Node.js prototypal inheritance

Question

Trying to wrap my mind around prototypes in Javascript, specifically Node.js, with a simple test.

function Lint() {
    this.input = 'foo';
    events.EventEmitter.call(this);
}

Lint.prototype.dirs = function (dirs) {
    _.each(dirs, this.files);
}

Lint.prototype.files = function (dir) {
    console.log(this.input); // trying to get 'foo', returns undefined
}

var lint = new Lint();

lint.dirs(['js', 'js/views']);

Lint.prototype.files logs undefined because this isn't referring to the instance of Lint. What am I missing here?

The only solution I can think of, that works, is passing around the initial this from Lint.prototype.dirs to each other function. I'm pretty sure there's a better way.

user1106925 · Accepted Answer · 2012-04-14 13:58:13Z

4

this refers to the lint object, but you're not passing the Lint object to _.each. You're detaching the function from the object and passing that.

You can bind the context of a function to the desired value using Function.prototype.bind...

_.each(dirs, this.files.bind(this));

Or, you could keep a reference to the this value, and pass an anonymous function to _.each...

var this_lint = this;

_.each(dirs, function(v) {
    this_lint.files(v);
}

answered Apr 14, 2012 at 13:58

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user1106925

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3 Comments

user1106925 Over a year ago

@alex: That's my thing now. All my answers are CW, so feel free to pitch in if you'd like. ;-)

Wayne Ashley Berry Over a year ago

Thanks so much! Interestingly enough, why does this variation return false? var this_lint = this; _.each(dirs, this_lint.files);

user1106925 Over a year ago

@wayne: In that case, you're really doing the same thing as you were in your original code. You're just passing the .files function to the _.each. The this_lint does not stay with the function when you pass it. That's where .bind is handy. It creates a new function with the first argument given to .bind set as the this value of the new function.

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