Develop Reference

How to Programmatically Provide and Consume Context?

So my question is a simple one. In React js I want to pass some states and handlers from a parent to its 3rd grandchild using Context. I have implemented this within the jsx but I want to use the states within the javascript o that I have some logic before I completely output my states.
I have divided my question into 2 parts. 1.) What I have done so far. 2.) What I want to do essentially.
1.)
// this file just stores the Context
MyContext.js
import React, { Component } from 'react';
export const MyContext = React.createContext();
MyProvider.js // this class is used by the parent and the child to have access to the provider
import React, { Component } from 'react';
import {MyContext} from '../MyContext'
class MyProvider extends Component {
state = {
name: 'Wes',
age: 100,
cool: true
}
render() {
return (
<MyContext.Provider value={{
state: this.state,
growAYearOlder: () => this.setState({
age: this.state.age + 1
})
}}>
{this.props.children}
</MyContext.Provider>
)
}
}
export default MyProvider;
// Ok so now I am basically skipping the parent and showing you the consumer grand-child
Person.js
import React, { Component } from 'react';
// first we will make a new context
import { MyContext } from '../MyContext';
class Person extends Component {
render() {
return (
<div className="person">
<MyContext.Consumer>
{(context) => (
<React.Fragment>
<p>Age: {context.state.age}</p>
<p>Name: {context.state.name}</p>
<button onClick={context.growAYearOlder}>🍰🍥🎂</button>
</React.Fragment>
)}
</MyContext.Consumer>
</div>
)
}
}
export default Person;
2.)
// Ok so as you can see here I have had to immediately use the context.growAYearOlder. What I want to do instead is have control of it using javascript and modify it as desired; So something like this:
Child.js
const parentContext = MyContext.getContext();
if(somethingHappens){
parentContext().growAYearOlder();
}
return(
// The now rendered component
);
I tried something like this but it doesnt work:
MyContext.Consumer.context.growAYearOlder();

There are many similar questions with proper answers, docs, examples and so on - but this question kept popping up for me.
So, in case you want to get the context value and use it within your component's render() just import it (export context itself not only provider) and use _currentValue e.g.
const contextData = MyContext._currentValue;
Note that you still have to wrap your components with your given context provider.
Also note that for function components, you need to use useContext e.g.
const contextData = useContext(MyContext);
And for class components you can assign the context to a static var and then use it e.g.
class Main extends React.Component(){
static contextType = MyContext;
componentDidMount(){
const contextData = this.context;
}
render() {
return (
<p>Hey</p>
);
}
Note that the static var has to be called contextType otherwise this.context won't hold the MyContext data.

I've based my answer solely from the docs itself(https://reactjs.org/docs/context.html#updating-context-from-a-nested-component)
import React, { Component } from 'react';
import { MyContext } from '../MyContext'
class MyProvider extends Component {
constructor(props) {
super(props)
// I've moved the state declaration inside the constructor
this.state = {
name: 'Wes',
age: 100,
cool: true
}
// moved the function here and added prevState
this.growAYearOlder = () => {
this.setState(prevState => ({
age: prevState.age + 1,
}))
};
}
render() {
return (
<MyContext.Provider value={{
state: this.state,
growAYearOlder: this.growAYearOlder,
}}>
{this.props.children}
</MyContext.Provider>
)
}
}
export default MyProvider;

How to properly render Component after this.setState in React

I have this React component
import React, { Component } from "react";
export default class ResourceForField extends Component {
constructor() {
super();
this.state = {
resources: [],
};
}
componentDidMount() {
// get the resources from the Link props and save it into the state
this.setState({
resources: this.props.location.resources,
});
}
// This component gets the id of current learningField from the url
// and the rest(like the resources) from the Link component
render() {
return (
<div>
{this.state.resources.map(res => (
<div>test</div>
))}
</div>
);
}
}
It gets the resources from the Link component, and that works fine. If I check out the state of the Component from the dev tools, the state looks right. And I thought with my logic this should work. So firstly, the state is empty, the component gets rendered, since the state is empty it doesn't render any components. Then, setState gets called, it gets all the resources and saves them into the state, and then the component would re-render, and it should work, but it doesn't. I'm getting a TypeError: Cannot read property 'map' of undefined error. What is the correct way to do this and how do I fix this?

