This content originally appeared on DEV Community and was authored by Suprabha
Define your functions in many ways.
One way is using function keyword:
// function declaration
function test(msg) {
return `Hey ${msg}`
}
// function expression
const test = function(msg) {
return `Hey ${msg}`
}
You can call both function declaration and expression as Normal/Regular Function
Arrow function is introduced in ES6 and also known as fat arrow function.
const arrowFunction = (msg) => {
return `Hey ${msg}`
}
As you see both functions work same by above example. Now the question comes why do we need regular or arrow function.
Let's discuss below ?
1. Syntax
2. Arguments binding
3. this
4. new keyword
5. No duplicate named parameters
6. Function Hoisting
7. Methods
1️⃣ Syntax
We can write normal and arrow function in this way ?
// ES5
var add = function(x, y) {
return x + y
};
// ES6
let add = (x, y) => x + y
Implicit Return
In regular function, you have to use return keyword to return any value. If you don't return anything then the function will return undefined.
function regFunc() {
return "Regular Function";
}
regFunc();
// Regular Function
function regFunc() {
console.log("Regular Function")
}
regFunc();
// Regular Function
// undefined
Arrow functions behave in the same way when returning values.
If the arrow function contains one expression, you can omit the curly braces, and then the expression will be implicitly returned.
{} not required if its only one line of statement
const addOne = (number) => number + 1;
addOne(10);
() not required if you pass only one argument
let add = x => x + x;
If there is no arguments
let arrowFunc = _ => console.log("Arrow Function");
2️⃣ Arguments binding
In regular function, Arguments keywords can be used to access the arguments of which passed to function.
Example:
function regularFunction(a,b) {
console.log(arguments)
}
regularFunction(1,2)
// Arguments[1,2]
Arrow functions do not have an arguments binding.
const arrowFunction = (a,b) => {
console.log(arguments)
}
arrowFunction(1,2)
//ReferenceError: argumnets is not defined
However, if you want to access arguments in an arrow function, you can use the rest operator:
var arrowFunction = (...args) => {
console.log(...args)
}
arrowFunction(1,2)
// 1 2
3️⃣ this
In regular function, this changes according to the way that function is invoked.
-
Simple Invocation:
thisequals the global object or maybe undefined if you are using strict mode. -
Method Invocation:
thisequals the object that owns the method. -
Indirect Invocation:
thisequals the first argument. -
Constructor Invocation:
thisequals the newly created instance.
// 1️⃣ Simple Invocation
function simpleInvocation() {
console.log(this);
}
simpleInvocation();
// Window Object
// 2️⃣ Method Invocation
const methodInvocation = {
method() {
console.log(this);
}
};
methodInvocation.method();
// logs methodInvocation object
// 3️⃣ Indirect Invocation
const context = { aVal: 'A', bVal: 'B' };
function indirectInvocation() {
console.log(this);
}
indirectInvocation.call(context); // logs { aVal: 'A' }
indirectInvocation.apply(context); // logs { bVal: 'A' }
// 4️⃣ Constructor Invocation
function constructorInvocation() {
console.log(this);
}
new constructorInvocation();
// logs an instance of constructorInvocation
Arrow functions don't have their own this, and they don’t redefine the value of this within the function.
this inside an arrow function always refers to this from the outer context.
var name = "Suprabha"
let newObject = {
name : "supi",
arrowFunc: () => {
console.log(this.name);
},
regularFunc() {
console.log(this.name);
}
}
newObject.arrowFunc(); // Suprabha
newObject.regularFunc(); // supi
4️⃣ new
Regular functions are constructible, they can be called using the new keyword.
function add (x, y) {
console.log(x + y)
}
let sum = new add(2,3);
// 5
However, arrow functions can never be used as constructor functions. Hence, they can never be invoked with the new keyword
let add = (x, y) => console.log(x + y);
const sum = new add(2,4);
// TypeError: add is not a constructor
5️⃣ No duplicate named parameters
In normal function, we can do this:
// ✅ will work
function add(a, a) {}
// ❌ will not work
'use strict';
function add(a, a) {}
// Uncaught SyntaxError: Duplicate parameter name not allowed in this context
Arrow functions can never have duplicate named parameters, whether in strict or non-strict mode.
const arrowFunc = (a,a) => {}
// Uncaught SyntaxError: Duplicate parameter name not allowed in this context
6️⃣ Function Hoisting
In regular function, function gets hoisting at top.
normalFunc()
function normalFunc() {
return "Normal Function"
}
// "Normal Function"
In arrow function, function get hoisted where you define. So, if you call the function before initialisation you will get referenceError.
arrowFunc()
const arrowFunc = () => {
return "Arrow Function"
}
// ReferenceError: Cannot access 'arrowFunc' before initialization
7️⃣ Methods
You can define methods in class using regular function.
class FullName {
constructor(name) {
this.name = name;
}
result() {
console.log(this.name)
}
}
let name = new FullName("Suprabha")
console.log(name)
// FullName {name: "Suprabha"}
You need to apply method as callback also.
setTimeout(name.result, 2000)
// after 1 second logs ""
But if you bind this
setTimeout(name.result.bind(name), 2000)
// Suprabha
By above example, you can see that you have to bind the this to there context.
In arrow function, you don't have to bind with context.
class FullName {
constructor(name) {
this.name = name;
}
result = () => {
console.log(this.name)
}
}
let name = new FullName("Suprabha")
setTimeout(name.result, 2000) // Suprabha
When not to use Arrow function ???
Object Methods
let dog = {
count: 3,
jumps: () => {
this.count++
}
}
When you call dog.jumps, the number of count does not increase. It is because this is not bound to anything, and will inherit the value of this from its parent scope.
Reference ?
Summary
In regular function, this value is dynamic, In arrow function it equals to this of the outer function.
In regular function, arguments will give you list of parameter passed in function, In arrow function arguments is not defined.
In regular function, you always have to return any value, but in Arrow function you can skip return keyword and write in single line.
In arrow function parameters should be unique.
Hoisting matters in arrow function as function get not be invoked before initialisations.
Thanks for reading the article ❤️
| ??? suprabha.me |
This content originally appeared on DEV Community and was authored by Suprabha
Suprabha | Sciencx (2021-07-29T06:36:18+00:00) Regular vs Arrow Function. Retrieved from https://www.scien.cx/2021/07/29/regular-vs-arrow-function/
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