gistfile1.txt

Legacy Generators
=================

Allowed only in versioned JS >= 1.7

The syntax and semantics as currently supported by SpiderMonkey are as follows.

Syntax and static semantics
---------------------------

A plain function that sees `yield` becomes a legacy generator.

It is an early error if a `yield` is seen in the following kinds of function bodies:
  - Arrows
  - Methods
  - Getters
  - Setters
  - Constructors
  - Expression bodies

- It is an early error to use `arguments` in a legacy generator expression.

Runtime semantics
-----------------

4 methods, as below. Does not follow the ES6 iterable protocol. E.g., yielded values are bare and not `{value, done}`.

  - `next`
    - Like ES6 generator `next`, but throws a `StopIteration` object if generator is already closed. Also throws a `StopIteration` object if generator is already done.

  - `send` (alias for `next`)

  - `throw`
    - Like ES6 generator `throw`

  - `close`
    - Unlike ES6 generator.close, always returns `undefined`.

Can be used in for-in loops or for-of loops. When used in for-of loops, an automatic shim is applied.

Different closing semantics when used as a legacy iterator in for-in:

  - Any exceptions thrown with the legacy generator used as an iterator on the stack closes it.

  - Any non-local control flow (break, return) with the legacy generator used as an iterator on the stack closes it.