小编Pie*_*ska的帖子

    var exemples =  [
            {
                'name'     : 'd',
                'index'    : 3
            },
            {
                'name'     : 'c',
                'index'     : 2
            },
            {
                'name'     : 'a',
                'index'    : 0
            },
            {
                'name'     : 'b',
                'index'    : 1
            }
        ];

      const list = exemples.map((exemple, index, array) => exemple.name)

      console.log(list)

    import Link from 'next/link'
    import React, { Component } from 'react'
    import firebase from 'firebase'

    export default class extends Page {

        constructor () {
          super()
          this.state = {
            datas: ''
          }
       }

        componentDidMount () {

          firebase.initializeApp(clientCredentials)

      // -------  Connect to the firebase DATABASE -------- ////

      firebase.database().ref().orderByChild("sectionIndex")
          .on("child_added",
            snapshot => {this.setState({datas: snapshot.val().category});},
            error => {console.log("Error: " + error.code);});

        }

        render () {

          return (
            <ul>
              {this.state.datas.map((data, i) => 
              <li key={i}> {data.category} </li>
            </ul>
          )
        }
      }