Hoisting

Hoisting

  • Variable declarations get hoisted to the top of their scope, but their assignment does not.

    // we know this wouldn't work (assuming there
    // is no notDefined global variable)
    function example() {
      console.log(notDefined); // => throws a ReferenceError
    }
    
    // creating a variable declaration after you
    // reference the variable will work due to
    // variable hoisting. Note: the assignment
    // value of `true` is not hoisted.
    function example() {
      console.log(declaredButNotAssigned); // => undefined
      var declaredButNotAssigned = true;
    }
    
    // The interpreter is hoisting the variable
    // declaration to the top of the scope,
    // which means our example could be rewritten as:
    function example() {
      var declaredButNotAssigned;
      console.log(declaredButNotAssigned); // => undefined
      declaredButNotAssigned = true;
    }
    
  • Anonymous function expressions hoist their variable name, but not the function assignment.

    function example() {
      console.log(anonymous); // => undefined
    
      anonymous(); // => TypeError anonymous is not a function
    
      var anonymous = function () {
        console.log('anonymous function expression');
      };
    }
    
  • Named function expressions hoist the variable name, not the function name or the function body.

    function example() {
      console.log(named); // => undefined
    
      named(); // => TypeError named is not a function
    
      superPower(); // => ReferenceError superPower is not defined
    
      var named = function superPower() {
        console.log('Flying');
      };
    }
    
    // the same is true when the function name
    // is the same as the variable name.
    function example() {
      console.log(named); // => undefined
    
      named(); // => TypeError named is not a function
    
      var named = function named() {
        console.log('named');
      }
    }
    
  • Function declarations hoist their name and the function body.

    function example() {
      superPower(); // => Flying
    
      function superPower() {
        console.log('Flying');
      }
    }
    
  • For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

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