There are several types of JavaScript variables

JavaScript variables have 9 types: 1. string type; 2. number type; 3. boolean type; 4. null type; 5. undefined type; 6. Object type; 7. Array type; 8. Function type; 9. Symbol type.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

What are the variable types in JavaScript?

• Value type: string, number, boolean, null, undefined

• ## Reference types: Object, Array, Function

• ES6 introduces a new primitive data type: Symbol, which represents a unique value.

  
  

The difference between value type and reference type

Value type:

• The occupied space is fixed and stored in the stack (when a method is executed, each method will establish its own memory stack, and the variables defined in this method will be put into this stack memory one by one. As When the execution of the method ends, the memory stack of this method will be naturally destroyed. Therefore, all variables defined in the method are placed in the stack memory; the stack stores basic variables and reference variables of some objects. Basic variables The value is stored on the stack, while the reference variable stored on the stack points to the address of an array or object in the heap. This is why modifying the reference type will always affect other reference variables pointing to this address.

• What is saved and copied is the value itself

• Use typeof to detect the type of data

• Basic type data is a value type

Reference type:

• The occupied space is not fixed and is saved in the heap (when we create a When an object is created, the object will be saved to the runtime data area for repeated use (because the creation cost of the object is usually large). This runtime data area is the heap memory. The objects in the heap memory will not be deleted when the method ends. Destroyed, even after the method ends, the object may be referenced by another reference variable (very common when passing method parameters), the object will still not be destroyed. Only when an object does not have any reference variables referring to it, the system The garbage collection mechanism will recycle it during verification.

• What is saved and copied is a pointer to the object

• Use instanceof to detect Data type

• The object constructed using the new() method is a reference type

// 值类型：Number、string、bollean、undefined
var a = 100
var b = a
a = 200
console.log(b) // 100


// 引用类型：对象、数组、函数、null(空指针)
// 可以扩展属性
var a = {age:20}
var b = a
b.age = 21
console.log(a.age) // 21

typeof undefined // undefined
typeof 'abc' // string
typeof 123 // number
typeof true // boolean
// typeof 区分不出来引用类型（除了函数）
typeof {} // object
typeof [] // object
typeof null // object
typeof console.log //function

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