ES6+ Syntax

Joel Ross

Arrow Functions

Joel Ross

Arrow Functions

A shortcut syntax for declaring functions using =>

//normal function declaration (literal format)
const foo = function(params) {
  return 'foo '+params;
}

//arrow function declaration (block body)
const foo = (params) => {
  return 'foo '+params;
}

//arrow function declaration (concise body)
const foo = (params) => 'foo '+params;

single expression implies return

//arrow function declaration (concise, no params)
const sayHello = () => console.log('Hello world!');

function literal assigned to a variable

Arrow Functions

Arrow functions are particularly nice for anonymous callback functions (literals), and in fact should always be used.

const array = [1,2,3]; //an array to work with

//normal function
array.map(function(num) {
  return num*2; //multiply each item by 2
});

//arrow syntax
array.map((num) => {
  return num*2; //multiply each item by 2
});

//concise body - ONLY FOR TINY EXPRESSIONS
//(better to avoid entirely)
array.map((num) => num*2);

const doggy = {
  name: "Fido",
  bark: () => console.log("woof")
}

Check your understanding

/* A */
const myFunction => (param1, param2) = {
  console.log(param1, param2);
  return param1 + param2;
}

/* B */
const function myFunction(param1, param2) => {
  console.log(param1, param2);
  return param1 + param2;
}

/* C */
const myFunction = (param1, param2) => {
  console.log(param1, param2);
  param1 + param2;
}

/* D */
const myFunction = (param1, param2) => {
  console.log(param1, param2);
  return param1 + param2;
}

Which arrow function has the correct syntax?

/* function declaration */
function myFunction(param1, param2) {
  return param1 + param2;
}

/* function literal */
const myFunction = function(param1, param2) {
  return param1 + param2;
}

/* arrow function */
const myFunction = (param1, param2) => {
  return param1 + param2;
}

/* arrow function (concise body) */
const myFunction = (param1, param2) => param1 + param2;

We now have 4 different ways to write functions. They all have different syntax, but do the same work.

Function Syntax

Code Style

Recommendation: don't use concise body syntax. Always include the explicit block.

It's easier to read (more clearly a function)
It's easier to modify (can add additional statements)
It's easier to debug (can add console logs)

/* Do this */
const exclaimed = stringArray.map((aString) => {
  return aString + "!!" //add exclamation points
})

/* Do not do this */
const exclaimed = stringArray.map((aString) => aString + "!!")

Destructuring and Spreading

Joel Ross

Destructuring

Destructuring assignment lets you assign (unpack) each value of an array or object into into separate variables in a single operation.

//destructuring arrays
const myArray = [1, 2, 3];
const [x, y, z] = myArray; //myArray elements to `x`, `y`, `z`
console.log(x); //=> 1;
console.log(y); //=> 2;
console.log(z); //=> 3;

//destructuring objects
const myObject = {a: 1, b: 2, c: 3};
const {a, b, c} = myObject; //myObject.a to a, etc.
console.log(a); //=> 1
console.log(b); //=> 2;
console.log(c); //=> 3;

Check your understanding

/* A */
const a, b, c = [1, 2, 3];

/* B */
const [a, b, c] = {a: 1, b: 2, c: 3};

/* C */
const {a, b} = {a: 1, b: 2, c: 3};

/* D */
const [a, b, c] = [1, 2, 3];

/* E */
const [a, b, c] = 1, 2, 3;

Which statement will assign a value of 3 to the variable c?

Destructuring Params

It's possible to destructure function parameters. This is commonly used in React (with object params)... but can make code maintenance harder.

//an example person object
const person = {name: 'Ada', height: 64, weight: 135}

//a function that expects a person object
function getBMI(personObj) {
  const height = person.height;
  const weight = person.weight;
  return 703*weight/(height*height);
}




const adaBMI = getBMI(person);

//implicit
const personObj = {
   name: 'Ada', height: 64, weight: 135
}

//implicit - destructured
const {height, weight} = {
   name: 'Ada', height: 64, weight: 135
}

//an example person object
const person = {name: 'Ada', height: 64, weight: 135}

//a function that expects a person object
function getBMI({height, weight}) {

  
  return 703*weight/(height*height);
}




const adaBMI = getBMI(person);

/* standard argument */
function getFullName(personObj) {
  return personObj.first + " " + personObj.last;
}

/* destructured argument */
function getFullName({first, last}) {
  return first + " " + last;
}

We now have 2 different ways to write function arguments. They all have different syntax, but do the same work.

