10 minutes a day, learn JavaScript – (13) variable improvement and function improvement

Variable declaration promotion

• Variables defined (declared) by var are accessible before statements are defined
• Value: undefined

      console.log(a) // undefined
      var a = 4
Copy the code

Function declaration promotion

• Functions declared by function can be called directly before
• Value: Function definition (object)

    fn()
    function fn() {
      console.log('fn()') // fn()
    }
Copy the code

Questions and Reflections

• What does it output?

    var a = 3
    function fn() {
      console.log(a) // undefined
      var a = 4
    }
    fn()
Copy the code

• What does it output?

    fn() // The function declared by function can be called directly before
    var fn = function () {
      console.log('fn')}Copy the code