JavaScript Advanced Functions: Basic Functions Review

Learning Goals

Define a function using a function declaration
Define the term hoisting
Define a function using a function expression
Define the term "anonymous function"
Define an IIFE: Instantly-Invoked Function Expression
Define the term "function-level scope"
Define the term "scope chain"
Define the term "closure"

Introduction

This lab provides a summation of the basics of JavaScript functions. Most of these ideas should feel familiar. Be sure to take time to experiment or read up on a concept if you're not comfortable with the idea before moving on. If you're struggling here, the remainder of this module will be challenging. Fix any gaps now before moving on.

We also recommend that you resolve the lab as you read through the sections. Reinforcing what you read with what you know how to type will make sure the concepts are locked in. We'll prompt you when it's a good time to shift modes from "reading along" to coding.

Define a Function Using Function Declaration

In JavaScript, the most common way to define functions is with a function declaration:

function razzle() {
  console.log("You've been razzled!");
}

The word razzle becomes a pointer to some stored, potential, not-yet-actually-run bit of work (the function). We use the pointer to call or invoke the function. We call the function by adding () after the pointer.

function razzle() {
  console.log("You've been razzled!");
}
razzle()
//=> "You've been razzled!"

Interestingly, you can write function declarations after you call them:

razzle() //=> "You've been razzled!"
function razzle() {
  console.log("You've been razzled!");
}

Functions can be passed arguments, given default arguments, etc. Here's a brief code synopsis:

function razzle(lawyer="Billy", target="'em") {
  console.log(`${lawyer} razzle-dazzles ${target}!`);
}
razzle() //=> Billy razzle-dazzles 'em!
razzle("Methuselah", "T'challah") //=> Methuselah razzle-dazzles T'challah!

LAB: Implement a function called saturdayFun. It should return a String like "This Saturday, I want to ....!" Fill in the ... with the activity that's passed in as the first parameter. If nothing is passed in, default to "roller-skate". Use the learn program to verify you've gotten a working implementation. Come back here once you've gotten just this set of tests passing.

Define the Term Hoisting

JavaScript's ability to call functions before they appear in the code is called hoisting. For hoisting to work, the function must be defined as a function declaration.

Define a Function Using a Function Expression

Early in your programming career, you probably learned that programming languages feature expressions: arrangements of constants, variables, and symbols that, when interpreted by the language, produce a value.

1 + 1 //=> 2
"Razzle " + "dazzle!" //=> "Razzle dazzle!"

By analogy, if we agree that JavaScript has function expressions that look like this:

function() {
  console.log("Yet more razzling")
}

what is in the variable fn at the end of this block of code?

let fn = function() {
  console.log("Yet more razzling")
}

This function expression evaluates to "stored work." Instead of the value 2 when 1 + 1 is evaluated, in fn JavaScript stores work — something that's much harder to visualize than 2! If we ask JavaScript what's in fn, it says:

let fn = function() {
  console.log("Yet more razzling")
} //=> undefined
fn //=> ƒ () { console.log("Yet more razzling") }

Here, Chrome tells us that fn points to an un-executed function, a "frozen" function, a "potential" function, a "recipe" for work. The Chrome console symbolizes this "frozen work" by putting the function definition after this cool special ƒ character.

Here, fn is a pointer to the stored block of work. But the work is merely potential, it hasn't been done yet. Just as with function declaration, we need to invoke or call the function.

To do the work, to make the potential real, to "un-freeze" the function, we add () to the end, optionally adding arguments.

let fn = function() {
  console.log("Yet more razzling")
} //=> undefined
fn //=> ƒ () { console.log("Yet more razzling") }
fn() //=> "Yet more razzling"

In any case, a function expression comes about when we write function(){...} and assign it to a variable. Very importantly, function expressions are not hoisted. Since we assign these expressions to variables, we'd expect things to operate in the same way they do when we assign a String to a variable or the result of an arithmetic expression to a variable. Those assignments are not hoisted, thus neither is a function expression.

LAB: Implement a function called mondayWork. It should return a String like "This Monday, I will ... ." Fill in the ... with the activity that's passed in as the first parameter. If nothing is passed in, default to "go to the office". Use the learn program to verify you've gotten a working implementation. Come back here once you've gotten just this set of tests passing.