Function Syntax

Spread Operator

const originalArray = ['a', 'b', 'c', 'd'];

const newArray = [...originalArray, 'e', 'f'];


console.log(newArray) //['a', 'b', 'c', 'd', 'e', 'f']

The spread operator ... lets you reference "the elements of" an array or object when declaring a new value. This is particularly useful for duplicating arrays or objects.

const person = {name: 'Ada', height: 64, weight: 135}

const copyOfPerson = {...person}; //clone an object!
console.log(person === copyOfPerson); //false! different objects!

//all off the properties are "spread" into the new object
const personWithHat = {hat: 'bowler', ...person}
console.log(person); //has name, height, weight
console.log(personWithHat); //has name, height, weight, hat

think: "the elements of original array"

ES6 Modules

Joel Ross

ES 6 Modules

Modules are self-contained, isolated scripts that are able to run in their own namespace (without polluting the global environment). Modules can export values that may be imported and used by other modules.

Web pages load modules by including a script tag with the type="module" attribute.

<!-- load a script as a module -->
<script type="module" src="path/to/module.js"></script>

Live-Server

cd path/to/project
npx live-server .

where index.html is!

dot to serve entire folder

When an HTML page is opened with the file:// protocol, JavaScript cannot access system files. You need to run a local web server instead to use the http:// protocol.
You can do this with the live-server package

Access the webpage at http://localhost:8080/index.html

Stop the server by using ctrl+c in the command shell.

Importing

It is "possible" to import external libraries and make them available in your script from JavaScript (not HTML)

CommonJS (used by Node.js)

ES 6 Modules (used by browsers)

const util = require('util');

node version of "import"

global to refer
to the library

module library name

import { ArrayList } from 'util'; //named import

module library name

Java

import java.util.ArrayList;

module name

variable to import

Exporting Values

export variables, functions, and classes to be available for other modules to import.

/*** my-module.js ***/
export function foo() { return 'foo'; } //named export

export const bar = "bar"; //export a variable

//will not be available (a "private" function)
function baz() { return 'baz'; }

/*** index.js ***/
//example of named imports:
import {foo} from './my-module.js'; //single named import
import {foo, bar} from './my-module.js'; //multiple named import
foo() //=> 'foo'
console.log(bar) //=> 'bar'

relative path to file (start with ./)
.js extension will be implied by default in Node

Importing Options

Use the as keyword to "alias" a value.

Use import * syntax to import all values from a module.

/*** my-module.js ***/
export function foo() { return 'foo'; } //"named" export

//provide an "alias" (consumer name) for value
export { bar as yourBar };

/*** index.js ***/
import { yourBar } from './my-module.js'; //import value by name
yourBar() //=> 'bar'

import { bar } from './my-module.js'; //error, no value `bar` exported

//provide "alias" for value when importing!
import {foo as myFoo} from './my-module.js';
myFoo(); //=> 'foo'

//import everything that was exported
//loads as a single object with values as properties
import * as theModule from './my-module.js'; 
theModule.foo(); //=> 'foo'
theModule.yourBar(); //=> 'bar'

Default Exports

Each module can have a single default export, which provides a shortcut when importing.

/*** my-module.js ***/
export default function sayHello() {
    return 'Hello world!';
}

/*** index.js ***/

//default import can assign own alias without needing name!
//think: "import {default as greet} from './mymodule.js'"
import greet from './my-module.js';

greet(); //=> "Hello world!"

Be careful about whether an export is named or default!

External libraries especially are not consistent.

When to export?

Only export a value if it must be used by another module. When in doubt, keep values private.
In general, prefer named exports. This makes it easier to export additional values later if needed.
If a module is designed around a single value or function (e.g., a single React Component), make it a default export. But a single exported value isn't necessarily a default export!

info340-es6

By Joel Ross

ES6+ Syntax

Joel Ross

Arrow Functions

Joel Ross

Arrow Functions

Arrow Functions

Check your understanding

Function Syntax

Code Style

Destructuring and Spreading

Joel Ross

Destructuring

Check your understanding

Destructuring Params

Function Syntax

Spread Operator

ES6 Modules

Joel Ross

ES 6 Modules

Live-Server

Importing

CommonJS (used by Node.js)

ES 6 Modules (used by browsers)

Java

Exporting Values

Importing Options

Default Exports

When to export?

info340-es6

More from Joel Ross