javascript - Q promises - How does scope work? - Stack Overflow

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How does the scope of the Q promises work? As far as I know the callback of the "then" is called by window, like a setTimeout.

In this example (just an example to understand how it works):

var getFileText = function() {
    var deferred = Q.defer();
    Server.readFile("foo.txt", "utf-8", function (error, text) {
        if (error) {
            deferred.reject(new Error(error));
        } else {
            deferred.resolve(text);
        }
    });
    return deferred.promise;
};

var Foo =  function () {
    getFileText().then(this.showFile);
};

Foo.prototype.showFile = function(text) {
    this.text = text;
    console.log(text);
};

var foo = new Foo();

To have the text in the instance of foo I'm using bind:

var Foo =  function () {
    getFileText().then(this.showFile.bind(this));
};

Is there any other way?

How does the scope of the Q promises work? As far as I know the callback of the "then" is called by window, like a setTimeout.

In this example (just an example to understand how it works):

var getFileText = function() {
    var deferred = Q.defer();
    Server.readFile("foo.txt", "utf-8", function (error, text) {
        if (error) {
            deferred.reject(new Error(error));
        } else {
            deferred.resolve(text);
        }
    });
    return deferred.promise;
};

var Foo =  function () {
    getFileText().then(this.showFile);
};

Foo.prototype.showFile = function(text) {
    this.text = text;
    console.log(text);
};

var foo = new Foo();

To have the text in the instance of foo I'm using bind:

var Foo =  function () {
    getFileText().then(this.showFile.bind(this));
};

Is there any other way?

• javascript
• scope
• q

Share Improve this question Follow edited Jul 4, 2013 at 17:46 Kaizo asked Jun 20, 2013 at 18:08 KaizoKaizo 4,19522 gold badges2525 silver badges2626 bronze badges

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1 Answer 1

Sorted by: Reset to default 8

How does the scope of the Q promises work?

You're looking for the context.

I know the callback of the "then" is called by window

Well, on the global context, yes. It is specified to be called with undefined as the thisArg.

I'm using bind. Is there any other way?

Only the lengthy one, with a variable referencing the instance:

var that = this;
getFileText().then(function(text) {
    that.text = text;
    // or even longer:
    // that.showFile(text);
});

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