Question

I have a class that is structured something like this

function myClass () {
    this.varA = 0;
}

myClass.prototype.consts = {
    someConst : 1,
    anotherConst : 1000,
}

myClass.prototype.startIt = function () {
    window.setTimeout(this.brokenFunc.bind(this), this.consts.anotherConst);
}

myClass.prototype.brokenFunc = function () {
    this.varA += this.consts.someConst;

    window.setTimeout(this.brokenFunc.bind(this), this.consts.anotherConst); 
}

// Example call
var myObj = new myClass();
myObj.startIt();

This works fine on most Android devices — but a user running Android 2.3 has now informed me that it doesn’t work and I could reproduce the error in an emulator. First, it says TypeError: Result of expression 'this.brokenFunc.bind' [undefined] is not a function for this line (within startIt):

window.setTimeout(this.brokenFunc.bind(this), this.consts.anotherConst); 

Fair enough, I thought, and did the old var _this = this trick to get around the bind call. But now it says TypeError: Result of expression 'this.consts' [undefined] is not an object in this line

this.varA += this.consts.someConst;

And I’m a little lost. How can this piece of code not work? Especially since it works in most Android versions.

Answer 1

By default, setTimeout invokes functions with this of the global object (i.e., window, in the browser). (Side note: in strict mode, it’s undefined instead.) Thus, when you don’t use bind, the this brokenFunc is now window instead of the object that registered the timeout.

In order to preserve the this from startIt, you’ll need to wrap your call in an anonymous function:

myClass.prototype.startIt = function () {
    var that = this;

    window.setTimeout(function() {
        // call brokenFunc with the outer this
        that.brokenFunc();
    }, this.consts.anotherConst);
}

Your first error happened because the Android 2.3 browser doesn’t support the EMCAScript 5 bind function. Your second error appeared because this.consts is really window.consts, which doesn’t exist. Just wrap the call in an anonymous function that has that in its scope, and call the function from that.