React＆Firebase-未捕获的TypeError：无法读取未定义的属性'setState'

Question

I am getting the following error after compiling in the console

Uncaught TypeError: Cannot read property 'setState' of undefined

this is my code please can u help me to find the mistake ? i wanna save the data of childData.pic on x then i read it on i tried this.x , this.state.x , x ... and every time i get the same error

import React, { Component } from 'react'
import * as firebase from 'firebase'
import './scss/style.scss';
import QuickView from './QuickView';

export default class Dashboard extends Component {
    constructor() {
        super();
        this.state = {
            speed: '', snapshot: undefined, x: ''
        }
    }
    componentDidMount() {
        var w = firebase.auth().currentUser
        var rootRef = firebase.database().ref(`restaus/`).orderByKey();
        rootRef.once("value")
            .then(function (snapshot) {
                snapshot.forEach(function (childSnapshot) {
                    var key = childSnapshot.key;
                    var childData = childSnapshot.val();
                    var x;
                    this.setState({
                        x: childData.pic,
                    })
                });
            });
        this.setState({
            email: w.email,
        })
    }

    render() {
        return (
            <div>
                <h4>your email is {this.state.email}</h4>

                <div className="product">
                    <div className="product-image">
                        <img src={this.x} /*onClick={this.quickView.bind(this, name)}*/ />
                    </div>
                    <h4 className="product-name">{this.state.x}</h4>
                    <p className="product-price">{this.props.x}</p>
                    <div className="product-action">
                        <button type="button" >Add To cart</button>
                    </div>
                </div>
            </div>)
    }
}

Answer 1

Please format your code properly, it is hard for people to follow. The problem is you use function(snapshot) , in this case this not point to your Component, it point to function which call it.

You can use arrow function to void this issue https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

For example:

rootRef.once("value").then((snapshot) => {
   snapshot.forEach((childSnapshot) => {
       ... 
       this.setState(...)
   })
})