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ES6+ Syntax
Joel Ross
Spring 2024
View of the Day
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Review Q&A
- We have a little time to go over past things!
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JavaScript Versions (lecture + demo)
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Arrow Functions (lecture + pollev)
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Destructure & Spread (lecture + pollev)
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ES6 Modules (demo, if time)
Today is a syntax day (again)!
Review Q&A
JavaScript History
The official name for JavaScript is ECMAScript, often abbreviated "ES"
- 1995: developed by Brendan Eich (co-founder of Mozilla) for the Netscape browser--took 10 days.
- 2009: version 5; first major revision since 1999.
- 2015: version 6 ("ES 6") released
- 2016: "ES 2016" released
- Annual releases since then, named for the year
As with CSS, actually providing specified features depends on the browser.
ES6+ Features
ES 6 is now supported by most major browsers, and introduces some key language features that are commonly used in React. In particular:
- Arrow Functions (ES 6)
- Destructuring and Spreading (ES 6; ES 2018)
- Modules (ES 6, only recently supported in browser)
Syntactic Sugar causes cancer of the semicolon
Arrow Functions
A
shortcut syntax for declaring functions using
=>
//normal function declaration
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!');
anonymous function 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
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
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
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);
What do you think?
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"
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
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.
Check your understanding
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!
Action Items!
Action Items!
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Review JavaScript chapters (10-12, 14)
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Read Ch 15: Introduction to React
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Problem Set 05 due Friday
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Don't delay on this!
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Problem Set 06 due next week
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This one is bigger and challenging (problem-c especially)! Give yourself time.
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Next time: React! Finally!
/*** module.js ***/
export function foo() { console.log('foo') }
export function bar() { console.log('bar') }
export default function baz() { console.log('baz') }
/*** index.js ***/
/* A */
import bar from './module.js';
/* B */
import { bar } from './module.js';
/* C */
import * as bar from './module.js';
/* D */
import { * as bar } from './module.js';
//call bar to print "bar"
bar();
info340sp24-es6
By Joel Ross
info340sp24-es6
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