JavaScript prototype problem

Question

If I call myRobot.Speak.sayHi() it always returns undefined. Please, what am I doing wrong? Thanks for reply!

var Factory = (function() {

    // Constructor  
    var Robot = function() {

    };

    // Public
    return {
        extendRobot: function(power, methods) {
            Robot.prototype[power] = methods;
        },
        createRobot: function() {
            return new Robot();
        }
    };

}());

Factory.extendRobot('Speak', {
    sayHi: function() {
        return 'Hi, ' + this.name;
    }
});

var myRobot = Factory.createRobot();
myRobot.name = 'Robin';
myRobot.Speak.sayHi() // => ‘Hi, Robin’

You don't return anything so the method returns undefined. That's not a problem at all in this case. — ThiefMaster
– ThiefMaster, Commented Jun 4, 2011 at 0:17

Raynos · Accepted Answer · 2011-06-04 00:47:10Z

createRobot: function() {
    var r = new Robot();
    for (var k in r.Speak) {
        if (typeof r.Speak[k] === "function") {
            r.Speak[k] = r.speak[k].bind(r);
        }
    }
    return r;
}

Rather then returning a new robot, make sure to bind all the methods in your powers to the robot.

To avoid hard coding in the loops try this:

Robot.powers = [];
...
extendRobot: function(power, methods) {
    Robot.powers.push(power);
    Robot.prototype[power] = methods;
},
createRobot: function() {
    var robot = new Robot();
    Robot.powers.forEach(function(power) {
        for (var method in robot[power]) {
            if (typeof robot[power][method] === "function") {
                robot[power][method] = robot[power][method].bind(robot);
            }
        }
    });
    return robot;
}

This relies on Function.prototype.bind so use the ES5 shim or use underscore for supporting old browsers (IE<9)

brymck · Accepted Answer · 2011-06-04 00:20:00Z

2

myRobot.Speak.name = 'Robin';

In your case, this refers to Robot.Speak, not the parent object Robot.

answered Jun 4, 2011 at 0:20

brymck

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3 Comments

Emmitt Gregory Over a year ago

Thanks for reply! Should I use apply or call?

Emmitt Gregory Over a year ago

But... I would like to access variable myRobot.name from myRobot.Speak.sayHi. :-| Is it even possible?

icktoofay Over a year ago

@Emmitt: Yes, it's possible, but it may make your code a little more complex.

AsherMaximum · Accepted Answer · 2011-06-04 00:33:04Z

0

insert name : this.name before the line sayHi: function() { and then it should work

answered Jun 4, 2011 at 0:33

AsherMaximum

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2 Comments

kqnr Over a year ago

No it won't, because this.name will refer to Factory.name when calling Factory.extendRobot

icktoofay Over a year ago

@chomp: No, it won't; it will refer to window.name.

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