Day 39 - 18/10

Plan:

Progress:

JSON - JavaScript Object Notation

What:

  • a lightweight data interchange format

  • language independent (JSON format is text only; code for reading and generating JSON data can be written in any programming language)

  • "self-describing" and easy to understand

The JSON format is syntactically identical to the code for creating JavaScript objects.

Syntax rules:

  • data is in name/value pair

  • data is separated by commas

  • curly braces hold objects

  • square brackets hold arrays

Common use of JSON is to read data from web server and display it in a web page

Convert a JSON text to a Js Object

  • Create a Js string containing JSON syntax

let text = '{ ... JSON ... }';

  • Use JSON.parse() to convert the string into a JavaScript object

const obj = JSON.parse(text);

  • Use the new JavaScript object in your page with the new name

<p id="demo"></p>

<script>

document.getElementById("demo").innerHTML = obj.employees[1].firstName + " " + obj.employees[1].lastName;

</script>

Coding conventions

Naming

  • names written as camelCase

  • global var written in UPPERCASE

  • constants written in UPPERCASE

  • hyphens ( - ) are not allowed

  • dont start with $ sign

Operators

Always put spaces around operators and after commas

Code indentation

Always use 2 spaces for indentation of code blocks

Statement rules

  • Always end a simple statement with a semicolon

  • Put the opening bracket at the end of the first line

  • Use one space before the opening bracket

  • Put the closing bracket on a new line, without leading spaces

  • Do not end a complex statement with a semicolon

Object rules

  • Place the opening bracket on the same line as the object name

  • Use colon plus one space between each property and its value

  • Use quotes around string values, not around numeric values

  • Do not add a comma after the last property-value pair

  • Place the closing bracket on a new line, without leading spaces

  • Always end an object definition with a semicolon

Line length

Line length < 80

Loading JavaScript in HTML

Use simple syntax for loading external scripts

<script src="myscript.js"></script>

Use lowercase file name

Best practice

Avoid global variable

Global variables and functions can be overwritten by other scripts.

Use local variables instead, and learn how to use closures.

Always declare local variable

Declarations on top

Declare object with const

Prevent any accidental change of type

Declare array with const

Prevent any accidental change of type

Don't Use new Object()

  • Use "" instead of new String()

let x1 = "";

  • Use 0 instead of new Number()

let x2 = 0;

  • Use false instead of new Boolean()

let x3 = false;

  • Use {} instead of new Object()

const x4 = {};

  • Use [] instead of new Array()

const x5 = [];

  • Use /()/ instead of new RegExp()

const x6 = /()/;

  • Use function (){} instead of new Function()

const x7 = function(){};

Beware of Automatic Type Conversions

A variable can contain all data types.

A variable can change its data type:

let x = "Hello"; // typeof x is a string

x = 5; // changes typeof x to a number

Use Parameter Defaults

If a function is called with a missing argument, the value of the missing argument is set to undefined.

It is a good habit to assign default values to arguments

End Switches with Defaults

Always end your switch statements with a default

Avoid Number, String, and Boolean as Objects

Always treat numbers, strings, or booleans as primitive values. Not as objects.

Common Mistakes

Accidentally Using the Assignment Operator

accidentally use ( = ) instead of ( == )

Loose Comparison

In regular comparison, data type does not matter; but in strict comparison, it does matter

Case switch use strict comparison

Confusing Addition (add numbers) & Concatenation (add strings)

Both uses the same ( + ) operator but adding a number as a number will produce a different result from adding a number as a string

let x = 10;

x = 10 + 5; // Now x is 15

let y = 10;

y += "5"; // Now y is "105"

Misunderstanding Floats

All programming languages, including JavaScript, have difficulties with precise floating point values

let x = 0.1;

let y = 0.2;

let z = x + y // the result in z will not be 0.3

to solve

let z = (x * 10 + y * 10) / 10; // z will be 0.3

Breaking a JavaScript String

Allowed:

let x =

"Hello World!";

Not allowed:

let x = "Hello

World!";

Allowed:

let x = "Hello \

World!";

