JS basic Syntax value type (data type) (variable type)

I. Non-changing raw values (stack data) (five)

Number, String (string), Boolean value (Boolean), Undefined,null

The undefined is undefined meaning, and null is empty meaning, the difference between them is that null has value, but this value is null, and undefined is undefined, there is no value at all. Null is generally used as a placeholder.

var a = 4; var b == ten; Console.log (b); // 4 // The original value is put a value A in another value B inside, changed the first value A, the second value B is unchanged

Two. Reference value (heap data)

Arrays (array), objects (object), Functions (function)

Note: The memory will be re-applied as long as the variable is assigned

1.

var arr = [1, 2]; var arr1=Arr;arr.push (3)//[1, 2, 3]//  When referencing a value, place the first value inside the second value, change the first value, and the second value changes
The reference value is the address of the heap in the stack memory, the copy is the address, so changed arr, actually changed the address that arr points to the contents of the store, and Arr and arr1 point to the same address, so arr1 also changed

Address	Variable	Store value
0x0001	Arr	[All ] [<--arr.push] (3)
0x0002	Arr1	0x0001

2.

 var  arr = [1,2 var  arr1 = Arr;arr  = [1,3];console.log (arr1);  //    [1,2];  // arr = [1,3] actually built another address within the heap, and arr points to the new address, The arr1 still points to the original address, so the ARR1 value is unchanged  

address	variable	storage value
0x0001	a rr	[1,2]
0x0002	arr1	0x0001
0x0003	arr	[1,3]

3.

var a = {N:1}; var b = A;A.N= a={m:1};console.log (a); // {m:1}Console.log (b); // {N:{m:1}}

1.var a = {N:1}, request two addresses in memory, storing A and n respectively;

2.var B = A; Apply for an address that holds B, and the value points to the address of A;

3.a.n= a={m:1};

(1) (the variable is assigned, so to re-request memory) first requested memory address N (0x0004) and A (0x0005), because the N value occupies a new memory, so 0x0001 storage value from n:0x0002 to n:0x0004;

(2) Assign value; a={m:1}, Request M address (0x0006),

Therefore: A: {m:0x0006}--->{m:1}

B: {0x0001}--->{n:0x0004}---->{n:0x0005}---->{n:{m:0x0006}}--->{n:{m:1}}

Address	Variable	Store value
0x0001	 A    (1)	n:0x0002    n:0x0004 Span style= "font-family: Song body; font-size:10.5pt; Color: #ff0000 "> ---(1)
0x0002	- N (1)	1
0x0003	B	0x0001
0x0004	n (1)	0x0005
0x0005	A (1)	M:0X0006 (2)
0x0006	m (2)	1 (2)

JS basic Syntax value type (data type) (variable type)