JS Iterators, generators, and destructuring #103

Implement the iterator protocol, generator functions, and destructuring syntax.

Scope#

Iterator Protocol#

• Symbol.iterator method on iterables (Array, String, Map, Set, etc.)
• Iterator objects: { next() → { value, done } }
• for...of loop consuming iterators
• Spread operator (...) in array literals and function calls
• Array.from() consuming iterables

Generator Functions#

• function* declaration and expression syntax
• yield expression — pause execution and produce value
• Generator objects conform to both iterator and iterable protocols
• generator.next(value) — resume with value
• generator.return(value) — force completion
• generator.throw(error) — throw into generator
• yield* delegation to sub-iterators

Destructuring#

• Array destructuring: let [a, b, ...rest] = arr
• Object destructuring: let { x, y: alias, ...rest } = obj
• Default values: let [a = 1] = [], let { x = 2 } = {}
• Nested destructuring: let { a: { b } } = obj
• Destructuring in function parameters
• Destructuring in for...of: for (let [key, val] of map)
• Computed property names in destructuring: let { [expr]: val } = obj

Acceptance Criteria#

• for...of works with arrays, strings, Maps, Sets
• Custom iterables work (objects with Symbol.iterator)
• Spread operator works in arrays and function calls
• Generator functions pause and resume correctly
• yield* delegates to sub-iterators
• generator.next(value) passes value into generator
• generator.return and generator.throw work
• Array destructuring with rest and defaults
• Object destructuring with aliases, rest, and defaults
• Nested destructuring works
• Destructuring in function parameters
• Unit tests for iterators, generators, and destructuring

Phase 10 — JavaScript Engine (issue 14 of 15). Depends on: JS Functions/closures, Symbol built-in.