Closures

closures.js

/*
  A "closure" is created whenever 
    - a function is executed and variables are scoped inside of it, either as params or with var
    - that function returns an executable function, either by itself or as part of an object/array
    
  The locally-scoped variables are then "closed over." Like any other variables defined inside of a function,
  they are not accessible outside of that scope, but in the special case of a closure they are still accessible
  by the **returned function**.
*/

// ------
// This function does not return a function. Variables defined inside of it go away.

var simpleFunction = function(str) {
  var base = "Hello, "
  return base + str
}

console.log(simpleFunction("World")) // prints "Hello, World"
console.log(base) // this line errors -- `base` is not defined outside of simpleFunction
console.log(str) // this line errors -- `str` is not defined outside of simpleFunction

// -----
// This function uses a globally-scoped variable, which persists outside of the function's scope.
// It does not return a function, so the variables defined inside of it go away.

var salutation = "Hi, "
var greetingFunction = function(str) {
  return salutation + str
}

console.log(greetingFunction("JavaScript")) // prints "Hi, JavaScript"
console.log(salutation) // prints "Hi, " -- globally scoped
console.log(str) // this line errors -- `str` is not defined outside of greetingFunction

// -----
// This function returns a function. Variables defined inside of the parent function are
// "closed over," and accessible only by the child function.

var closureFunction = function(num) {
  var multiplier = 2
  
  return function() {
    return num * multiplier;
  }
}

var twoTimes5 = closureFunction(5)
var twoTimes10 = closureFunction(10)

console.log(twoTimes5()) // prints 10
console.log(twoTimes10()) // prints 20

console.log(num) // this line errors -- "num" is not defined outside of closureFunction
console.log(multiplier) // this line errors -- `multiplier` is not defined outside of closureFunction