Try this code:
import React, { Component } from "react";
export default class ResourceForField extends Component {
constructor() {
super();
this.state = {
resources: this.props && this.props.location && this.props.location.resources?this.props.location.resources:[],
};
}
componentDidMount() {
}
// This component gets the id of current learningField from the url
// and the rest(like the resources) from the Link component
render() {
return (
<div>
{this.state.resources.map(res => (
<div>test</div>
))}
</div>
);
}
}
Or use directly props
import React, { Component } from "react";
export default class ResourceForField extends Component {
constructor() {
super();
}
// This component gets the id of current learningField from the url
// and the rest(like the resources) from the Link component
render() {
return (
<div>
{
this.props && this.props.location &&
this.props.location.resources
?this.props.location.resources.map(res => (
<div>test</div>
))
:null
}
</div>
);
}
}
Or use componentWillReceiveProps or getDerivedStateFromProps life cycle methods.
Check this.props.location.resources is array.
See more: https://hackernoon.com/replacing-componentwillreceiveprops-with-getderivedstatefromprops-c3956f7ce607

For first check is this.props.location.resources array, or if data type changes you can add checking, you can use lodash isArray or with js like this:
import React, { Component } from "react";
export default class ResourceForField extends Component {
constructor() {
super();
this.state = {
resources: [],
};
}
componentDidMount() {
// get the resources from the Link props and save it into the state
Array.isArray(this.props.location.resources) {
this.setState({
resources: this.props.location.resources,
});
}
}
// This component gets the id of current learningField from the url
// and the rest(like the resources) from the Link component
render() {
return (
<div>
{this.state.resources.map(res => (
<div>test</div>
))}
</div>
);
}
}
Or you can just use hooks like this:
import React, { useState, useEffect } from "react";
export default function ResourceForField({location}) {
const [ resources, setResources ] = useState([]);
useEffect(() => {
if (location && Array.isArray(location.resources)) {
setResources(location.resources)
}
}, [location]);
return (
<div>
{resources.map(res => (
<div>test</div>
))}
</div>
);
}

If the internal state of ResourceForField doesn't change and always equals to its prop, you shouldn't save the prop in the state. You can instead create a pure functional component.
Also note that there's nothing preventing you from initializing the state from the props in constructor method. i.e. you're not required to wait for the component to mount in order to access the props.
So, I'd write the following component for ResourceForField:
function ResourceForField({resources = []}) {
return (
<div>
{
resources.map(res => (<div>test</div>))
}
</div>
);
}

Display a new component with button click

Im making my first react ptoject. Im new in JS, HTML, CSS and even web app programing.
What i try to do, is to display some infomration on button click.
I have an API, that looks like this:
endpoint: https://localhost:44344/api/Projects
My Data from it:
[{"id":1,"name":"Mini Jira","description":"Description for first project in list","tasks":null},{"id":2,"name":"Farm","description":"Description for second one","tasks":null}]
And im fine with that, i can get it easily by axios in my react app.
Now i will show you my Project.js Component:
import React, { Component } from "react";
import { ListGroupItem, Button, ButtonToolbar } from "react-bootstrap";
import ProjectDetails from "./ProjectDetails";
class Project extends Component {
render() {
return (
<ButtonToolbar>
<ListGroupItem>{this.props.project.name}</ListGroupItem>
<Button onClick={Here i want to display new component with details }bsStyle="primary">Details</Button>
</ButtonToolbar>
);
}
}
export default Project;
I have all data from api in project type.
My question is, how to display component that i named ProjectDetails.js on button click? I want to show all data stored in project from my api in separate view (new page or somethig like that).
View looks like this:
Thanks for any advices!
EDIT:
based on #Axnyff answer, i edited Project.js. it works ok. But when i want to (for testing) displat project.name, i get error map of undefined. My ProjectDetails.js:
import React, { Component } from "react";
class ProjectDetails extends Component {
state = {};
render() {
return <li>{this.props.project.name}</li>;
}
}
export default ProjectDetails;
EDIT2:
In Project.js in #Axnyff answet i just edited that line:
{this.state.showDetails && (
<ProjectDetails project={this.props.project} />
)}
i passed project by props, now it works like i want too. After click it displays project.name that i clicked on.

You should use state in your React component.
Let's create a field called showDetails in your state.
You can initialize it in your constructor with
constructor(props) {
super(props); // needed in javascript constructors
this.state = {
showDetails: false,
};
}
Then you need to modify the onClick to set that state to true
<Button onClick={() => this.setState({ showDetails : true })} bsStyle="primary">Details</Button>
And then use that state to show or not the ProjectDetails:
{ showDetails && <ProjectDetails /> }
The full component should look like
import React, { Component } from "react";
import { ListGroupItem, Button, ButtonToolbar } from "react-bootstrap";
import ProjectDetails from "./ProjectDetails";
class Project extends Component {
constructor(props) {
super(props); // needed in javascript constructors
this.state = {
showDetails: false,
};
}
render() {
return (
<ButtonToolbar>
<ListGroupItem>{this.props.project.name}</ListGroupItem>
<Button onClick={() => this.setState({ showDetails : true })} bsStyle="primary">Details</Button>
{ this.state.showDetails && <ProjectDetails /> }
</ButtonToolbar>
);
}
}
export default Project;
You can then modify the logic to add a toggling effect etc.
If you haven't done it, you should probably follow the official tutorial

function Bar() {
return <h1>I will be shown on click!</h1>;
}
class Foo extends React.Component {
constructor() {
super();
this.state = { showComponent: false };
}
handleClick = () => {
this.setState({ showComponent: !this.state.showComponent });
};
render() {
return (
<div>
{this.state.showComponent && <Bar />}
<button onClick={this.handleClick}>click</button>
</div>
);
}
}
ReactDOM.render(<Foo />, document.getElementById("root"));
<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="root"></div>

