You assume it is valid, until it isn't and you can have different strategies to handle that, like just skipping the broken part and carrying on.

Anyway, you write a state machine that processes the string in chunks – as you would do with a regular parser – but the difference is that the parser is eager to spit out a stream of data that matches the query as soon as you find it.

The objective is to reduce the memory consumption as much as possible, so that your program can handle an unbounded JSON string and only keep track of where in the structure it currently is – like a jQuery selector.