Babel-plugin-tailcall-optimization NPM

babel-plugin-tailcall-optimization

Tail call optimization for JavaScript!

Installation

*if you use babel@6 use babel-plugin-tailcall-optimization@1 package.

npm install babel-plugin-tailcall-optimization --save-dev

and add to your .babelrc:

"plugins": ["tailcall-optimization"]

How does it work?

We rewrite functions with tail calls to ones using while loops. Original function with tail call:

function counter (n, acc = 0) {
  if (n === 0) {
    return acc
  } else {
    return counter(n - 1, acc + 1)
  }
}

gets rewritten to this:

function counter(n, acc = 0) {
  var _repeat = true;

  var _n, _acc;

  while (_repeat) {
    _repeat = false;

    if (n === 0) {
      return acc;
    } else {
      _n = n - 1
      _acc = acc + 1
      n = _n
      acc = _acc
      _repeat = true;
      continue;
    }
  }
}

Plugin does not affect functions without TCOs so it's safe to use.

Benchmarks

For Fibonacci Sequence example benchmark.js results are:

Fibonacci Sequence without TCO x 270,170 ops/sec ±1.14% (85 runs sampled)
Fibonacci Sequence with TCO x 1,298,276 ops/sec ±1.24% (83 runs sampled)

So function after TCO optimization is almost 5 times faster.

Benchmark code

Known issues

Currently when plugin detects function creation within tailcalled function it does not optimize it. It's related to difficulties in implementation (function scoping rules). Read more: https://phabricator.babeljs.io/T6869
It does not work for mutual recursive functions. I guess it's not super big problem - even JVM does not do this.