JavaScript Symbol type: (non-string object keys)

问题:

What is the "Symbol" javascript type as mentioned in this ECMAScript 6 draft specification?

To quote the spec:

The Symbol type is the set of all non-String values that may be used as the key of an Object property.

Each possible Symbol values is unique and immutable.

Symbol values have a single observable attribute called [[Private]] whose immutable value is either true or false. A private symbol is a Symbol value whose [[Private]] attribute has the value true.

I thought object keys were strings only, and I'm not alone. To quote this accepted SO answer:

…object keys are always strings…

Could you explain what the symbol type is, and demonstrate its usage. I'm trying to make sense of the spec.

Thanks!

回答1:

I thought object keys were strings only

You're right, but that was true for EcmaScript 5 only. ES 6 / harmony is a draft for something new!

I'm trying to make sense of the spec

It's a draft only, rapidly changing. How symbols are used and whether or how they can be created by arbitrary scripts does not seem to have settled yet (scan through the versions for changes).

If you scroll down to the very end of that document (even below Annex F), you for example will see a Section 8.4.4: Symbol Exotic Objects that has been moved out there. It states

Exotic Symbol objects provide alternative definitions for all of the essential internal methods.

You can see them used at section 8.1.7.4 Well-Known Symbols and Intrinsics for example. For proposed uses (and still existing problems / open questions) of Symbol constructors have a look at these strawman pages or this wiki site.

回答2:

Symbol is a new addition to the language proposed as part of ECMAScript 6:

Current work on ECMAScript™

Work on future ECMAScript™ editions continues as part of the previously announced ECMAScript™ "Harmony" project. More details of the current work on ECMAScript "Harmony" are described on this Wiki. A sixth edition of the standard is currently under development with a target date of December 2013 for completion.