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What are generators and how do they differ from regular functions?

function* can pause with yield and resume with .next().

Ideal Answer

Generators are functions that can pause and resume execution, returning multiple values over time. Declared with function* and using yield.

JavaScript 
function* idGenerator() {
  let id = 0
  while (true) {
    yield id++
  }
}

const gen = idGenerator()
gen.next() // { value: 0, done: false }
gen.next() // { value: 1, done: false }

vs regular functions

Regular function Generator
Runs to completion Always Can pause at each yield
Return Single return value Iterator of { value, done } objects
yield N/A Pauses and produces a value

Use cases

  • Lazy sequences (infinite streams, paginated data)
  • Custom iterables implementing Symbol.iterator
  • Coroutine-style control flow (largely replaced by async/await for I/O)
  • Delegating iteration with yield*
JavaScript 
function* range(start, end) {
  for (let i = start; i <= end; i++) yield i
}

for (const n of range(1, 3)) console.log(n) // 1, 2, 3

Generators are synchronous by default. For async iteration, use async generators (async function*) with for await...of.