Detailed explanation of Javascript assignment mechanism_Basic knowledge

Today I answered a question about Javascript, which involved assignment issues, so I wanted to summarize this question well.

Copy code The code is as follows:

var a = 'test';
var b = function() {};
b.a = 'test';
function change(m, n) {
m = 'change';
n.a = 'change';
}
change(a, b);

After executing the above code, will the values ​​of variables a and b change?

Original and reference values

In the previous article, we introduced original values ​​and reference values. Original values ​​refer to Undefined, Null, Boolean, Number, String, etc. They are stored in the stack, while reference values ​​are integrated from Object and are stored in the heap. middle.
Here we need to clearly distinguish between the two:

Copy code The code is as follows:

var a = 'test';
var b = new String('test');
var A = 'true';
var B = new Boolean('true');

For the above four variables, a and A are original values, while b and B are reference values.

Assignment mechanism

After knowing the difference between original value and reference value, we can introduce the assignment mechanism of Javascript in detail:

In Javascript, for variables of primitive value types, each assignment will generate a copy, while for reference values, as the name suggests, assignment by reference points to the memory of the same storage object.
Assignment of original values:

Copy code The code is as follows:

var a = 1;//Original value
var b = a;//Generate a copy to variable b
b = 2;//It has nothing to do with a
alert(a);//output 1

Assignment of reference values:

Copy code The code is as follows:

var A= new Object();//Reference value
A.x = 1;
var B = A; //Reference assignment, pointing to the same memory
B.x = 2;//Modifying B will affect A
alert(A.x);//Output 2

Parameter passing

Now let’s take a look at how to handle passing two types of values ​​to function parameters.
1. Pass the original value

Copy code The code is as follows:

var a = 1;
function test(m) {
m = 2;
}
test(a);
alert(a);//output 1

The output is 1, so we know that the function only passes the value of the variable in, so m in the function body gets the passed value 1, and then is assigned a value of 2. This process does not affect the external variable a.

2. Pass reference value

Copy code The code is as follows:

var A= new Object();
A.x = 1
function test(M) {
M.x = 2;
}
test(A);
alert(A.x);//Output 2

The output is 2, so we know that the function passes the address of the variable, so M in the function body gets the passed address, so the attribute x is assigned a value of 2 and will also affect A pointing to the same memory address.

Summary

Now back to the opening question:

Copy code The code is as follows:

var a = 'test';
var b = function() {};
b.a = 'test';
function change(m, n) {
m = 'change';
n.a = 'change';
}
change(a, b);

Variable a is the original value and variable b is the reference value. One is the value and the other is the address passed into the function body. Therefore, after the function is run, the variable a will not change, but the value of variable b will change.