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So because we're going to be writing an interpreter for JavaScript in HTML,
we're only really going to be able to interpret good programs successfully.
That means we want to recognize and rule out bad programs.
So here you can conceptually imagine some sort of border patrol or
gateway crossing where our diligent customs agent is going to keep out bad programs,
like this "1"+2, while allowing in super-happy-fun programs like 1+2.
The process of looking at a program's source code, looking at its text,
looking at the letters that make it up, and trying to see if it's going to be well-behaved or not
is known as type checking or semantic analysis.
And as you may already guess, it's not going to be perfect
but it's going to be pretty good.
Type checking and semantic analysis.
Semantic means "meaning."
We're going to break a program down into its meaning.
And I'm going to explain types in just a minute.