Generators in ES6

Question

I have created a function to generate fibonacci series using es6 generators:

//WARNING CAUSES INFINITE LOOP
function* fibonacci(limit = Infinity) {
  let current = 0
  let next = 1

  while (current < limit) {
    yield current
      [current, next] = [next, current + next]
  }
}

for (let n of fibonacci(200)) {
  console.log(n)
}

The above function doesn't swap the two numbers while if done normally in any other function swaps the two. On running this function I get an infinite loop. Why doesn't the variable swap work?

score 2 · Accepted Answer · answered Nov 02 '17 at 12:01

You've got a syntax mistake: a missing semicolon makes the engine parse your statement as

yield (current [ current, next ] = [ next, current + next ])
//             ^        ^
// property access    comma operator

If you want to omit semicolons and let them be automatically inserted where ever ~~possible~~ needed, you will need to put one at the begin of every line that starts with (, [, /, +, - or `:

function* fibonacci(limit = Infinity) {
  let current = 0
  let next = 1

  while (current < limit) {
    yield current
    ;[current, next] = [next, current + next]
  }
}

for (let n of fibonacci(200)) {
  console.log(n)
}

We should only allow people who understand ASI to omit semicolons :D — Felix Kling, Nov 02 '17 at 17:36

score -1 · Answer 2 · answered Nov 02 '17 at 11:58

You have to first swap and then yield. Yield will give control back to the caller, so the method kindad stops executing there..

This will work (tested in Firefox

function* fibonacci(limit = Infinity) {
  let current = 0
  let next = 1

  while (current < limit) {
    [current, next] = [next, current + next];
    yield current;

  }
}

for (let n of fibonacci(200)) {
  console.log(n)
}

Generators in ES6

2 Answers2