javascript中的变量声明（全局与本地）

Question

I'm not able to understand the following code

function test() {
  let a = b = 0;
  a++;
  return a;
}

test();
typeof a;
typeof b;

getting type of a as undefined whereas typeof b as "number"

Answer 1

The main difference is scoping rules. variables that are declared by the keyword var are scoped to the immediate function body while let variables are scoped to the immediate enclosing block denoted by { }, meaning blocker scoped. This is the reason why let keyword was introduced to the language. the function scope can be very confusing and can cause many bugs in JavaScript.

So, what you're seeing in the code is a decleration of let a that makes it scoped to the function. hence, the typeof a outside of the function is indeed undefined (since that area of code is not aware of a). b, on the other hand, is not declared with let. it's being assigned to the number 0, meaning it's an implicit global variable, hence existing out of the function.

Answer 2

Here when the foo() function executions starts java script interpreter

1. creates variable a in the scope of f00() function
2. for variable b the interpreter thinks it is accidentally created so the interpreter creates a global variable b and assigns value 0 eg: (window.b = 0; or b = 0;) this is because scoping rules here var keyword will be scoped to the immediate variable store (eg: a) so
3. type of variable a is undefined as it is in the scope of function foo()
4. type of variable b is number as it is global variable and is available anywhere in the code