Why is it like this in javascript?

Question

Why In first Code it says x is not initialized
While in second code logic does not work
Third can be used to avoid such mis-matched logic???
https://sololearn.com/compiler-playground/cOY0E0I6cEY7/?ref=app
https://sololearn.com/compiler-playground/cn4mk4sJ972w/?ref=app
https://sololearn.com/compiler-playground/cq4m2aaEFp8I/?ref=app

Answer 1

I assume it has to do with how Javascript inserts left-out semicolons (Automatic Semicolon Insertion).

The interpreter "guesses" where the semicolon needs to be. Perhaps for your example 1, it expects something like
let x = [1,2][0];
which might be a plausible interpretation attempt but not the one you intend it to actually have.

We would need to dig deeper into the specification of ES6...

Answer 2

Agree with Lisa. I had unexpected bugs that happens when lines of code get unexpectedly concatenated. It's good practice to always end statements with semicolons in js.

the simple version should be

let x = [1, 2];

x = [x[1], x[0]];

console.log(x);