Rewriting functions in java scripts

Question

Just came across a funky function rewriting concept in Javascript.

var foo = function () {
    alert("Hello");
    foo = function () {alert("World !");};
};
foo();
foo();

In what situations are these helpfull and is there any other scripting language which support this kind of code ?

Fiddler link : http://jsfiddle.net/4t2Bh/

This is called lazy function definition, similar to lazy loading except that the variable happens to hold a function. — jbabey
– jbabey, Commented Jun 6, 2013 at 18:53
That just gives you a warning that variable is already defined and will just alert "Hello" jsbin.com/ocezar/1/edit — elclanrs
– elclanrs, Commented Jun 6, 2013 at 18:55
@elclanrs but call foo() a second time and see the alert says. It will not be hello. — epascarello
– epascarello, Commented Jun 6, 2013 at 18:58
It is helpful if you need to run a expensive lookup/check that will not change and you can make it so the following calls use the previously found data. Sort of like a getter type of pattern. jsfiddle.net/NHHFH — epascarello
– epascarello, Commented Jun 6, 2013 at 19:00
I'd rather see a one-shot flag (if(!ran){ran=true; ...}) than this self-rewriting code. — John Dvorak
– John Dvorak, Commented Jun 6, 2013 at 19:17

aggsol · Accepted Answer · 2013-06-07 09:31:15Z

1

You can use this idiom to initialize a LUT on the first call like this

var getBase32Value = function (dummy) {

    var base32Lut = {};
    var alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";

    for(var i=0; i<alphabet.length; i+=1) {
        base32Lut[ alphabet[i] ] = i;
    }

    getBase32Value = function (v) {
        return base32Lut[ v ];
    }
    return base32Lut[ dummy ];
}

answered Jun 7, 2013 at 9:31

aggsol

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