javascript - Typescript/Angular2 TypeError: Cannot read property of undefined -

ang2 component:

import { component, oninit } '@angular/core'; import { angularfire, firebaselistobservable, firebaseobjectobservable } 'angularfire2'; import chart 'chart.js'  @component({   selector: 'app-prchart',   templateurl: './app-prchart.component.html',   styleurls: ['./app-prchart.component.css'] }) export class appprchartcomponent implements oninit {   useremail: any;   email: string;   uid: any;   records = [];   newrecords = [];   filteredrecords = [];   labels = [];    public barchartoptions:any = {     scaleshowverticallines: false,     responsive: true,     legend: {         display: false      }   };   public barchartcolors: [] =[     {         backgroundcolor:'rgba(30, 136, 229, 1)'     }   ];    public barcharttype:string = 'bar';    constructor(public af: angularfire) {     var auth = this.af.auth.subscribe( (user) => {       if (user) {           this.useremail = this.af.auth.subscribe(auth => {           const queryobservable = af.database.list('/users/'+ auth.auth.uid +'/records/', {           });           queryobservable.subscribe(querieditems => {             this.records.push(querieditems);           });            // filter records pr's           this.newrecords =           this.records[0].sort((a, b) => {             if (a.movement === b.movement) {               return a.weight >= b.weight ? -1 : 1             }             return a.movement > b.movement ? 1 : -1           })           .filter((rec, i, arr) => {             if (i === 0) return true             return rec.movement !== arr[i - 1].movement           });           let recordstring = json.stringify(this.newrecords);           let recordparse = json.parse(recordstring);           this.filteredrecords.push(recordparse);         });       } else {       }     });      this.filteredrecords[0].foreach(function(snapshot) {         this.labels.push(snapshot.movement);         //barchartdata.push(snapshot.weight);     });     //console.log(barchartlabels);     //console.log(barchartdata);   }    ngoninit() {    }  } 

i'm trying push items array keep getting following error:

typeerror: cannot read property 'labels' of undefined 

the error occurs towards bottom line of code being run:

this.labels.push(snapshot.movement); 

i've been playing hours , can't figure out i'm doing wrong, appreciated.

the issues this changes inside callback functions. can fix using arrow functions capture correct this:

this.filteredrecords[0].foreach((snapshot) => {     this.labels.push(snapshot.movement);     //barchartdata.push(snapshot.weight); }); 

or capturing this in variable:

let = this; this.filteredrecords[0].foreach(function (snapshot) {     that.labels.push(snapshot.movement);     //barchartdata.push(snapshot.weight); }); 

this might useful: what 'this' in typescript