1. Basic package type
To facilitate the operation of basic type values, ECMAScript provides three special reference types (basic wrapper types): Boolean, number and string.
Whenever a basic type value is read, a corresponding basic wrapper type object will be created in the background, so that some methods can be called to operate the data.
//1. Basic data types -- variables and values are stored in the stack -- no methods and properties can be called
var str='hello'; //string type
str.split("");
//Auto boxing and auto unpacking when a basic data type can call a method, the basic data type is packaged into a reference data type
str.split("");
//The basic data type has no methods and properties to call. When it can be called, it is not the basic data type, but the wrapped String type -- js automatic help
//The method in String.prototype -- automatic boxing becomes the basic data string type after calling -- automatic unpacking
Why is the basic data type not an object but can call the method of the object?
var str='hello';
str.split("");
//Background js will execute automatically
//1. Create an instance of String type -- auto boxing
//2. Call the specified method on the instance
//3. Destroy this instance -- the string type of automatic unpacking has changed to string type
Call the basic wrapper type constructor with new
It is different from directly calling a conversion function with the same name
var str='100';
var str=Number(str);//Converter to convert other data types to number type
var str =new Number();//Once you use new, you create an instance Number instance that references a data type
Difference between reference type and basic wrapper class
- Reference type: the reference type instance created with the new operator is always saved in memory before the execution flow leaves the current scope
- Basic packing type: there is only one line of code at the moment of execution, and then it is destroyed immediately
2. String type properties and methods
| method | describe |
|---|
| charAt(index) | Returns the character at the specified index position |
| charCodeAt(index) | Returns the character at the specified index position in Unicode encoding |
| method | describe |
|---|
| concat() | Connection string, concat(str1,str2... strx) |
| slice() | Method extracts a part of a string and returns the extracted part as a new string. slice(start,end) does not include end. Start and end can be negative numbers. If there is no end, extract the rest |
| substring() | Extracts the character between two specified index numbers in a string. substring(start,end) does not include end. Start and end cannot be negative numbers. If there is no end, extract the rest |
| substr() | Extracts the specified number of characters from the string from the starting index number. substr(start,length) |
| method | describe |
|---|
| indexOf(str,n) | Search the first str from n and return the index value of the search |
| lastIndexOf(str,n) | The last str to start the search from n and return the index value of the search |
| method | describe |
|---|
| toLowerCase() | Converts a string to lowercase. |
| toUpperCase() | Converts a string to uppercase. |
| toLocaleLowerCase() | Converts a string to lowercase. localization |
| toLocaleUpperCase() | Converts a string to uppercase. localization |
-
Pattern matching method of string
| method | describe |
|---|
| match() | A match was found for one or more regular expressions. |
| replace() | Replace substrings that match regular expressions. |
| search() | Retrieves the value that matches the regular expression. |
| split() | Splits a string into an array of strings. |
3. Math object
//Common properties
Math.PI //Returns the PI (approximately equal to 3.14159).
Math.sqrt //Return square root
//Common methods
Math.min() //Find the minimum of a set of numbers
Math.max() //Find the maximum value in a set of numbers
Math.ceil() //Round up
Math.floor() //round down
Math.round() //rounding
Math.random() //Returns a random number greater than 0 but less than 1 [0, 1)
4. Date object
Creating Date Objects
var myDate = new Date();
console.log(myDate); //2021-09-02T11:52:31.903Z
//The output is different in node environment and browser environment
console.log(myDate.getFullYear()); //2021
Common Date object methods
getFullYear() //Return year, such as 2020
getMonth() //Returns the number of months in the date. The return value is 0 (January) - 11 (December)
getDate() //Returns the number in the date object.
getHours() //Returns the hour in the date
getMinutes() //Returns the number of minutes in a date
getSeconds() //Returns the number of seconds of a date
getDay() //Returns the day of the week in the date
getMilliseconds() //Returns the number of milliseconds in a date
getTime() //Returns a date object in milliseconds