Basic knowledge of JavaScript-variables, data types
What is JavaScript?
JavaScript is a literal scripting language, a dynamically typed, weakly typed, prototype-based language. Popularity: JS is the most important part of the front-end code (behavior layer). It is commonly used to manipulate HTML pages , respond to user operations , and verify data transmission .
Declare variable
Use letor vardeclare variables, which can be used to store various types of data for subsequent use.
Variable naming rules
Strictly case sensitive;
Can only contain alphanumeric _ $ , can not start with a number;
Cannot use keywords/reserved words;
Unless needed, do not overwrite the existing API;
See the name knows the meaning.
The difference between var and let:
**var:** When the page loads, there are two processes: 1. First define the stage
Will find the variables defined by var in the current scope (only define variables, not assign values in advance)
2. Execution stage
Start from the beginning and execute the code down
1) It is not regarded as a scope in if, for, while···. Variables defined in these are equal to global variables, but only as a scope in the function() function.
console.log(a); //undefined
if(false){
var a = 10;
}
console.log(a); //undefined
a:是全局变量
2) When defining the same variable, the latter will overwrite the former.
**let: **1) Not only the function is regarded as a scope, but also as a scope within if, for, ···, the variables defined are not global variables
2) Undefined variables cannot be used in advance
console.log(a);
let a = 10;
3) The same scope cannot let the same variable
let a = 1;
let a = 2;
type of data
The data types are divided into:
Basic data types: number, string, boolean, undefined, null
Complex data types: reference data types
Basic data type
number type
Number types include integers, decimals (floating point numbers)
Special: NAN is a number type
let a = 10;
let b = 1.1;
string string type
String types are enclosed in single quotes, double quotes, and template string brackets.
The brackets can be Chinese, English, numbers, and symbols, but the single quotes, double quotes, and template strings that define the string type cannot be nested. If you must add them, you need to use escape symbols.
let a = "中文";
let b = 'english';
let c = `111`;
let d = "中"国""; //这种写法错误
let f = "中\"国\""; //正确写法
boolean Boolean type
The Boolean type has only true and false: true and false;
let a = false;
let b = true;
undefined undefined type
An undefined type is just a declaration of unassigned. Not pointing to anything
let a ;
let b = undefined;
null pointer
I don't know what the value of the variable is at the beginning, but it is certain that the variable is stored as an object type
let a = null;
Reference data type
Object type
Array
An array is a piece of data, which is used to store a group of data
let a = [];//定义
let b = [100,"数组",1,true];
//取数据，数组里面的数都有一个下标，利用下标取数据，从0开始
console.log( b[2] ); //结果是1
console.log( b.length ); //取长度，为4
//给某一项重新赋值
a[0] = 20;
Object object data
Object: the description of a thing
Two attribute names cannot be the same, the same will be overwritten;
The data in the object has no order.
let a = {
//(属性)键 : 值
name : "姓名",
age : 20,
height : "175cm",
};
//取值
console.log(a.name); //方法一
console.log(a["name"]); //方法二，[]里面表示一个数据，没有加“”号表示是一个变量
//修改某个属性
a.age = 22;
//删除某个属性
delete a.age;
//添加新属性
a.tel = "12345678901";
Same and different with array:
Same: can store different data; can store multiple data
Different: There is a description for each data, called attribute
Nested value
let a = {
//(属性)键 : 值
name : "姓名",
age : 20,
height : "175cm"
friend:{
name : "名字",
hobby : ["唱歌","画画","弹琴"]
}
};
//取数据
console.log( a.friend.hobby[2]); //弹琴
function function
Encapsulate a bunch of code so that it will not be called immediately after the browser is run, and will be executed unless the function is called
function a (){
//定义一个函数，a为函数名
}
//函数调用
a();
Built-in object node object
It has been defined in JavaScript in advance and can be obtained without definition.
For example: window, document
typeof operator
Used to detect the type of basic data, the return is a string
console.log( typeof 10 ); //number
console.log( typeof "10" ); //string
console.log( typeof true ); //boolean
console.log( typeof undefined ); //undefined
console.log( typeof null ); //特殊：返回的是object
console.log( typeof [] ); // object
console.log( typeof {} ); //object
console.log( typeof function a(){} ); //返回function，但这个是函数，属于的是object类型
The difference between value type and reference data type
Value type
Value type refers to the value of primitive type, also called basic type.
Data type: number, string, boolean, null, undefined
Stored in the stack, occupies a fixed memory space, and is destroyed after use
Assignment method: 1) Copy the value and create a new object
2) Save and copy are the value itself
3) The two pieces of data are completely independent in memory
Immutable value
The scope is in the scope of the function. It takes effect when the function is modified inside, and it becomes invalid when the function is destroyed.
The comparison of objects and objects is: comparison of values
Type detection: typeof operator
Reference type
The reference value is the value of the guiding type.
Data type: object, function, array, data, regexp
Stored in the heap, it occupies an unstable memory space, and may not be destroyed after use. Only when an object has no references, the system's garbage collection mechanism will recycle and destroy it.
Assignment method: 1) Copy of reference, create a new reference
2) Save and copy is a pointer to the object
3) The object constructed using the new() method is a reference type
Variable value
The scope is the value in the runtime data area that is modified when the function is modified. Even if the function is destroyed, the value of the variable is still changed.
The comparison of objects and objects is: comparison of addresses
Type detection: instanceof operator