Why is my variable undefined inside the Underscore

问题:

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.

回答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);
   }
   

回答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

回答3:

this in javascript does not work the same way as you would expect. read this article: http://www.digital-web.com/articles/scope_in_javascript/

short version:

the value of this changes every time you call a function. to fix, set another variable equal to this and reference that instead

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

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

回答4:

Note that you can also pass things other that "this". For example, I do something like:

var layerGroupMasterData = [[0],[1,2,3],[4,5],[6,7,8,9],[10]];

_.each(layerGroupMasterData,function(layerGroup,groupNum){
    _.each(layerGroup, function (layer, i) {
            doSomethingThatComparesOneThingWithTheOverallGroup(layerGroupMasterData,layer);
    },layerGroups);
},layerGroupMasterData);