What's the value of x?

Placed before a variable, "++" adds one to the variable and the expression with "++" equals the new value of the variable + 1. This can be seen in line 4, where "x = ++x;" really means "x = (x + 1)".

Placed after a variable, however, "++" adds one to the variable and the expression with "++" equals the first value of the variable, without 1 added. This is why line 3, "x = x++;" has no effect — because the "x = x + 1" execution that the "++" gives is executed but then replaced by the the previous value of x.

In the end, you have:

let x = 0; // x = 0

x++; // x = x + 1 --> x = 1
x = x++; // x = x --> x = 1
x = ++x; // x = x + 1 --> x = 2

console.log(x); // 2

I hope my explanation made sense to you, was a bit hard to explain.

— Gabriel

3?
Dangit. I believe it would be 3 in dart