Optimize CoffeeScript-generated JS for V8

Question

coffee-generated JS of the following simple code snippet:

console.log 'b' if 'b' in arr

is

var __indexOf = [].indexOf || function(item) {
    for (var i = 0, l = this.length; i < l; i++) {
        if (i in this && this[i] === item) return i;
    } return -1;
};

if (__indexOf.call(arr, 'b') >= 0) {
  console.log('b');
}

I can understand why it is so. IE doesn't support indexOf, and we want to make sure our CS code runs smoothly on all browsers. But, when writing the code for a Node.js server, we know exactly what the JS engine supports (ECMA-262, 5th edition), so we wouldn't need the above trick.

I'm not terribly familiar with different JavaScript implementations, but I'm sure it's not the only non-optimal code coffee -c produces because of browser incompatibilities, and if we consider all of them in a production server with thousands of concurrent connections, they add a considerable unnecessary overhead to the code.

Is there a way to remedy this? More and more Node.js code is written in CS these days, and with SourceMap on the horizon, the number would be even greater...

Asherah · Accepted Answer · 2012-05-27 12:12:33Z

3

This is barely non-optimal; the __indexOf declaration is evaluated once, at the beginning, and it's immediately resolved to [].indexOf, i.e. using the underlying implementation's Array.prototype.indexOf. That's not exactly a huge expense, surely.

I'd need to see some other examples of "non-optimal" code, but I'm sure most of them fall into the same basket. Number of concurrent connections doesn't scale the effect of this at all.

answered May 27, 2012 at 12:12

Asherah

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2 Comments

Pooria Azimi Over a year ago

Thanks. I'm not really familiar with how JS interpreters work under the hood, and didn't know that the above resolution needs to be done only once. I guess it has something to do with JS being prorotype-based... In that case, you're right that it's really a non-isuue and doesn't scale.

Asherah Over a year ago

It's not that it's prototype-based, but rather the assignment __indexOf = [].indexOf || … will only happen once—when the assignment is executed. At any rate, there's unlikely to be any perf difference between __indexOf.call(x, y) and x.indexOf(y)—in fact, I'd imagine the former might be faster, because it doesn't require looking up in x's prototype chain (!). So, if anything, CoffeeScript is giving you an optimisation. :)

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