How to transfer functions in js

In the process of learning js recently, I encountered the problem of passing objects in js function parameters. I also studied the issues of reference passing and value passing. Although it took some time, I finally understood it.

Data type

Data types in JavaScript can be divided into two categories:

Basic type value primitive type, such as Undefined, Null, Boolean, Number, String.

Reference type value, that is, object type Object type, such as Object, Array, Function, Date, etc.

Copying of variables

As we all know, the basic types and reference types of variables in js are saved in different ways, which results in different variables when copied. If a basic type value is copied from one variable to another variable, the former value will be cloned, and then the cloned value will be assigned to the latter. Therefore, the two values are completely independent, but their values are the same.

var num1 = 10;var num2 = num1; console.log(num2);//10

The value saved in num1 above is 10. When the value of num1 is assigned to num2, the value of num2 is also 10. But these two 10s are completely independent. The 10 in num2 is just cloned. It is equivalent to me writing a word document and putting it in the folder of num1. Then I copy the word document and it is called word. Make a copy, and then put this copy in the folder of num2. The two word documents are exactly the same, and modifying either one will not affect both.

num2 += 1; console.log(num1); //10 console.log(num2); //11

It can be seen from the above that the value of num2 has been modified, but the value of num1 has not changed. Let’s look at the copying of reference types. When a value of a reference type is copied from one variable to another, a copy of the value stored in the variable object is also copied into the space allocated for the new variable.

var obj1 = { name : "111"};var obj2 = obj1; console.log(obj2.name); //111 obj2.name = "222"; console.log(obj1.name); //222

The result printed out for the first time is "111", which is easy for us to understand, but the result printed out the second time is "222", which is a bit puzzling. This is the difference between reference types and basic types. When copying an object, an identical object will not be created in the heap memory. It will just have an additional variable that holds a pointer to the object. Copy the value of obj1 to obj2, and the copy of this value is actually a pointer. This pointer points to an object stored in the heap. That is to say, a new memory address is created and passed to obj2, obj1 and obj2. The variables point to the same Object at the same time. When the object is changed, their values will change, which means that changes made by any one of them will be reflected in the other. The simplified diagram below may be more clear.

Passing of function parameters

"JS Advanced Programming" describes parameter passing like this: All function parameters are passed by value, that is to say, the parameters outside the function are Copying the value to the parameter inside the function is the same as copying the value from one variable to another. So if you can understand the copying of variables, then the passing of parameters will be very simple. Let’s start with an example of basic types.

var count = 10;function num(num1){ num1 = 1; return num1; }var result = num(count);console.log(result);//1 console.log(count);//10，并未变成1

This example is easy to understand. It actually creates a copy of count, and then passes the value of count into the parameter. Because the value of the parameter is defined in the function, 1 overwrites 10. Finally The result returns 1, but the count has not changed. Let's look at an example of passing objects.

var person = { name : "Tom"};function obj(peo){ peo.name = "Jerry"; return peo; }var result = obj(person); console.log(result.name);// Jerry console.log(person.name);// Jerry

In the above example, person is copied and passed into obj(). peo and person point to the same object, and modifying the name attribute in peo actually modifies the object they jointly point to. The name attribute and the name attribute referenced by the corresponding external person also change, so the printed one is Jerry. In fact, at first glance, it seems that reference type parameters are passed by reference. This was the mistake I made initially. Let’s look at another example.

var person = { name : "Tom"}; function obj(peo){ peo = { name : "Jerry" }; return peo; } var result = obj(person);console.log(result.name);// Jerry console.log(person.name);// Tom

In the above example, an object is redefined in the function, that is, there are now two objects in the heap memory. The external person points to the old object. After the parameters are passed in Points to the newly defined object, so the value returned after the call is the value of the newly defined object. If the parameters are passed by reference, then the printed result of person.name is Jerry. From this point, it can be concluded that the parameters are passed by value (some places call it passed by sharing).

We used "A Brief History of Humanity" recommended by Lao Luo to visualize it, but the description is not very good. The title of the first chapter of the brief history is "Cognitive Revolution". We changed its name to "person". According to the number of pages behind, we can directly find the content of "Cognitive Revolution" "that is, the object pointed by peoson". Second The chapter is "Agricultural Revolution", we call it "result", and its sub-directory has a section "Memory Overload" (renamed "peo"). You can also find the content of this section directly based on the page number. Now we copy "person" to "peo", the "peo" section in Chapter 2 becomes "person", and what we find based on "peoson" in Chapter 1 is still the one in Chapter 1 Content, this is because they point to different content sections and do not interfere with each other. Here, the heap memory is the content of each chapter, and the content of Chapter 1 and Chapter 2 are two different objects, and the two are unrelated to each other, so when printing external person.name, the result is still the previous object attribute value.

in conclusion

In short, parameters in js are passed by value. The examples I wrote are a bit rough, but the examples in "JavaScript Advanced Programming" are more clear and easier to understand.

Related recommendations:

The implementation principle of function parameters in js

The actual parameters, formal parameters and Understanding closure

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