Functions, Scope and Closures

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Functions in Javascript are first-class objects and can be manipulated and passed around just like any other object.

// JavaScript functions are declared with the function keyword.
function myFunction(word){
    return word.toUpperCase();
myFunction("foo"); // = "FOO"

// functions can also be declared like variables:
var sayHello = function(name){
  return "Hi " + name; 

// Note that the value to be returned must start on the same line as the 'return' keyword,
// or it will return 'undefined' due to automatic semicolon insertion: 
function myFunction()
    return // <- semicolon automatically inserted here
        thisIsAn: 'object literal'
myFunction(); // = undefined

// JavaScript functions can be reassigned to different variable names
// and passed to other functions as arguments - e.g. when supplying an event handler:
function myFunction(){
    console.log("this code will be executed after 5 seconds");
setTimeout(myFunction, 5000);
// Note: setTimeout isn't part of the JS language, but is provided by browsers and Node.js.

// Function objects don't even have to be declared with a name - you can write
// an anonymous function definition directly into the arguments of another.
    console.log("this code will be executed after 5 seconds");
}, 5000);


As mentioned in JS Basics, variables should be declared with var so that they're not automatically placed in the global scope. Their scope will then depend on where they're declared:

// JavaScript has function scope; functions get their own scope but other blocks do not.
if (true){
    var i = 5;
i; // = 5  //the variable is within scope here, unlike a block-scoped language
// To prevent temporary variables from leaking into the global scope,
// one-time anonymous functions can be used to wrap the variables:
    var temporary = 5;
    // We can access the global scope with the 'global object', which is named 'window' in a web browser.
    //The global object may have a different name in non-browser environments such as Node.js.
    window.permanent = 10;
temporary; // raises ReferenceError
permanent; // = 10


One of JavaScript's most powerful features is closures: When a function is defined inside another function, the inner function has access to all the outer function's variables, even after the outer function exits.

function delayedGreet(name){
  var prompt = "Hello, " + name + "!"; // #2

  //inner function (they're put in the local scope by default, as if declared with 'var')
  function inner(){
    console.log(prompt);             // #4 (after delay)
  setTimeout(inner, 5000);          // #3

delayedGreet("Adam");              // #1

After 5 sec, this gets printed to the console:

Hello, Adam!

What happened:

  1. The bottom line calls delayedGreet(),
  2. delayedGreet assigns a value to prompt.
    2b - The inner() function is created but not executed)
  3. setTimeout() runs with inner() as a parameter. The code then exits delayedGreet and its scope.
  4. After 5 seconds, inner actually runs and accesses prompt. Without closures, prompt would be out of scope, since we left the function 5 seconds earlier. However, because inner() is "closed over" delayedGreet(), it still has access to the prompt variable when it is finally called.


Given the following code:

function doThrice(action){
function greet(){
  console.log("Hello, how are you doing?");

...write one line of code that will print "Hello, how are you doing?" three times.

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You found a mysterious piece of convoluted code that calculates and prints theAnswer. What will it print?

universe = 1;  //global variable

function getEverything(){
    var life = 22; //declared in function
    return life;

function doubleUniverse(){
    universe = universe * 2;

//double universe 4 times
for(var life=0; life<4; life++){  

var everything = getEverything();
var theAnswer = life + universe + everything;

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The function makeAdder, takes in one parameter, x. Fill in the function body so it returns a function that takes in one parameter y and returns the sum of x and y.

For example, calling makeAdder(3); should return a function that returns 3+y, while calling makeAdder(10) should return a function that returns 10+y.

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