React JS - Passing functions for child components

I have an App component and a function 'modalToggled' inside its.
I want to pass the function to multiple child components until I get to the last one, the 'interiores' component.
Like this:
<App> -> <Coluna1> -> <MenuPrincipal> -> <Portfolio> -> <PortfolioMenu> -> <interiores>
App Component, the parent of all components:
import React, { Component } from 'react';
import Coluna1 from './Coluna1'
class App extends Component {
constructor(props) {
super(props)
this.state = {
modalOn: false
}
this.modalToggled = this.modalToggled.bind(this)
}
modalToggled = (on) => {
this.setState({modalOn: on});
}
render() {
return (
<div>
<Coluna1 onModalToggle={this.modalToggled}/>
</div>
)
}
}
export default App;
This is the 'Coluna1' the first child component. I did the same thing in the another ones: 'MenuPrincipal', 'Portfolio', 'PortfolioMenu'
class Coluna1 extends Component {
constructor(props){
super(props)
}
render() {
return (
<div>
<Header />
<MenuPrincipal onModalToggle={this.props.modalToggled} />
</div>
)
}
}
export default Coluna1
Therefore here is the last component interiores, when I click on the button there appears an error message:
TypeError: _this.props.onModalToggle is not a function
import React, { Component } from 'react'
import Modal from 'react-responsive-modal';
class Interiores extends Component {
constructor(props) {
super(props)
this.state = {
open: false
}
}
onOpenModal = () => {
this.setState({ open: true });
this.props.onModalToggle(true);
};
onCloseModal = () => {
this.setState({ open: false });
this.props.onModalToggle(false);
};
render() {
const { open } = this.state;
return (
<div>
<button onClick={this.onOpenModal}>Open modal</button>
<Modal open={open} onClose={this.onCloseModal} center></Modal>
</div>
)
}
}
export default Interiores;
Does anybody know how to solve it? Thank you

It happens, because in App class you pass prop with name onModalToggle:
<Coluna1 onModalToggle={this.modalToggled}/>
But in Coluna1 you receive this props with wrong name, modalToggled:
<MenuPrincipal onModalToggle={this.props.modalToggled} />
Just make the names of props equal. In Coluna1 and other intermediate components pass and receive this props as onModalToggle:
<MenuPrincipal onModalToggle={this.props.onModalToggle} />

This is the problem
modalToggled = (on) => {
this.setState({modalOn: on});
}
Since this is a class function it needs to be defined like
modalToggled(on) {
this.setState({modalOn: on});
}

React - How to not run function inside component children if user has no permission

I'm working on a react based app and I am checking user's permission before rendering some piece of jsx using a HasRole component. Everything works fine except if I have a function as a child of the HasRole component, the function is getting called every time the parent component renders, if the user doesn't have permission also. How do I prevent this from happening.
HasRole.js
import React, { Component } from 'react';
import { connect } from 'react-redux';
class HasRole extends Component {
render() {
const { children, role, requiredRole } = this.props;
if(!requiredRole.includes(role)) return null;
return children;
}
}
const getMapStateToProps = (extendWith = {}) => state => {
return {
role: state.auth.userType,
...extendWith
}
}
export default connect(getMapStateToProps)(HasRole);
And when I use the HasRole component to check permission, {this.renderUsers} is executed if user don't have permission also.
MovieShow.js
import React, { Component } from 'react';
import HasRole from '../has-role';
class MovieShow extends Component{
constructor(props){
super(props);
}
renderUsers(){
...
}
render(){
return(
...
<HasRole requireRole={["admin"]}>
{this.renderUsers} // this will run if user is not admin also but the component wont be shown
</HasRole>
}
}
Thanks in advance!

Make the rendersUsers a stateless functional component by dragging it out of the class instead.
https://reactjs.org/docs/components-and-props.html
By doing this it should enable you to pass the requireRole param to it, and lock it behind permissions aswell.

The reason that renderUsers is executed is because it is evaluated in Parent although rendered as children in HasRole. You could write your HasRole component to use used with renderProps like
class HasRole extends Component {
render() {
const { children, role, requiredRole } = this.props;
if(!requiredRole.includes(role)) return null;
return children();
}
}
and then you would use it like
class MovieShow extends Component{
constructor(props){
super(props);
}
renderUsers(){
...
}
render(){
return(
...
<HasRole requireRole={["admin"]}>
{() => {return this.renderUsers()}}
</HasRole>
}
}