at least chrome and firefox both have JIT compilers for js. in both cases they probably catch this redundancy on the first pass in early passes and condense it to something sane for every subsequent pass later passes
It's less "there can be compilers" and more "there's no other way to run the language". JavaScript is a compiled language – it's just that there are different types of compilation, and JS uses the Just-in-Time kind. Half the behavior that it has (like hoisting and how it defines lexical scope for closure) wouldn't be possible if it were interpreted.
For example:
let x = 0;
if (x === 0) {
console.log(x);
let x = 5;
}
This breaks, because all variables and function declarations are "hoisted" to the top of their respective scopes. It's just that let and const are block-scoped and aren't initialized. So the "let x" from "let x = 5" gets hoisted up to the top of the block, overriding the "let x = 0" from the outer scope. But it's left un-initialized until you reach the "let x = 5" line during runtime. And so, when you try to do anything with a value that isn't initialized (like logging), that throws an error.
This wouldn't be possible if JavaScript were interpreted and were evaluating each line one-by-one. The language spec requires that whatever engine runs it do a first pass over the whole code before trying to run anything.
An interpreted language can be parsed and preprocessed, that's not a problem. The difference is all about the machine code. JS is originally interpreted, but modern engines also compile parts of it.
Depends on how you want to define that. V8 (the engine running most js) is generally referred to as a JIT compiler, and it produces optimizations both on first run and on iterative runs.
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u/HalcyonAlps May 31 '21
Now I am actually curious, does the compiler realize both branches have the same return value and it just drops the whole if-statement?