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This gives “[object Object]”:
<!doctype html>
<script>
var a={};
a.valueOf=function(){
return "asd";
};
alert(a);
</script>
but this gives “asd”:
<!doctype html>
<script>
var a={};
a.valueOf=function(){
return "asd";
};
alert(""+a);
</script>
i can understand why i get “asd” in the second example. But in the first example, shoudn’t i also get “asd” since i’m putting the object through the alert function (which basically ends up calling the value-of anyway) ?
When you use the + operator, JavaScript uses valueOf, and otherwise when a String is expected from an object, JavaScript uses toString(). Internally, perhaps in the Object.prototype.valueOf, it is designed to be the same as toString(). So, if you define only toString(), but not valueOf(), JavaScript will use toString() for both ‘+’ (because internally valueOf calls toString) and in contexts where a String is expected. However, if you define valueOf, then that will be used where there is a ‘+’ (or a – or / or any other arithmatic operator). But it won’t be used when it expects a String (as in case of alert). It will call toString().