Introduction to Javascript

Topics

  • What is Javascript
  • Js basics: variables, functions, data types...
  • Working with simple types
  • Objects and arrays
  • Conditionals
  • Loops
  • Introduction to functional programming
  • Extra: managing asyncronism

What is Javascript?

Is

  • A programming language
  • Interpreted
  • Dynamic (not compiled)
  • Weakly typed
  • Prototype based (object oriented)
  • Multi-paradigm: event driven, functional, or imperative

Isn't

  • Java
  • Only for the client side
  • NodeJs

A bit of history

Year Version Support
1997 ECMAScript 1 All browsers
1998 ECMAScript 2 All browsers
1999 ECMAScript 3 All browsers
2009 ECMAScript 5 All modern browsers
2015 ECMAScript 6 Partial
? ECMAScript 7 Poor

Retrocompatible

JS basics

A variable is a named container for a value

Three ways to declare it

  • var
  • let
  • const

But you can leave it empty right now! (except constant)

var imperator = 'Furiosa';
let auntyEntity = 'Tina Turner';
const beyondThunderdome = 3;
var master;
let blaster;
const hasToBeInitialized = true;

Data types

string

let text = 'mediocre';

number

let pi = 3.141592;

null

let what = null;

undefined

let none = undefined;

boolean

let yesOrNo = true;

object

let complex = {
    key: 'value',
    evenMoreObjects: {
        yes: false
    }
};

Array

Function

built-in objects

error

console.log(Math.PI) // 3.141592653589793
console.log(Date.now()) // 1525979416506
const warlords = ['Immortan Joe', 'Bullet Farmer'];
new Error('I live, I die, I live again');

Play time!

A function is a fragment of code that can be invoked any time

When invoked, it can receive a list of arguments

 

Two different sintax, almost the same

function greet() { // ES5
  console.log('hi!');
}
greet();

const sayBye = () => { // ES6
  console.log('bye bye')
}
sayBye();
function ask(question) {
  console.log(question + '?');
}
ask('who killed the world');

const yell = (text) => {
  console.log(text + '!!')
}
yell('two enter, one leaves');

An array is a list of values, usually of the same type

Is an object with special methods, where the keys are automatically set

const originalSaga = [3, 'Mad Max', { bestOne: false }];

console.log(originalSaga[0]); // 3

console.log(originalSaga[2].bestOne); // false

working with simple types

arithmetic operators

+ Addition
- Subtraction
* Multiplication
/ Division
% Remainder
** Exponential
-- Decrement
++ Increment
function addOne(a) {
    return ++a;
}

function squaredAdd(a, b) {
    return (a ** 2) + (b ** 2) + (a * b);
}

Comparison operators

function areLoselyEqual(a, b) {
    return a == b;
}

function isNegativeNumber(a) {
    return a < 0;
}
< Less
<= Less or equal
> Greater
>= Greater or equal
== or === Equal
!= or !== Not equal

Logical operators

function isTheLetterA(letter) {
    return letter === 'a' || letter === 'A';
}

function isBetween(target, min, max) {
    return target >= min && target <= max;
}
&& And
|| Or
! Not

Practice time!

objects && arrays

accessing values

Object values can be accessed using the dot notation or the brackets notation

const madMax4 = {
    year: new Date('7/May/2015'),
    title: 'Mad Max: Fury Road',
    director: 'George Miller'
};

console.log(madMax4.title);

console.log(madMax4['director']);

const someProp = 'year';
console.log(madMax4[someProp]);

accessing values

madMax4.stars = [
'Tom Hardy', 
'Charlize Theron'
];

Array values can only be accessed with the brackets notation, using the index

We can add or modify values using the assignment operator (=)

console.log(madMax4.stars[1]);

madMax4.stars[1] = 'Charlize <3';

Object methods

entries

assign

keys

values

console.log(Object.values(madMax2));
console.log(Object.entries(madMax2));
console.log(Object.keys(madMax2));
const filmBase = {director: 'George Miller'};
const madMax2 = Object.assign(
    {title: 'Road Warrior'},
    filmBase
);

Practice time!

array props and methods

length

pop

push

soundtrack.push('Buzzards Arrive');
soundtrack.pop();
const soundtrack =  [
    'Survive',
    'Escape',
    'Immortan\'s citadel',
    'Blood bag'
];
soundtrack.length; // 4

array props and methods

indexof

soundtrack.indexOf('Escape');

sort

soundtrack.sort();

objectsArray.sort((a, b) => {
    return a.value - b.value;
});

concat

soundtrack.concat(['Spikey Cars']);

join

soundtrack.join(' - ')

Practice time!

conditionals

Conditional structures allow to execute or not a piece of code depending on a decision

if else

The if...else structure is usefull to execute statements if a condition is true

function whoIsMadMax(film) {
    if (film <= 3) {
        return 'Mel Gibson';
    } else {
        return 'Tom Hardy';
    }
}

if else

We can use only the if part, or chain any number of else if

function getArea(poligon, side) {
    if (poligon === 'square') {
        return side * side;
    } else if (poligon === 'triangle') {
        return side * side / 2;
    } else {
        return new Error('Sorry')
    }
}

Ternary operator

The ternary operator is an abbreviated if else.

function whoIsMadMax(film) {
    return film <= 3
        ? 'Mel Gibson'
        : 'Tom Hardy';
}

Practice time!

