Question

160

Accessing this in a forEach loop results in undefined

rated 0 times [ 167] [ 7] / answers: 1 / hits: 27097 / 11 Years ago, fri, october 18, 2013, 12:00:00

I'm iterating through an array using forEach in one of my Class's methods. I need access to the instance of the class inside the forEach but this is undefined.

var aGlobalVar = {};



(function () {



    use strict;



  aGlobalVar.thing = function() {

      this.value = thing;   

  }



  aGlobalVar.thing.prototype.amethod = function() {

      data.forEach(function(d) {

      console.log(d);

      console.log(this.value);

  });

}

})();



var rr = new aGlobalVar.thing();

rr.amethod();

I have a fiddle I'm working on here: http://jsfiddle.net/NhdDS/1/ .

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rocioblancac

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4 Years ago

answered 11 Years ago susanajamiep · Accepted Answer

In strict mode if you call a function not through a property reference and without specifying what this should be, it's undefined.

forEach (spec | MDN) allows you to say what this should be, it's the (optional) second argument you pass it:

aGlobalVar.thing.prototype.amethod = function() {

  data.forEach(function(d) {

    console.log(d);

    console.log(this.value);

  }, this);

  // ^^^^

}

Alternately, arrow functions were added to JavaScript in 2015. Since arrows close over this, we could use one for this:

aGlobalVar.thing.prototype.amethod = function() {

  data.forEach(d => {

    console.log(d);

    console.log(this.value);

  });

}