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Basic knowledge of JavaScipt objects_Basic knowledge

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JavaScript uses "object-oriented programming", or "object-oriented programming". The so-called "object-oriented programming" means dividing the scope of JavaScript into large and small objects, and continuing to divide objects below the objects until they are very detailed. All programming takes objects as the starting point and is based on objects. As small as a variable, as large as a web page document, window or even screen, they are all objects. This chapter will describe the operation of JavaScript "object-oriented".
Basic knowledge of objects
An object is a small piece that can be divided from the "sphere of influence" of JavaScript. It can be a piece of text, a picture, a form, etc. Each object has its own properties, methods and events. The properties of an object reflect certain specific properties of the object, such as: the length of the string, the length and width of the image, the text in the text box (Textbox), etc.; the method of the object can do some things to the object, for example, The "Submit" of the form, the "Scrolling" of the window, etc.; and the events of the object can respond to things that happen on the object. For example, submitting a form generates a "Submit event" of the form, and clicking a connection generates a "Submit event" of the form. click event". Not all objects have the above three properties, some have no events, and some only have attributes. To reference any "property" of an object, use the method ".".

Basic objects
Now we have to review what we have learned above - re-learn some data types from the perspective of objects.
Number “Number” object. This object is rarely used and the author has never seen it once. However, there are many objects belonging to "Number", that is, "variables".

Attribute
MAX_VALUE Usage: Number.MAX_VALUE; Returns "maximum value".
MIN_VALUE Usage: Number.MIN_VALUE; returns the "minimum value".
NaN usage: Number.NaN or NaN; returns "NaN". "NaN" (not a number) was introduced very early on.
NEGATIVE_INFINITY Usage: Number.NEGATIVE_INFINITY; Return: negative infinity, a value smaller than the "minimum value".
POSITIVE_INFINITY Usage: Number.POSITIVE_INFINITY; Returns: positive infinity, a value greater than the "maximum value".
Method
toString() Usage: .toString(); Returns: Numeric value in string form. For example: if a == 123; then a.toString() == '123'.

String String object. The simplest, quickest, most effective, and commonly used way to declare a string object is to assign it directly.

Attribute
length Usage: .length; Returns the length of the string.
Method
charAt() Usage: .charAt(); Returns the single character of the string located at the position. Note: One character in the string is at position 0, the second character is at position 1, and the last character is at position length - 1.
charCodeAt() Usage: .charCodeAt(); Returns the ASCII code of a single character located at the position of the string.
fromCharCode() usage: String.fromCharCode(a, b, c...); returns a string, the ASCII code of each character in the string is determined by a, b, c... etc.
indexOf() usage: .indexOf([, ]); This method is derived from Search for (if is given, ignore the previous position), if found, return its position, if not found, return "-1". All "positions" start from scratch.
lastIndexOf() usage: .lastIndexOf([, ]); similar to indexOf(), but starting from the back try to find.
split() usage: .split(); returns an array, which is separated from the ,< The separator character > determines where to separate and is not itself included in the returned array. For example: '1&2&345&678'.split('&') returns array: 1,2,345,678. Regarding arrays, we will discuss them in a moment.
substring() Usage: .substring([, ]); Returns the substring of the original string, which is the original string starting from < The segment from the > position to the previous position of the position. - = Returns the length of the string (length). If is not specified or exceeds the string length, the substring is taken from the position to the end of the original string. If a string cannot be returned at the specified location, an empty string is returned.
substr() usage: .substr([, ]); returns the substring of the original string, which is the substring of the original string. A section that starts at the position and has a length of . If is not specified or exceeds the string length, the substring is taken from the position to the end of the original string. If a string cannot be returned at the specified location, an empty string is returned.
toLowerCase() Usage: .toLowerCase(); returns a string that changes all uppercase letters of the original string into lowercase.
toUpperCase() Usage: .toUpperCase(); returns a string that converts all lowercase letters of the original string into uppercase.

Array array object. An array object is a collection of objects, and the objects inside can be of different types. Each member object of the array has a "subscript" used to indicate its position in the array (since it is a "position", it also starts from zero).
How to define an array:

var = new Array();

This defines an empty array. To add array elements in the future, use:

[] = ...;

Note that the square brackets here are not "can be omitted" means that the subscript of an array is expressed by enclosing it in square brackets.
If you want to initialize the data directly when defining the array, please use:

var = new Array(, ,
For example, var myArray = new Array(1, 4.5, 'Hi'); defines an array myArray, and the elements inside are: myArray[0] == 1; myArray[ 1] == 4.5; myArray[2] == 'Hi'.
However, if there is only one element in the element list, and this element is a positive integer, this will define an array containing empty elements.
Note: JavaScript only has one-dimensional arrays! Never use the stupid method "Array(3,4)" to define a 4 x 5 two-dimensional array, or use the method "myArray[2,3]" to return the elements in a "two-dimensional array" . Any call of the form "myArray[...,3]" actually only returns "myArray[3]". To use a multidimensional array, use this virtual method:

var myArray = new Array(new Array(), new Array(), new Array(), ...);

