Why does adding a string to Date call toString () and not valueOf ()?
Just stumbled upon this today and I couldn't find where this behavior is defined in the spec.
Adding a string to an object in JavaScript (i.e. new Object() + "whatever"
) usually causes an object call valueOf()
for the object, but Date objects seem to be the exception to this rule: new Date() + "..."
calls toString()
on instead.
function test(constructor)
{
var name = constructor.name || /^\s*function ([\w\$]+)/.exec(constructor.toString())[1];
var obj = new constructor();
obj.toString = function () { return name + ' : toString()'; }
obj.valueOf = function () { return name + ' : valueOf()'; }
document.body.appendChild(document.createElement('DIV')).textContent = obj + '';
}
test(Array);
test(Boolean);
test(Date);
test(Function);
test(Number);
test(Object);
test(RegExp);
test(String);
The question is: what is happening here with Date objects?
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When you call the add statement of an object and the object is converted to a primitive , [[DefaultValue]]
the object's internal method is called . The method takes prompt type which determines whether the estimate must first toString
or valueOf
.
If the hint is "String", then toString
look up, up valueOf
. If "Number" is transmitted, it is the other way around.
However (my attention):
When the inner method
[[DefaultValue]]
O is called without a hint, it behaves as if the hint was a Number, unless O is a Date object (see 15.9.6 ), in which case it behaves as if the hint was a string .
This is also noted in the statement section:
NOTE 1 In steps 5 and 6.
ToPrimitive
not specified in calls . All original ECMAScript objects other than Date objects handle hint absence as if a hint number had been specified; Date objects handle the absence of a hint as if a hint string were supplied. Host objects can handle missing hints in some other way.
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