The google closure compiler does this. You can optionally annotate your javascript functions with type information (JavaDoc style) and the compiler will enforce

The google closure compiler does this. You can optionally annotate your javascript functions with type information (JavaDoc style) and the compiler will enforce it. For example: /** * @param {Function} fn * @param {string|null|undefined} str */ foo.bar = function(fn, str) { // do something } It is pretty powerful, you can also do some generics-like stuff like "@param {Array.<string>}" to enforce an array with only strings as elements.