Define the Term "Anonymous Function"

We also call the expression that produces a function that uses the template function(){....} an "anonymous function." In the previous example the following was the anonymous function:

function() {
  console.log("Yet more razzling")
}

Unlike a function declaration, there's no function name in front of the (). Since anonymous means, "without a name," this function is called, sensibly enough, an anonymous function.

Define an IIFE: Instantly-Invoked Function Expression

As a thought experiment, what happens here:

(function(x){ return x + 3 })(2) //=> ???

We recognize the first () as being those that we might use from arithmetic:

( 3 - 4 ) * 2 // => -2

So the first parentheses return the anonymous function, the potential to do work. The second () are the () of function invocation. So we're "invoking" the function immediately after defining it.

(function(x){ return x + 3 })(2) //=> 5

This bit of code is called an IIFE or "Instantly Invoked Function Expression." This stands to reason since it's a function expression that we run immediately. IIFEs are a good way to make sure that you're grasping the content of this lesson up to this point.

Interestingly, any variables, functions, Arrays, etc. that are defined inside of the function expression's body can't be seen outside of the IIFE. It's like opening up a micro-dimension, a bubble-universe, doing all the work you could ever want to do there, and then closing the space-time rift. IIFEs are definitely science fiction or comic book stuff recalling the plot of "Donnie Darko" or Dr. Strange's "mirror dimension."

(
  function(thingToAdd) {
    let base = 3
    return base + thingToAdd
  }
)(2) //=> 5
// console.log(base) //=> Uncaught ReferenceError: base is not defined

We'll see some of the practical power of "hiding things" in IIFEs a little later in this lesson.

(ADVANCED) SYNTAX QUESTION Some keen-eyed readers might ask, "Why add parentheses around the function expression?" Why not:
function(x){ return x + 2 }(2) //=> ERROR
instead of:
(function(x){ return x + 2 })(2) //=> 4
The reason is that JavaScript gets confused by all those bits of special symbols and operators sitting right next to each other. Just as we find the way ancient Romans wrote (all-caps, no spaces) VERYHARDTOREADANDHARDTOKEEPTRACKOF, JavaScript needs those "extra" parentheses to tell what's part of the function expression and what's part of the invocation. It shouldn't be necessary, but is.

Define the Term "Function-level Scope"

JavaScript exhibits "Function-level" scope. This means that if a function is defined inside another function, the inner function has access to all the parameters (variables passed in) as well as any variables defined in the function. This moves backward recursively too. Each of the enclosing parents' scopes are made available via the scope chain. Let's see things working before we define scope chain.

ASIDE: This is where people really start to get awed by JavaScript.

Consider this code:

function outer(greeting, msg="It's a fine day to learn") { // 2
  let innerFunction =  function(name, lang="Python") { // 3
    return `${greeting}, ${name}! ${msg} ${lang}` // 4
  }
  return innerFunction("student", "JavaScript") // 5
}

outer("Hello") // 1
//=> "Hello, student! It's a fine day to learn JavaScript"

We call outer, passing "Hello" as an argument
The argument ("Hello") is saved in outer's greeting parameter. The other parameter, msg is set to a default value
Here's our old friend the function expression. It expects two arguments which it stores in the parameters name and lang with lang being set as a default to "Python". This expression is saved in the local variable innerFunction
Inside innerFunction we make use of both the parameters name and lang as well as the parameters of innerFunction's containing (parent) function. innerFunction gets access to those variables
Inside outer, we invoke innerFunction

This might look a little bit weird, but it generally makes sense to our intuition about scopes: inner things can see their parent outer things. But with a simple change, something miraculous can happen:

function outer(greeting, msg="It's a fine day to learn") { // 2
  let innerFunction =  function(name, lang="Python") { // 3
    return `${greeting}, ${name}! ${msg} ${lang}` // 4
  }
  return innerFunction
}

outer("Hello")("student", "JavaScript") // 1, 5
//=> "Hello, student! It's a fine day to learn JavaScript"

Amazingly, this code works the exact same. Even if the inner function innerFunction is invoked outside the parent function, it still has access to those parent function's variables. It's like a little wormhole in space-time to the outer's scope!

