Are function arguments with the value `undefined` "present"?

Allen Wirfs-Brock allen at
Fri May 31 09:27:04 PDT 2013

If a term is not explicitly defined by the spec. then the English dictionary definition applies: "4) being with one or others or in the specified or understood place"

This is further clarified  by the introductory paragraphs of chapter 15 that say: "Unless otherwise specified...if a function or constructor described in this clause is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient additional arguments, each such argument being the undefined value."

Testing is an argument is present is an "otherwise specified" situation. In other words, of an argument is not present then it is treated as if the value is undefined.  In general, the specification test for undefined as an argument position if it doesn't care whether or not an explicit undefined was passed.

Any time the spec. says "if argument foo is present..." it could be restate by "if the number of explicitly passed arguments is >= foo's argument position..."

On May 31, 2013, at 4:33 AM, Till Schneidereit wrote:

> I couldn't find any precise definition for the spec term "present", so this is not entirely clear to me.
> Example:
> In step 7 of Array.prototype.reduce, a TypeError must be thrown if the optional argument `initialValue` isn't present. If a value of `undefined` does *not* cause the argument to be present, that function can be implemented with the following signature:
> `function ArrayReduce(callbackfn, initialValue = undefined)`
> If, however, the value `undefined` means that the argument is present, the signature should probably be:
> `function ArrayReduce(callbackfn,`

or you could define it as:

const missing = {};
function ArrayReduce(callbackfn, initValue=missing) {
   //step 7
   if (len === 0 && initialValue === missing) throw new TypeError("initial value required");

> The question isn't entirely academic, as I'm trying to get the self-hosted implementations of builtins in SpiderMonkey to be as close to the spec as possible.

that better be semantically identical to the spec...


> thanks,
> till

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