Misplacing Semicolon

Breaking a Return Statement

  • a statement is automatically closed at the end of a line

  • allow to break a statement into two lines except for return

Access array with named index

Js doesnt support array with named index, just numbered index

const person = [];

person[0] = "John"; //allowed

person["firstName"] = "John"; //not allowed

Undefined != Null

  • Can test if an object exists by testing if the type is undefined

  • cannot test if an object is null, because this will throw an error if the object is undefined. Instead, test if an object is not undefined, and not null

if (typeof myObj !== "undefined" && myObj !== null)

Performance

Reduce activity in loops

Place statements and assignments outside the loop will make the loop run faster.

let l = arr.length;

for (let i = 0; i < l; i++) {}

Reduce DOM access

Accessing the HTML DOM is very slow, if expect to access a DOM element several times, access it once, and use it as a local variable

const obj = document.getElementById("demo"); obj.innerHTML = "Hello";

Keep the number of elements in the HTML DOM small

Avoid Unnecessary Variables

Replace

let fullName = firstName + " " + lastName; document.getElementById("demo").innerHTML = fullName;

with

document.getElementById("demo").innerHTML = firstName + " " + lastName;

Delay JavaScript Loading

Putting scripts at the bottom, before the body close tag lets the browser load the page first, reduce the page loading time because Js file might be large .

Js ES6

Let

let declares var with block scope

Const

const declares a constant with block scope and unchangeable value

Arrow function

Don't need the function keyword, the return keyword, and the curly brackets in arrow function

const x = (x, y) => x * y;

  • not suitable for defining object method

  • dont have its own this

  • not hoisted

  • use const is safer than varbecause a function expression is always a constant value

  • can only omit the return keyword and the curly brackets if the function is a single statement, so just keep them

const x = (x, y) => { return x * y };

Spread (...) operator

  • When ...arr is used, it ‘expands’ an iterable object arr into the list of arguments.

const numbers = [23,55,21,87,56];

let maxValue = Math.max(...numbers);

The for/of loop

  • Loop through the values of an iterable object

const cars = ["BMW", "Volvo", "Mini"];

let text = "";

for (let x of cars) { text += x + " "; }

Maps

  • Able to use an Object as a key

const fruits = new Map([

["apples", 500],

["bananas", 300],

["oranges", 200] ]);

Sets

Classes

  • JavaScript Classes are templates for JavaScript Objects, not an object

class Car {

constructor(name, year) {

this.name = name;

this.year = year; }

}

  • Can use the class to create objects

const myCar1 = new Car("Ford", 2014);

Promises

An object that links "Producing Code" and "Consuming Code"

"Producing Code" can take some time and "Consuming Code" must wait for the result

Default parameter values

ES6 allows function parameters to have default values

function myFunction(x, y = 10) {

// y is 10 if not passed or undefined

return x + y; }

myFunction(5); // will return 15

Function Rest parameter

The rest parameter (...) allows a function to treat an indefinite number of arguments as an array:

function sum(...args) {

let sum = 0;

for (let arg of args) sum += arg;

return sum; }

let x = sum(4, 9, 16, 25, 29, 100, 66, 77);

String

  • .includes() : The includes() method returns true if a string contains a specified value, otherwise false

  • .startsWith() : returns true if a string begins with a specified value, otherwise false

  • .endsWith() : returns true if a string ends with a specified value, otherwise false

Array

  • .from() : returns an Array object from any object with a length property or any iterable object

Array.from("ABCDEFG") // Returns [A,B,C,D,E,F,G]

  • .keys() : returns an Array Iterator object with the keys of an array

  • .find() : returns the value of the first array element that passes a test function

  • .findIndex() : returns the index of the first array element that passes a test function

Math method

  • Math.trunc(x) : returns the integer part of x

  • Math.sign(x) : returns if x is negative, null or positive

  • Math.cbrt(x) : returns the cube root of x

  • Math.log2(x) : returns the log base 2 logarithm of x

  • Math.log10(x) : returns the log base 10 logarithm of x

New number properties

  • Number.EPSILON; //2.220446049250313e-16

  • Number.MIN_SAFE_INTEGER; //-9007199254740991

  • Number.MAX_SAFE_INTEGER; //9007199254740991

New number method

  • Number.isInteger() : returns true if the argument is an integer

  • Number.isSafeInteger() : returns true if the argument is a safe integer.

New global method

  • isFinite() : returns false if the argument is Infinity or NaN

  • isNaN() : returns true if the argument is NaN

Object entries() method

entries() returns an Array Iterator object with key/value pairs and doesnt change the original array.

Modules

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