LET and CONST

    Definições
    • const for values that we will not change change.
    • let like the old var, for values that we wanna change or mutate later on.

    Diferenças
    • Function-scoped and Block-scoped

      • Variables declared with var in the ES5 are function-scoped but variables
      declared with let on ES6 are block-scoped.


      // Everything inside the driversLicence6 function, is it's scope.
      // With that, in ES5 we have access to everything inside the scope.
      // However, in ES6, we only have access to what is in the blocks.
      // With that, you can not access firstName and yearOfBirth from outside the if statment,
      // being that you would be trying to access something from outside it's block.

      function driversLicence6(passedTest) {

      if (passedTest) {
        // this is a block - the if statement
        let firstName = 'John';
        const yearOfBirth = 1990; // impossible to change the birth year.
       }
       // this is a block - this will not work because it doesn't have access to firstName and yearOfBirth, being that they are in another block.
      console.log(firstName + ', born in ' + yearOfBirth + ', is now officially allowed to drive a car.');
      }

      How to make it work:
      Declare the variables outside the function.

      function driversLicence6(passedTest) {
        let firstName; let doesn't need to have it's value defined here.
        const yearOfBirth = 1990; const must have it's value defined here.

      if (passedTest) {
        let firstName = 'John'; defined value for let
      }
      console.log(firstName + ', born in ' + yearOfBirth + ', is now officially allowed to drive a car.');
      }
      driversLicence6(true); // the person passed the test.

    • Undefined

      • // ES 5
      function driversLicence5(passedTest) {

      console.log(firstName);
      if (passedTest) {
        var firstName = 'John';
        var yearOfBirth = 1990;
      }
      }
      Ocorre um erro: firstName is undefined, visto que o JS tenta executar a variable antes dela receber um valor, no momento ela só foi lida para o JS saber que ela existe.

    • Blocks


      • // ES 5

      var i = 23;

      for (var i = 0; i < 5; i++) {
      console.log(i); // exibe 0, 1, 2, 3, 4
      }

      console.log(i); //exibe 5, pois vai ser 4++, quando chegar no if vai ser false, não executando-o, porém será exibido nesse console.log.

      // ES 6

      let i = 23;

      for (let i = 0; i < 5; i++) {
      console.log(i); // exibe 0, 1, 2, 3, 4
      }

      console.log(i); // exibe 23, visto que as duas variáveis estão em diferentes blocks.
      Nesse caso, seria como possuir uma var dentro do for e uma var fora.