text)} />
</>
)
}
Now if you click the button in the child component, you will execute the function passed from the parent and have access to the child component's state variables.