Learn ECMAScript - Second Edition

Declaring block-scoped variables

Variables that are declared using the let keyword are called block-scoped variables. Block-scoped variables behave the same way as function-scoped variables when declared outside a function, that is, they are accessible globally. But when block-scoped variables are declared inside a block, they are accessible inside the block that they are defined in (and also any sub-blocks) but not outside the block:

 let a = 12; // accessible everywhere function myFunction() {   console.log(a); // alerts 12   let b = 13;   if(true) {     let c = 14; // this is NOT accessible throughout the function!     alert(b); // alerts 13   }   alert(c); // alerts undefined } myFunction(); alert(b); // alerts undefined

Study the code carefully. This ...