Functions

이 글은 자바스크립트의 Function(함수)에 관한 개념 및 문법을 정리하기 위해 작성하였습니다.

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< What are Functions? >

A function is a reusable block of code to perform a specific task.

 

 

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< Function Declarations >

 

function identifier() {
	// function body
}

 

1. The function keyword
2. Function's name (a.k.a identifier) : identifier
3. The block of statements required to perform a specific task : function body

 

< Hosting >
: Allow access to function declarations before they're defined.
 
ex)

greetWorld();   //   Hello World!

function greetWorld() {
	console.log('Hello World!');
}

 

 

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< Calling a Function >

The code inside a function body runs, or executes, only when the function is called.

 

• Type the function name

 

ex)

function sayThanks() {
	console.log('Thank you for your purchase! We appreciate your business.');
}

sayThanks();   //   Thank you for your purchase! We appreciate your business.
sayThanks();   //   Thank you for your purchase! We appreciate your business.
sayThanks();   //   Thank you for your purchase! We appreciate your business.

 

 

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< Parameters and Arguments >

Some functions can take inputs and use the inputs to perform task.

To allow functions to accept input(s) and perform a task using the input(s).

 

// Function Declaration
function area(A, B) {
	console.log(A * B);
}

// Function Calls
area(5, 4)   //   20

 

1. Parameters ( Variable ) : A, B
2. Arguments ( Value ) : 5, 4 

 

 

---

< Default Parameters >

To allow parameters to have a predetermined value in case there is no argument passed into the function.

 

ex)

function greeting(name = 'Stranger') {
	console.log(`Hello, ${name}!`)
}

greeting('LSH');   //  Hello, LSH!
greeting();   //   Hello, Stranger!

 

 

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< Return >

To pass back information from the function call, use return keyword

 

• Without return keyword, the resulting value is undefined
• When a return is used, the execution of the function is stopped and the code that follows it will not be executed.

 

ex)

function rectangleArea(width, height) {
	if (width < 0 || height < 0) {
    	return 'You need positive integers to calculate area!';
    }
    return width * height;
}

 

 

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< Helper Functions >

The function being called within another function.

To section off small bits of logic or tasks.

 

ex)

function multiplyByNineFifths(number) {
	return number * (9/5);
}

function getFahrenheit(celcius) {
	return multiplyByNineFifths(celcius) + 32;
}

getFahrenheit(15);   //   59

 

 

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< Function Expressions >

Another way to define a function.

 

const identifier = function() {
	// function body
}

 

< Not Hosting >
: Unlike function declaration, functions expressions are not hoisted,
  so they cannot be called before they are defined.

ex)

greetWorld();   //   ReferenceError : greetWorld is not defined

const greetWorld = function() {
	console.log('Hello World!');
}

 

 

---

< Arrow Functions >

A shorter way to define a function.

 

const identifier = () {
	// function body
}

 

• if the function have only one parameter : ' ( ) ' can be removed.
• if the function is a single line block : ' { } ', ' return ' can be removed.

 

ex)

// Basic Version
const squareNum = (num) => {
	return num * num;
}

// Shorter Version
const squareNum = num => num * num;