Why is my variable undefined inside the Underscore.js each function?

Question

Here is my code:

TextClass = function () {
    this._textArr = {};
};

TextClass.prototype = {
    SetTexts: function (texts) {
        for (var i = 0; i < texts.length; i++) {
            this._textArr[texts[i].Key] = texts[i].Value;
        }
    },
    GetText: function (key) {
        var value = this._textArr[key];
        return String.IsNullOrEmpty(value) ? 'N/A' : value;
    }
};

I'm using the Underscore.js library and would like to define my SetTexts function like this:

_.each(texts, function (text) {
    this._textArr[text.Key] = text.Value;
});

but _textArr is undefined when I get into the loop.

Answer 1

In JavaScript, the function context, known as this, works rather differently.

You can solve this in two ways:

1. Use a temporary variable to store the context:

   SetTexts: function (texts) {
     var that = this;
     _.each(texts, function (text) {
       that._textArr[text.Key] = text.Value;
     });
   }
   

2. Use the third parameter to _.each() to pass the context:

   SetTexts: function (texts) {
     _.each(texts, function (text) {
       this._textArr[text.Key] = text.Value;
     }, this);
   }
   

Answer 2

You have to pass this as context for _.each call like this:

_.each(texts, function (text) {
    this._textArr[text.Key] = text.Value;
}, this);

See the docs for http://underscorejs.org/#each