Let's tighten this code up once more instead of assigning the function expression to innerFunction, let's just return the function expression.

function outer(greeting, msg="It's a fine day to learn") {
  return function(name, lang="Python") {
    return `${greeting}, ${name}! ${msg} ${lang}`
  }
}

outer("Hello")("student", "JavaScript")
//=> "Hello, student! It's a fine day to learn JavaScript"

Our "inner" function is now a returned anonymous function. To repeat: When it came into existence, it inherited the values in outer's parameters greeting and msg. When we invoked outer we provided the arguments for greeting and left msg as the default. outer then returned an anonymous function that had its uses of greeting and msg set. It was almost as if outer returned:

return function(name, lang="Python") { // The "inner" function
  return `Hello, ${name}! It's a fine day to learn ${lang}`
}

We invoked this returned or "inner" function" by adding () and passing the arguments "student" and "JavaScript" which were then loaded into name and lang inside the inner function. This filled in the final two values inside of the template string and effectively returned:

  return `Hello, student! It's a fine day to learn JavaScript`
//=> "Hello, student! It's a fine day to learn JavaScript"

Keep in mind, it's not the case that we have to invoke functions like this:

outer("Hello")("student", "JavaScript")

We could:

let storedFunction = outer("Hello")
// ... lots of other code
storedFunction("student", "JavaScript")
//=> "Hello, student! It's a fine day to learn JavaScript"

Define the Term "Closure"

In the previous example, we could call the "inner" function, the anonymous function a "closure." It "encloses" the function-level scope of its parent. And, like a backpack, it can carry out the knowledge that it saw - even when you're out of the parent's scope.

Recall the IIFE discussion, since what's inside an IIFE can't be seen, if we wanted to let just tiny bits of information leak back out, we might want to pass that information back out, through a closure.

Note: We're also using destructuring assignment, don't forget your ES6 tools!

let [answer, theBase] = (
  function(thingToAdd) {
    let base = 3
    return [
      function() { return base + thingToAdd },
      function() { return base }
    ]
  }
)(2)
answer() //=> 5
console.log(`The base was ${theBase()}`)
// OUTPUT: The base was 3

Define the Term "Scope Chain"

The mechanism behind all the cool stuff we just saw is the scope chain which allows functions defined in functions to access all their parent scopes' variables. Here's a simple example:

function demoChain(name) {
  let part1 = 'hi'
  return function() {
    let part2 = 'there'
    return function() { // Innermost
      console.log(`${part1.toUpperCase()} ${part2} ${name}`);
    }
  }
}

demoChain("Dr. Stephen Strange")()() //=> HI there Dr. Stephen Strange

Through the scope chain, the function labeled //Innermost has access to name, part, and part2 when it is called and runs the console.log() statement. That's awesome wormhole, space-time, magic!

LAB:

Implement a function called wrapAdjective.
- It should return a function
  - This "inner" function should:
    - take a single parameter that should default to "special". Name it however you wish.
    - return a String of the form "You are ..." where ... should be the adjective this function received wrapped in visual flair
- It should take as parameter a String that will be used to create visual flair
- You may call the parameter whatever you like, but its default value should be "*"
- Call example: let encouragingPromptFunction = wrapAdjective("!!!")
Thus a total call should be: wrapAdjective("%")("a dedicated programmer") //=> "You are %a dedicated programmer%!"

Use the learn program to verify you've gotten a working implementation. Come back here once you've gotten just this set of tests passing.

Additional Practice in Lab-Driven Development for JavaScript Basics

Whew! That's a lot of recap with a lot of mind-bending stuff. Let's make sure that we review some of our easier basics.

The remainder of the tests are not new material. They're here to make sure you remember how to work with Objects and Arrays full of functions. Use the tests to guide you in feeling confident working with functions.

Conclusion

In this lesson, we've covered the basics of function declaration, invocation, and function scope. As a refresher on your skills, we've provided a simple lab to make sure that you're set for the new information coming up in the rest of this module.

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