JavaScript III
INFO 253A: Front End Web Architecture
Kay Ashaolu
ECMAScript, what is that?
- ECMAScript is technically the JavaScript language standard
- Other languages have adopted some of this standard (e.g. ActionScript)
- Lays out the features of JavaScript to be agreed upon
So many versions...
- Figuring out which browser is running which version of ECMAScript could get daunting
- Browsers also do not simply implement the entire version
- A browser update could add support to single particular feature
What if you actually want to use the newer features
- That person that never updates IE will not be able to execute your JavaScript
- That person that found a way to not automatically update Chrome will not be able to see your site
Solution: Transpiler
- Similar to a compiler, but converts JavaScript to JavaScript
- Converts Javascript code written in a higher version into lower version JavaScript code
- This enables developers to use newer features, and users with older browsers able to execute the code
Example ES6 Code
class Planet {
constructor (mass, moons) {
this.mass = mass;
this.moons = moons || 0;
}
reportMoons () {
console.log(`I have ${this.moons} moons.`)
}
}
Complied ES5 Code
var _createClass = function () { function defineProperties(target, props) {...
defineProperties(Constructor, staticProps); return Constructor; }; }();
function _classCallCheck(instance, Constructor) {
if (!(instance instanceof Constructor)) {
throw new TypeError("Cannot call a class as a function");
}
}
var Planet = function () {
function Planet(mass, moons) {
_classCallCheck(this, Planet);
this.mass = mass;
this.moons = moons || 0;
}
_createClass(Planet, [{
key: 'reportMoons',
value: function reportMoons() {
console.log('I have ' + this.moons + ' moons.');
}
}]);
return Planet;
}();
Using Babel
- Babel is a transpiler that accomplishes conversion
- There is an entire build environment, using webpack 4, babel, and npm to set up
- For this week, please use the latest version of Chrome or Firefox to run your Javascript
Any Questions?
Syntactic Sugar
A lot of improvements to language focuses on changing syntax to make it easier to accomplish a certain goal
Let's talk about some of those features in ES6
Let and Scope
- Let creates a variable with scope
- Scope is a term that defines a boundary where variables live
- Scope is how you can ensure content inside a function is not affected by the outside
- Scope in Javascript is largely defined by curly brackets ('{}')
Let example
let a = 50;
let b = 100;
if (true) {
let a = 60;
var c = 10;
console.log(a/c); // 6
console.log(b/c); // 10
}
console.log(c); // 10
console.log(a); // 50
Let example explained
- The variable a is found both in the scope of this script, and in the scope of the if statement block
- The variable a within the block can be considered a different variable than the variable a outside the block
Const
- There a times where you do not want a variable to change after assignment
- For example, if you have a variable that is set to the number PI
- You wouldn't want that variable PI to change during your program
Const Example
const b = "Constant variable";
b = "Assigning new value"; // shows error.
const LANGUAGES = ['Js', 'Ruby', 'Python', 'Go'];
LANGUAGES = "Javascript"; // shows error.
LANGUAGES.push('Java'); // Works fine.
console.log(LANGUAGES); // ['Js', 'Ruby', 'Python', 'Go', 'Java']
Const example explained
- The variable LANGUAGES can not be changed
- However, what LANGUAGES points to, if it is mutable can change
Why use let and const?
- Cleaner understanding of the lifespan of a variable
- Reduce coding mistakes by ensuring variables that shouldn't change does not
Arrow Functions
- There is a new way of defining functions
- There are a few reasons for this (and that's actually a pun, but you can look that up to figure it out)
- This new way of writing function also helps with clearly defining scope
Arrow Functions Example
function oldOne(name) {
console.log("Hello " + name);
}
oldOne("Kay");
// New Syntax
let newOne = (name) => {
console.log("Hello " + name);
}
newOne("Kay");
What did that do?
- The parameters are named in the parenthesies outside the name of the function
- Note how you assign a variable to a function (and can use let for scope)
Default Parameters
- Convenient ability to assign parameters to a function a value if not specified by the caller
Default Parameter Example
let Func = (a, b = 10) => {
return a + b;
}
console.log(Func(20)); // 20 + 10 = 30
console.log(Func(20, 50)); // 20 + 50 = 70
let NotWorkingFunction = (a = 10, b) => {
return a + b;
}
console.log(NotWorkingFunction(20)); // NAN. Not gonna work.
What did that do?
- The function Func sets a default value to the second parameter
- You can pass the second parameter or leave it blank
- However order matters. You can't define a default parameter and then the next parameter does not have a default value
For...loop
- Very nice way of looping through a list of elements
- No need to figure out index parameters and value conditions
For...loop
let arr = [2,3,4,1];
for (let value of arr) {
console.log(value);
}
For...loop explained
- The variable 'value' is assigned each element of that array once
- Note you do not have access to the index while using this construct
Spread Attributes
- Ability to define a function with a variable number of parameters
- You do not have to pass an array in order to have a variable number of parameters
Spread
let SumElements = (...arr) => {
console.log(arr); // [10, 20, 40, 60, 90]
let sum = 0;
for (let element of arr) {
sum += element;
}
console.log(sum);
}
SumElements(10, 20, 40, 60, 90);
SumElements(10, 20, 90);
What did that do?
- You can pass a variable number of parameters
- Those parameters are avaiable as an array inside the function
Template Literals
- Template literals makes adding variables to your strings much easier
- Many Languages (like Python and Ruby) has this built into the langauge
Template Literals Example
let name = "Jon Snow";
let msg = `My name is ${name}`;
console.log(msg);
Destructing Objects and Arrays
- Let’s just get into an example
Destructing Objects Example
let person = {firstName: "Jon", lastName: "Snow", age: 23}
const {firstName, age} = person
console.log(firstName);
console.log(age);
Destructing Arrays Example
let arr = [1,2,3,4,5,6]
let [a,b,,d,e] = arr
console.log(a);
console.log(b);
console.log(d);
console.log(e);
What did that do?
- You can do the same thing with arrays
- Order of the array that is the result of destructuring matters
- You can skip what you don't want by leaving that position blank
Questions?
JavaScript III - Frontend Webarch
By kayashaolu
JavaScript III - Frontend Webarch
Course Website: https://www.ischool.berkeley.edu/courses/info/253a
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