781422 - parameters should get defaults whenever they are undefined

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Dave Herman [:dherman]]

Currently the parameter default values implementation uses the arguments length to determine whether a parameter needs a default value. It should instead be based on whether the argument provided was undefined.

Dave

Comment 1

[Jason Orendorff [:jorendorff]]

In short:

    function f(x=3) { return x; }
    assertEq(f(undefined), 3);  // currently FAILS, f(undefined) === undefined.

In detail:
    function f(x=EXPR) { BODY }
will expand to:
    function f(x) {
        if (x === undefined)
            x = EXPR;
        BODY
    }

This will invalidate a bunch of our existing tests for default parameters. The tests are wrong (i.e. they are testing for what we *thought* the semantics were going to be).

Comment 2

[Allen Wirfs-Brock]

Attachment: interim ES6 draft, with desstructuring updates (zipped doc)

I haven't yet released a spec. draft with the parameter and destructuring default changes. However, the attached interim draft does have the changes.  Use this as the spec. until a release the next official draft sometime in Sept. 2012

Comment 3

[:Benjamin Peterson]

Attachment: use defaults when formals are undefined

This certainly simplifies the implementation.

I didn't update the decompiler except to delete some lines to make it compile. I'm not sure what you want me to do with it otherwise.

Comment 4

[Jason Orendorff [:jorendorff]]

Comment on attachment 659443 [details] [diff] [review]
use defaults when formals are undefined

Review of attachment 659443 [details] [diff] [review]:
-----------------------------------------------------------------

Great.

Comment 5

[Jason Orendorff [:jorendorff]]

That Word document caused Preview to peg the CPU, and Google Docs couldn't render it either, so basically I don't have a good way to view it right now. I'll wait for the September draft.

Comment 6

[:Benjamin Peterson]

https://hg.mozilla.org/integration/mozilla-inbound/rev/e9bd85f336b9

Comment 7

[Ed Morley [:emorley]]

https://hg.mozilla.org/mozilla-central/rev/e9bd85f336b9

Comment 8

[David Bruant]

I updated https://developer.mozilla.org/en-US/docs/JavaScript/Reference/default_parameters
Tell me if you expect other changes to be documented.

Comment 9

[:Benjamin Peterson]

I suppose bug 757676 can be dev-doc-complete, too then.,