How to perform less than/greater than comparisons on custom objects in javascript

Question

I have a custom class that has several members. I need to compare them to each other. javascript lets me write:

var a = new MyType(1); var b = new MyType(2); if (a < b) { ... 

but I don't understand the behavior of the logical comparison. Can someone explain the semantics of the < comparison in the above code? Is there a way to control what happens so that I can get right behavior? I know I can write a comparison method for the class, but since javascript lets me write it, I wondered what it thought it was doing.

Thanks.

Answer 1

You need to define a .valueOf method that returns a primitive that can be used for comparison:

function MyType( value ){     this.value = value; }  MyType.prototype.valueOf = function() {     return this.value; };  var a = new MyType(3),     b = new MyType(5);  a < b true a > b false a >= b false b < a false b > a true 

If you don't define it, the the string "[object Object]" is used for comparison:

"[object Object]" < "[object Object]" false "[object Object]" > "[object Object]" false "[object Object]" >= "[object Object]" true "[object Object]" <= "[object Object]" true