In fact, this is a one-dimensional array, and each element in it is an array. When calling the elements of this "two-dimensional array": myArray[2][3] = ...;

property
length Usage: .length; Returns: the length of the array , that is, how many elements there are in the array. It is equal to the index of the last element in the array plus one. So, if you want to add an element, just: myArray[myArray.length] = ....
Method
join() usage: .join(); returns a string that strings together the elements in the array, separated by < The symbol > is placed between elements. This method does not affect the original contents of the array.
reverse() usage: .reverse(); reverses the order of elements in the array. If you use this method on the array [1, 2, 3], it will change the array to: [3, 2, 1].
slice() usage: .slice([, ]); returns an array, which is a subset of the original array, starting from , finally . If is not given, the subset is taken until the end of the original array.
sort() usage: .sort([]); arranges the elements in the array in a certain order. If is not specified, alphabetical order is used. In this case, 80 is ranked higher than 9. If is specified, the sorting method specified by is sorted. are difficult to describe. Here we only introduce some useful to you.
Arrange numbers in ascending order:

function sortMethod(a, b) {
return a - b;
}

myArray.sort(sortMethod);

Sort the numbers in descending order: put the above "a - b" should become "b - a".
For functions, please see below.

Math "Math" object, providing mathematical calculations on data. The properties and methods mentioned below will not explain the "usage" in detail. When using them, please remember to use the format "Math.".

Attribute
E returns the constant e (2.718281828...).
LN2 returns the natural logarithm of 2 (ln 2).
LN10 returns the natural logarithm of 10 (ln 10).
LOG2E Returns the logarithm of e with 2 as the lowest value (log2e).
LOG10E returns the logarithm of e with 10 as the lowest value (log10e).
PI returns π (3.1415926535...).
SQRT1_2 returns the square root of 1/2.
SQRT2 returns the square root of 2.
Method
abs(x) returns the absolute value of x.
acos(x) returns the inverse cosine of x (the cosine is equal to the angle of x), expressed in radians.
asin(x) returns the arcsine of x.
atan(x) returns the arctangent of x.
atan2(x, y) returns the argument angle of the complex number corresponding to the point (x, y) in the complex plane, expressed in radians, and its value is between -π and π.
ceil(x) returns the smallest integer greater than or equal to x.
cos(x) returns the cosine of x.
exp(x) returns e raised to the x power (ex).
floor(x) returns the largest integer less than or equal to x.
log(x) returns the natural logarithm of x (ln x).
max(a, b) returns the larger number in a, b.
min(a, b) returns the smaller number of a, b.
pow(n, m) returns n raised to the m power (nm).
random() returns a random number greater than 0 and less than 1.
round(x) returns the rounded value of x.
sin(x) returns the sine of x.
sqrt(x) returns the square root of x.
tan(x) returns the tangent of x.

Date date object. This object can store any date from 0001 to 9999, and can be accurate to milliseconds (1/1000 of a second). Internally, a date object is an integer that is the number of milliseconds since midnight, January 1, 1970, to the date pointed to by the date object. If the referenced date is earlier than 1970, it is a negative number. All dates and times, if no time zone is specified, use the "UTC" (Universal Time) time zone, which is numerically the same as "GMT" (Greenwich Mean Time).
Define a date object:

var d = new Date;

This method makes d a date object and has an initial value: the current time. If you want to customize the initial value, you can use:

var d = new Date(99, 10, 1); //October 1, 1999
var d = new Date('Oct 1, 1999'); // October 1, 1999

and other methods. The best way is to strictly define time using the "method" introduced below.

