How to declare a global variable in React?

问题

I initialized i18n translation object once in a component ( first component that loads in the app ) . That same object is required In all other components. I don't want to re-initialize it in every component. What's the way around ? Making it available to window scope doesn't help as I need to use it in render() method.

Please suggest a generic solution for these problem and not i18n specific solution .

回答1:

Why don't you try using Context?

You can declare a global context variable in any of the parent components and this variable will be accessible across the component tree by this.context.varname. You only have to specify childContextTypes and getChildContext in the parent component and thereafter you can use/modify this from any component by just specifying contextTypes in the child component.

However, please take a note of this as mentioned in docs:

Just as global variables are best avoided when writing clear code, you should avoid using context in most cases. In particular, think twice before using it to "save typing" and using it instead of passing explicit props.

回答2:

Is not recommended but.... you can use componentWillMount from your app class to add your global variables trough it... a bit like so:

componentWillMount: function () {
    window.MyVars = {
        ajax: require('../helpers/ajax.jsx'),
        utils: require('../helpers/utils.jsx')
    };
}

still consider this a hack... but it will get your job done

btw componentWillMount executes once before rendering, see more here: https://reactjs.org/docs/react-component.html#mounting-componentwillmount

回答3:

Beyond React

You might not be aware that an import is global already. If you export an object (singleton) it is then globally accessible as an import statement and it can also be modified globally.

If you want to initialize something globally but ensure its only modified once, you can use this singleton approach that initially has modifiable properties but then you can use Object.freeze after its first use to ensure its immutable in your init scenario.

const myInitObject = {}
export default myInitObject

then in your init method referencing it:

import myInitObject from './myInitObject'
myInitObject.someProp = 'i am about to get cold'
Object.freeze(myInitObject)

The myInitObject will still be global as it can be referenced anywhere as an import but will remain frozen and throw if anyone attempts to modify it.

If using react-create-app

(what I was looking for actually) In this scenario you can also initialize global objects cleanly when referencing environment variables.

Creating a .env file at the root of your project with prefixed REACT_APP_ variables inside does quite nicely. You can reference within your JS and JSX process.env.REACT_APP_SOME_VAR as you need AND it's immutable by design.

This avoids having to set window.myVar = %REACT_APP_MY_VAR% in HTML.

See more useful details about this from Facebook directly:

https://facebook.github.io/create-react-app/docs/adding-custom-environment-variables

回答4:

You can use mixins in react https://facebook.github.io/react/docs/reusable-components.html#mixins .

回答5:

Can keep global variables in webpack i.e. in webpack.config.js

externals: {
  'config': JSON.stringify(GLOBAL_VARIABLE: "global var value")
}

In js module can read like

var config = require('config')
var GLOBAL_VARIABLE = config.GLOBAL_VARIABLE

Hope this will help.

回答6:

The best way I have found so far is to use React Context but to isolate it inside a high order provider component.

回答7:

I don't know what they're trying to say with this "React Context" stuff - they're talking Greek, to me, but here's how I did it:

Carrying values between functions, on the same page

In your constructor, bind your setter:

this.setSomeVariable = this.setSomeVariable.bind(this);

Then declare a function just below your constructor:

setSomeVariable(propertyTextToAdd) {
    this.setState({
        myProperty: propertyTextToAdd
    });
}

When you want to set it, call this.setSomeVariable("some value");

(You might even be able to get away with this.state.myProperty = "some value";)

When you want to get it, call var myProp = this.state.myProperty;

Using alert(myProp); should give you some value .

Extra scaffolding method to carry values across pages/components

You can assign a model to this (technically this.stores), so you can then reference it with this.state:

import Reflux from 'reflux'
import Actions from '~/actions/actions`

class YourForm extends Reflux.Store
{
    constructor()
    {
        super();
        this.state = {
            someGlobalVariable: '',
        };
        this.listenables = Actions;
        this.baseState = {
            someGlobalVariable: '',
        };
    }

    onUpdateFields(name, value) {
        this.setState({
            [name]: value,
        });
    }

    onResetFields() {
        this.setState({
           someGlobalVariable: '',
        });
    }
}
const reqformdata = new YourForm

export default reqformdata

Save this to a folder called stores as yourForm.jsx.

Then you can do this in another page:

import React from 'react'
import Reflux from 'reflux'
import {Form} from 'reactstrap'
import YourForm from '~/stores/yourForm.jsx'

Reflux.defineReact(React)

class SomePage extends Reflux.Component {
    constructor(props) {
        super(props);
        this.state = {
            someLocalVariable: '',
        }
        this.stores = [
            YourForm,
        ]
    }
    render() {

        const myVar = this.state.someGlobalVariable;
        return (
            <Form>
                <div>{myVar}</div>
            </Form>
        )
    }
}
export default SomePage

If you had set this.state.someGlobalVariable in another component using a function like:

setSomeVariable(propertyTextToAdd) {
    this.setState({
       myGlobalVariable: propertyTextToAdd
   });
}

that you bind in the constructor with:

this.setSomeVariable = this.setSomeVariable.bind(this);

the value in propertyTextToAdd would be displayed in SomePage using the code shown above.

来源：https://stackoverflow.com/questions/34351804/how-to-declare-a-global-variable-in-react