lecture-9.md

Episode 9 : Block Scope & Shadowing in JS

What is a Block?

• Block aka compound statement is used to group JS statements together into 1 group. We group them within {...}

  {
      var a = 10;
      let b = 20;
      const c = 30;
      // Here let and const are hoisted in Block scope,
      // While, var is hoisted in Global scope.
  }

• Block Scope and its accessibility example

  {
      var a = 10;
      let b = 20;
      const c = 30;
  }
  console.log(a); // 10
  console.log(b); // Uncaught ReferenceError: b is not defined

  • Reason?

    • In the BLOCK SCOPE; we get b and c inside it initialized as undefined as a part of hoisting (in a seperate memory space called block)

    • While, a is stored inside a GLOBAL scope.

    • Thus we say, let and const are BLOCK SCOPED. They are stored in a separate mem space which is reserved for this block. Also, they can't be accessed outside this block. But var a can be accessed anywhere as it is in global scope. Thus, we can't access them outside the Block.

What is Shadowing?

•   var a = 100;
    {
        var a = 10; // same name as global var
        let b = 20;
        const c = 30;
        console.log(a); // 10
        console.log(b); // 20
        console.log(c); // 30 
    }
    console.log(a); // 10, instead of the 100 we were expecting. So block "a" modified val of global "a" as well. In console, only b and c are in block space. a initially is in global space(a = 100), and when a = 10 line is run, a is not created in block space, but replaces 100 with 10 in global space itself. 

• So, If one has same named variable outside the block, the variable inside the block shadows the outside variable. This happens only for var

• Let's observe the behaviour in case of let and const and understand it's reason.

  let b = 100;
  {
      var a = 10;
      let b = 20;
      const c = 30;
      console.log(b); // 20
  }
  console.log(b); // 100, Both b's are in separate spaces (one in Block(20) and one in Script(another arbitrary mem space)(100)). Same is also true for *const* declarations.

  

• Same logic is true even for functions

  const c = 100;
  function x() {
      const c = 10;
      console.log(c); // 10
  }
  x();
  console.log(c); // 100

What is Illegal Shadowing?

•   let a = 20;
    {
        var a = 20;
    }
    // Uncaught SyntaxError: Identifier 'a' has already been declared

  • We cannot shadow let with var. But it is valid to shadow a let using a let. However, we can shadow var with let.
  • All scope rules that work in function are same in arrow functions too.
  • Since var is function scoped, it is not a problem with the code below.

    let a = 20;
    function x() {
        var a = 20;
    }

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