Methods
There are many methods like "g/set[UTC]XXX" below, which means there are both "getXXX" method and "setXXX" method. "Get" is to get a certain value, and "set" is to set a certain value. If there is a "UTC" letter, it means that the value obtained/set is based on UTC time, if not, it means that it is based on local time or the default time of the browsing period.
If there is no description, the usage format of the method is: ".", the same below.
g/set[UTC]FullYear() Returns/sets the year, represented by four digits. If "x.set[UTC]FullYear(99)" is used, the year is set to 0099.
g/set[UTC]Year() returns/sets the year, expressed as two digits. When setting, the browser automatically adds "19" at the beginning, so use "x.set[UTC]Year(00)" to set the year to 1900.
g/set[UTC]Month() returns/sets the month.
g/set[UTC]Date() returns/sets the date.
g/set[UTC]Day() returns/sets the day of the week, 0 means Sunday.
g/set[UTC]Hours() returns/sets the number of hours in 24-hour format.
g/set[UTC]Minutes() returns/sets the minutes.
g/set[UTC]Seconds() returns/sets the number of seconds.
g/set[UTC]Milliseconds() returns/sets the number of milliseconds.
g/setTime() returns/sets the time, which is the internal processing method of the date object: the number of milliseconds calculated from 0:00 on January 1, 1970 to the date pointed to by the date object. If you want to delay the time pointed by a date object by 1 hour, use: "x.setTime(x.getTime() 60 * 60 * 1000);" (one hour 60 minutes, one minute 60 seconds, one second 1000 milliseconds ).
getTimezoneOffset() returns the number of minutes difference between the time zone used by the date object and Greenwich Mean Time. In urban areas east of Greenwich, the value is negative, for example: China time zone (GMT 0800) returns "-480".
toString() returns a string describing the date pointed to by the date object. The format of this string is similar to: "Fri Jul 21 15:43:46 UTC 0800 2000".
toLocaleString() returns a string describing the date pointed to by the date object, expressed in local time format. For example: "2000-07-21 15:43:46".
toGMTString() returns a string describing the date pointed to by the date object, in GMT format.
toUTCString() returns a string describing the date pointed to by the date object, in UTC format.
parse() Usage: Date.parse(); Returns the internal expression of the date object.



Global object
The global object never appears. It can be said to be virtual, with the purpose of "objectifying" the global function. In the Microsoft JScript Language Reference, it is called "Global Object", but its methods and properties are never referenced with "Global.xxx" (and doing so will cause an error), but directly with "xxx".

Attribute
NaN I said it early.
Method
eval() operates the string enclosed in brackets as a standard statement or expression.
isFinite() returns true if the number in the brackets is "finite" (between Number.MIN_VALUE and Number.MAX_VALUE); otherwise it returns false.
isNaN() returns true if the value in the brackets is "NaN" otherwise returns false.
parseInt() returns the value after converting the content in parentheses into an integer. If the brackets are a string, the numeric part at the beginning of the string is converted to an integer, if it starts with a letter, "NaN" is returned.
parseFloat() returns the value after converting the string in parentheses into a floating point number. The number at the beginning of the string is converted into a floating point number. If it starts with a letter, "NaN" is returned.
toString() Usage: .toString(); Convert the object into a string. If you specify a value in parentheses, all values ​​are converted to the specific base during the conversion process.
escape() returns the new string encoded by the string in the brackets. This encoding is applied to the URL, that is, spaces are written in the format of " ". " " is not encoded. If you want " " to be encoded, please use: escape('...', 1).
unescape() is the reverse process of escape(). Decode the string in parentheses into a normal string.


Definition of function
The so-called "function" is an object or method of an object that returns a value.
Types of functions
Common functions include: constructors, such as Array(), which can construct an array; global functions, which are methods in global objects; custom functions; etc.
Custom function
Use the following statement to define a function:

function function name ([parameter set]) {
...
[return[ ]; ]
...
}

Among them, the braces used after function and at the end of the function cannot be omitted, even if the entire function has only one sentence.
Function names have the same naming rules as variable names, that is, they only contain letters, numbers, underscores, leading letters, and cannot be repeated with reserved words, etc.
The parameter set is optional, but the parentheses must be present.
Parameters are a bridge that transmits information from outside the function to inside the function. For example, if you want to ask a function to return the cube of 3, you have to let the function know the value "3". At this time, there must be a variable to receive the value. This Variables are called parameters.
A parameter set is a set of one or more parameters separated by commas, such as: a, b, c.
There are one or more lines of statements inside the function. These statements will not be executed immediately, but will only be executed when other programs call it. These statements may contain "return" statements. When executing a function, when a return statement is encountered, the function immediately stops execution and returns to the program that called it. If "return" is followed by , the value will be returned when exiting the function.
Inside the function, parameters can be used directly as variables, and var statements can be used to create new variables, but these variables cannot be called by procedures outside the function. To make the information inside the function available for external calls, either use a "return" return value or use a global variable.
Global variables The variables defined by the "var" statement at the "root" (not inside the function) of Script are global variables, which can be called and changed anywhere in the entire process.
Example

function addAll(a, b, c) {
return a b c;
}

var total = addAll(3, 4, 5);

This example creates a function called "addAll", which has 3 parameters: a, b, c. Its function is to return the result of adding three numbers. Outside the function, use "var total = addAll(3, 4, 5);" to receive the return value of the function.
More often, functions have no return value. This kind of function is called "procedure" in some languages ​​that emphasize strictness, such as "Sub" in Basic language and "procedure" in Pascal language.

Attribute
arguments An array that reflects the parameters specified when an external program calls a function. Usage: Call "arguments" directly inside the function.

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