# Iterators and generators — Advanced generators

> Generators compute their yielded values _on demand_, which allows them to efficiently represent sequences that are expensive to compute (or even infinite sequences, as demonstrated above).

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## Knowledge

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Generators compute their yielded values _on demand_, which allows them to efficiently represent sequences that are expensive to compute (or even infinite sequences, as demonstrated above).

The {{jsxref("Generator/next", "next()")}} method also accepts a value, which can be used to modify the internal state of the generator. A value passed to next() will be received by yield.

&gt; [!NOTE] &gt; A value passed to the _first_ invocation of next() is always ignored.

Here is the fibonacci generator using next(x) to restart the sequence

You can force a generator to throw an exception by calling its {{jsxref("Generator/throw", "throw()")}} method and passing the exception value it should throw. This exception will be thrown from the current suspended context of the generator, as if the yield that is currently suspended were instead a throw value statement.

If the exception is not caught from within the generator, it will propagate up through the call to throw(), and subsequent calls to next() will result in the done property being true.

Generators have a {{jsxref("Generator/return", "return()")}} method that returns the given value and finishes the generator itself.

{{PreviousNext("Web/JavaScript/Guide/Typed_arrays", "Web/JavaScript/Guide/Resource_management")}}

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