javascript - Passing scope to forEach

Question

I'm trying to use a callback method addToCount instead of anonymous function in forEach. But I can't access this.count in it (returns undefined).

function Words(sentence) {
  this.sentence = sentence;
  this.count = {};
  this.countWords();
}

Words.prototype = {
  countWords: function() {
    var words = this.sentence.split(/W+/);
    words.forEach(this.addToCount);
  },
  addToCount: function(word) {
    word = word.toLowerCase();
    if (word == '') return;
    if (word in this.count)
      this.count[word] += 1;
    else
      this.count[word] = 1;
  }
}

I think the problem is the scope. How can I pass this to addToCount or is there any other way to make it work?

Answer 1

You need to use Function#bind to bind a scope:

words.forEach(this.addToCount.bind(this));

Note that this is not available in all browsers: you should use a shim (as provided in the link above) to add it in the browsers that don't support Function#bind.

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As dandavis points out in the comments, you can pass a value to Array#forEach as the context for the callback:

words.forEach(this.addToCount, this);