Day 34 - 9/10

Plan: Continue learning JavaScript

Progress:

Data types:

• Js types are dynamic, which means the same var can have different data types

• when add number and string, the number will be treated as string; caculation still be done if there is no string before that

• array: const name = [a, b, c];

• objects: const name = {a, b, c}; : works like struct

• undefined: a var without value hold value undefined and type undefined; string without value hold value "" and type undefined

Function

• function name(parameter1, parameter2, parameter3) { // code to be executed }

• Accessing a function without () returns the whole function, not the result: let value = toCelsius;

• Functions can be used the same way as variables, in all types of formulas, assignments, and calculations.

Objects

• const name = {property:property name, ... }

• A method is a function stored as a property.

• this keyword

• do not declare Strings, numbers, and booleans as object

Events

• Js event is how Js react when sth happen to Html elements (click a button, web page finished loading, ...)

• event are commonly used in function rather than directly used

String

• let string = string.length;

• turn special characters to string: \' ~ ' ; \" ~ "

String method

Extract string parts:

• slice(start, end); :

  • if 'end' is omitted, the method will slice out from 'start' to the rest

  • if 'end' is omitted, 'start' is negative, the position will be counted from the end

  • 'end' and 'start' is negative works like positive

• substring(start, end); : works as slice but negative values are treated as 0

• substr(start, length); similar to slice

Replace string

let text = "Please visit Microsoft!"; let newText = text.replace("Microsoft", "W3Schools");

• replaces a specified value with another value in a string; returns a new string, does not change the string it is called on; and only replace the first match; case sensitive

• use /specified value/i flag to remove case sensitive

• use /specified value/g flag to replace all matches

• text = text.replaceAll("Cats","Dogs");

Uppercase / Lowercase

• toUpperCase() : the whole string to uppercase

• toLowerCase() ; the whole string to lower case

Concat

• join 2 or more string

let text3 = text1.concat(" ", text2);

Trim

• remove whitespace from head and tail

• trimStart() / trimEnd() : remove whitespace from head or tail only

String padding

• padStart() / padEnd(): pad head or tail with character "y" until reach length x

let padded = text.padStart(x,"y");

• to padding number, it needed to convert to string first

let numb = 5;

let text = numb.toString();

let padded = text.padStart(4,"0");

Extracting characters

• charAt(position) : return the character

• charCodeAt(position) : return the UTF-16 code of the character

String search

• indexOf() return the first position of a string in a string; search forward

• lastIndexOf() return the last position of a string in a string; search backward

• Both indexOf(), and lastIndexOf() return -1 if the text is not found

• a second parameter is added to declare the position to start searching:

text.lastIndexOf("locate", 15);

• search()

String match

• .match("string"); : returns an array containing the results of matching a string against a string

• .match(/ain/gi); : use /g , /i flag to find all matches despite uppercase or lowercase

• .matchAll() : return every matches in order

• .includes("specified value", position) : return true / false whether the value is included or not, search from position forward (optional); case sensitive

• .startsWith() : return True / false; check if the string starts with specified value or not; a second parameter to set the start position (optional); case sensitive

• .endsWith()

Numbers

Integer Precision

• Integers (numbers without a period or exponent notation) are accurate up to 15 digits

Floating Precision

• Floating point arithmetic is not always 100% accurate; to solve this, multiply the parameter, calculate then divide

Not a Number

• do arithmetic with a non-numeric string result in NaN

• isNaN() check a value is or isn't a number

• typeof NaN is number

Infinity (or -Infinity)

• the value return when calculate a number outside the largest possible number

• Infinity is a number

Hexadecimal

• hexadecimal constant preceded by 0x

• .toString() : to convert number among bases (base 2, base 10, base 8, ...)

Numbers as Objects

• numbers can be defined as objects but DON'T

• objects can't be compared with objects

• compare 2 objects alway return false

Number Method

• .toString() : convert number to string

• .toExponential(x) : rounded number to x numbers behind decimal point

• .toFixed(x) : number of decimal

• .toPrecision(x) : number with length x

• .valueOf() : return a number as a number

• Number() : convert JavaScript variables to numbers

• parseInt() : parse a string and return the first number, float not included

• parseFloat() : parse a string and return the first number