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/

• jbabey: This is called lazy function definition, similar to lazy loading except that the variable happens to hold a function.
• elclanrs: That just gives you a warning that variable is already defined and will just alert "Hello" jsbin.com/ocezar/1/edit
• epascarello: @elclanrs but call foo() a second time and see the alert says. It will not be hello.
• epascarello: 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
• John Dvorak: I'd rather see a one-shot flag (if(!ran){ran=true; ...}) than this self-rewriting code.

Answer 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 ];
}