Weird Javascript expression

Question

I try to understand how Protovis works, and I stumbled upon code like this:

force.node.add(pv.Dot)
    .size(function(d) (d.linkDegree + 4) * Math.pow(this.scale, -1.5)) // notice this
    .fillStyle(function(d) d.fix ? "brown" : colors(d.group)) // and this
    .strokeStyle(function() this.fillStyle().darker()) // and even this
    .lineWidth(1)
    .title(function(d) d.nodeName)
    .event("mousedown", pv.Behavior.drag())
    .event("drag", force);

I tried rolling my own short functions, like this:

(function(a) a+2)

I am NOT asking about anonymous functions declared like function(){stuff();}. The code in question looks like function() stuff; and it works. I want to know why. I don't want to learn about constructs like myvar = function(a){return a+1;}, but about constructs like myvar = (function(a) a+1). Please look at the above code more carefully.

But, as I suspected, it threw a syntax error.

How can such code work?

(Note: the protovis code does work as intended.)

I don't get it. I even checked the grammar specified in the spec - the braces *are* required for anonymous functions: http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf "A.5 Functions and Programs". — , Nov 17 '10 at 16:39

score 10 · Accepted Answer · answered Nov 17 '10 at 16:41

10

This is a Expression Closure that was introduced in JavaScript 1.8. It is an extension to ECMAScript.

https://developer.mozilla.org/en/JavaScript/New_in_JavaScript/1.8

answered Nov 17 '10 at 16:41

ChaosPandion

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Finally. an answer that understood the question! – SLaks Nov 17 '10 at 16:42
@lincolnk - For compatibilities sake that is true but something like this should really be built into ECMAScript. – ChaosPandion Nov 17 '10 at 16:51
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why? trading explicitness for shorter code seems like a bad trade. – lincolnk Nov 17 '10 at 16:52
@lincolnk - I am of the camp that feels that core language features should be as short as possible but no shorter. Have you ever seen a considerably sized project that didn't use lambdas all over the place? The expression closure syntax is still a bit heavy because of the `function` keyword but the majority of the noise is removed. The same idea is used in C# with lambda expressions. – ChaosPandion Nov 17 '10 at 16:55
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I'm all for short lambdas. But this still sounds like a bad idea, especially since they didn't shorten the `function`, which is about as long the braces and the `return`. Real solution: use Coffeescript :) – Nov 17 '10 at 17:10

score 2 · Answer 2 · answered Nov 17 '10 at 16:49

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Protovis also has its own code to handle the case where the browser you're running does not yet support the Expression Closure format, here: http://vis.stanford.edu/protovis/jsdoc/symbols/src/src_pv-internals.js.html

answered Nov 17 '10 at 16:49

Hellion

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Thanks! I was stumped trying to determine what ` – RobM Jan 06 '11 at 14:52

score 1 · Answer 3 · answered Nov 17 '10 at 16:43

1

This is an Expression Closure, see this:

http://asserttrue.blogspot.com/2009/07/javascript-expression-closures-and-why.html

Here's the jsfiddle that works:

http://jsfiddle.net/fyB9h/

answered Nov 17 '10 at 16:43

icyrock.com

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Weird Javascript expression

3 Answers3