A generator is a special type of function that works as a factory for iterators and it allows you to define an iterative algorithm by writing a single function which can maintain its own state. A function becomes a generator if it contains one or more yield statements. When a generator function is called, the body of the function does not execute straight away; instead, it returns a generator-iterator object. Each call to the generator-iterator's next() method will execute the body of the function up to the next yield statement and return its result. When either the end of the function or a return statement is reached, a StopIteration exception is thrown. For example, the following fib() function is a Fibonacci number generator, that returns the generator when it encounters the yield statement:

 
 function fib() {
    var fibNum = 0, j = 1;
    while (true) {
       yield fibNum;
       var t = fibNum;
       fibNum = j;
       j += t;
    }
 }
 
 

To use the generator, simply call the next() method to access the values returned by the function:

 
  var gen = fib();
  for (var i = 0; i < 10; i++) {
    document.write(gen.next() " ");
  }