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