Loops

A loop is a structure that allow us to repeat the same code a certain number of times

Loops can be placed inside other loops, which is called nested loop. Be aware, so they can increase the execution time exponentially.

A loop has (implicit or explicitly) three elements:

  1. Counter
  2. Exit condition
  3. Iterator

for

The for loop is the most basic form of loop in javascript

const wives = [
    'Angharad',
    'Toast the Knowing',
    'Capable',
    'The Dag',
    'Cheedo the Fragile'
];

for (let i = 0; i < wives.length; i++) {
    console.log(`${i}: ${wives[i]}`); 
}

Counter, exit condition and iterator are easily identifiable

for eACH

forEach is a shorter way to iterate over the elements of an array

const wives = [
    'Angharad',
    'Toast the Knowing',
    'Capable',
    'The Dag',
    'Cheedo the Fragile'
];

wives.forEach((wife, i) => {
    console.log(`${i}: ${wife}); 
});

for of

for ... in is the object-equivalent of forEach

const gangs = {
    theAquifer: 'Immortan Joe',
    bulletFarm: 'Bullet Farmer',
    gasTown: 'People Eater'
};

for (const place in gangs) {
    console.log(
    `${place} is lead by ${gangs[place]}`
    );
}

while

The while statement does not need an iterator, it will run until a condition is true

let theWivesAreSave = false;

while (immortanJoe.isAlive) {
    tryToKill(immortanJoe);
}

theWivesAreSave = true;

Practice time!

Functional programing 101

disclaimer: as a frontend dev, i'm not good at this

Functional programming (often abbreviated FP) is the process of building software by composing pure functions, avoiding shared state, mutable data, and side-effects.

Eric Elliot

Advantages

  • Easier to test
  • More predictable
  • Smaller pieces = reusability

Pure functions

// very bad
const truck = { rider: 'Furiosa', gear: 0, started: false };

function startTruck() {
    if (truck.rider !== 'Furiosa') {
        return;
    } else {
        truck.gear = 1;
        truck.started = true
    }
}

startTruck();
  • Given the same input, return the same output
  • No side effects

shared state

// still bad

function startTruck(truck) {
    if (truck.rider !== 'Furiosa') {
        return false;
    } else {
        truck.gear = 1;
        truck.started = true
    }
}

startTruck({ rider: 'Furiosa', gear: 0, started: false });

Shared state is a variable, object, etc that exists in a shared scope.

// still bad

function startTruck(truck) {
    if (truck.rider !== 'Furiosa') {
        return;
    } else {
        truck.changeGear(1);
        truck.started = true
    }
}

startTruck({ rider: 'Furiosa', gear: 0, started: false });

Mutable data

Data shouldn't be modified after it has been created.

Be aware, that const does not mean the data is immutable

// not so bad

function startTruck(truck) {
    if (truck.rider !== 'Furiosa') {
        return Object.assign({}, truck, {started: false});
    } else {
        truck.changeGear(1);
        return Object.assign({}, truck, {started: true});
    }
}

startTruck({ rider: 'Furiosa', gear: 0, started: false });

side effects

A side effect is any change visible outside the called function

(apart from the returned value)

// done!

function startTruck(truck) {
    return Object.assign(
        {}, truck, {started: truck.rider === 'Furiosa'}
    );
}

const truck = startTruck(
    { rider: 'Furiosa', gear: 0, started: false }
).changeGear(1);

map

Creates a new array with the results of calling a function over the elements of other array

const characters = [
    'Imperator',
    'Mad Max',
    'Auntie',
    'Toecutter'
];
const everybodyGetsACar = characters
.map(name => `${name} gets a car!`);

The function needs to return the new element

reduce

Similar to map, but reduces the array to a single value

const characters = [
    'Imperator',
    'Mad Max',
    'Auntie',
    'Toecutter'
];
const everybodyGetsACar = characters
.reduce((sentence, name) => {
    return `${sentence} ${name}, `;
}, 'All of them get a car: ');

The initial value is optional

filter

Filter returns a new array containing only some values, depending on a filtering condition

const someNumbers= [15, 8, 4, 42, 23, 16];
const smallNumbers= someNumbers
.filter(num => num <= 15);

Practice time!

Managing asyncronism

Sometimes, a line of code will schedule something to be executed later, while the rest of the code continues running. This is called asyncronism

 

In javascript this is usually related to http calls or database modifications

callbacks (ES5)

promises (ES6)

async await (ES7)

asynFunction('p', result => {
    console.log(result);
});
asynFunction('p').then(result => {
    console.log(result);
});
async function asynFunction(p) {...}

const result = await asynFunction('p');
console.log(result);

Introduction to javascript

By Paqui Calabria

Introduction to javascript

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