Some supplementary summary about JavaScript functions and methods.

Supplementary explanation of data types, constructors, prototypes, and prototype chains

I. Data Types#

1. 5 Primitive Types#

string
number
boolean
undefined
null
symbol

The most common ones are strings, numbers, and booleans.

2. Common Reference Types#

Reference types are data structures used to organize data and functionality.

Object - Object, Array - Array, Function - Function, RegExp - RegExp, Date - Date, etc.

II. Functions#

1. What is a Function?#

A code block with independent functionality, defined using the function keyword in JavaScript.
Makes the code structure clearer and improves code usability.
Categories of JavaScript functions: custom functions and built-in functions.

2. Custom Functions#

There is a type of function called an anonymous function, which is a function without a name. It creates closures to avoid polluting the global scope.

Anonymous self-invoking function

Concept: Immediately executes the definition of the anonymous function, executing the function expression.
Purpose: Implement closures and create independent namespaces.
Usage: Grouping operator (), void operator, ~ operator, ! operator, etc.
Use cases: function expressions, object properties, events, event parameters, return values.
After defining an anonymous function, it must be called.

// Function expression
window.onload = function() {
    let funcobj = function() {
        alert("Anonymous function in function expression")
    }
    funcobj();
}

// Object property
window.onload = function() {
    let obj = {
        name: function() {
            alert('Anonymous function in object property')
        }
    }
    obj.name();
}

3. Built-in Functions#

Functions placed under the global scope are called functions, while functions placed inside objects are called methods, which are object methods.

Regular Functions

alert() // Displays an alert box
confirm() // Displays a confirmation box
prompt() // Displays an input box
isNaN() // Checks if a value is NaN
parseInt() // Converts a string or floating-point number to an integer
parseFloat() // Converts a string to an integer or floating-point number
eval() // Evaluates the result of an expression

Array Functions

Traversing an array using a for loop

var arr = [1,2,3,4];
for(var i = 0; i<arr.length; i++) {
    console.log(arr[i])
}

// Output: 1,2,3,4

Traversing an array using for...in

var arr = [1,2,3,4];
for (var i in arr) {
    console.log(arr[i]);
}

// Output: 1,2,3,4

// Adding elements
unshift() // Adds elements to the beginning of an array and returns the new length of the array
push() // Adds elements to the end of an array and returns the new length of the array
concat() // Joins two arrays and returns the resulting array
    var arr1 = [1];
    var arr2 = [2];
    let arr = arr1.concat(arr2);
    console.log(arr); // [1,2]

/*****************************************/
// Removing elements
pop() // Removes the last element of an array and returns the removed element
shift() // Removes the first element of an array and returns the removed element
splice(a,b) // Removes b elements starting from position a in the array and returns the removed elements
slice(a,b) // Removes elements between positions a and b in the array

/*****************************************/
// Searching
indexOf() // Checks if an array contains a specified element. Returns the element if it exists, or -1 if it doesn't
includes() // Checks if an array contains a specified element. Returns true if it exists, or false if it doesn't

/*****************************************/
// Others
sort() // Sorts the array according to a specified rule
    var arr1 = [4,5,6];
    var arr2 = [1,2,3];

    var arrAscSort = arr5.sort((a, b) => a-b); // Ascending sort
    console.log(arrAscSort); // [ 1, 1, 2, 3, 5, 6 ]
    
    var arrDescSort = arr5.sort((a, b) => b-a); // Descending sort
    console.log(arrDescSort); // [ 6, 5, 3, 2, 1, 1 ]

reverse() // Reverses the array
    var arr = [1,2,3,4,5,6];
    // Directly calling the reverse() method
    console.log(arr.reverse())  // [6,5,4,3,2,1]

Array.from() // Converts a string of characters into an array
    var str = 'Converts a string of characters into an array'
    console.log(Array.from(str))
    // ["C", "o", "n", "v", "e", "r", "t", "s", " ", "a", " ", "s", "t", "r", "i", "n", "g", " ", "o", "f", " ", "c", "h", "a", "r", "a", "c", "t", "e", "r", "s", " ", "i", "n", "t", "o", " ", "a", "n", " ", "a", "r", "r", "a", "y"]

Array.isArray() // Checks if a variable is an array
    var str = 'Converts a string of characters into an array'
    console.log(Array.isArray(str))
    // false

Date Functions Date()

Getting the time

var time = new Date()
// Get the current time
// Fri Nov 13 2020 20:21:35 GMT+0800 (China Standard Time)

getFullYear() // Get the current year
getMonth() // Get the current month (0-11)
getDate() // Get the current day of the month
getHours() // Get the current hour
getMinutes() // Get the current minute
getSeconds() // Get the current second
getMilliseconds() // Get the current millisecond
getTime() // Get the timestamp - the number of milliseconds since January 1, 1970

Setting the time

setYear() // Set the year
setMonth() // Set the month
setDate() // Set the day of the month
setHours() // Set the hour
setMinutes() // Set the minute
setSeconds() // Set the second

Math Functions

Mainly used mathematical functions and methods

Math.abs() // Absolute value
Math.ceil() // Rounds up to the nearest integer
Math.floor() // Rounds down to the nearest integer
Math.round() // Rounds to the nearest integer
Math.random() // Generates a random number between 0 and 1

Extended usage of random() - generating a random number with a specified number of digits

function getRandomNumber(min, max){
    return Math.floor(Math.random()*(max - min)) + min;
}
console.log(getRandomNumber(1000, 9999));

String Functions

indexOf() // Searches for a string and returns the index value
    var arr = ["C", "h", "a", "r", "a", "c", "t", "e", "r", " ", "F", "u", "n", "c", "t", "i", "o", "n", "s"]
    console.log(arr.indexOf("F"))  // 10

split() // Splits a string into an array of substrings using a specified separator
    var str = "Character Functions"
    console.log(str.split(''))      // ["C", "h", "a", "r", "a", "c", "t", "e", "r", " ", "F", "u", "n", "c", "t", "i", "o", "n", "s"]

trim() // Removes whitespace from both ends of a string
    var str = "   Character Functions   "
    console.log(str)        // [   Character Functions   ]
    console.log(str.trim()) // [Character Functions]

match() // Searches for a value according to a specified pattern
    var str = "Character Functions"
    console.log(str.match(/Character/))    // Character
    document.write(str.match(/Character/)) // Character

search() // Returns the position of the first occurrence of a specified value in a string
    var str = "Character Functions"
    console.log(str.search('F'))   // 10

replace() // Replaces a specified string with another string
    var str = "Character Functions"
    console.log(str.replace('C','F'))
    // Fharacter Functions

substring(a,b) // Extracts the characters from a string, between two specified positions (left-closed, right-open)
    // Extracts from index a to b
    var str = "Character Functions"
    console.log(str.substring(0,3)) // Cha

substr(a,b) // Extracts a specified number of characters from a string, starting from a specified position
    // Extracts b characters starting from index a
    var str = "Character Functions"
    console.log(str.substr(0,3))    // Cha
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