search
HomeWeb Front-endJS TutorialJs parameter passing and variable copying

Js parameter passing and variable copying

Oct 26, 2017 am 10:05 AM
javascriptvariablecopy

  ECMAScript variables may contain values ​​of two different data types: primitive type values ​​and reference type values. Primitive type values ​​refer to simple data segments, while reference type values ​​refer to objects that may be composed of multiple values.

5 basic data types: Undefined, Null, Boolean, Number and String. These 5 basic data types are accessed by value because the actual value stored in the variable can be manipulated. ES6 has one more Symbol type.

 The value of a reference type is an object stored in memory. JavaScript does not allow direct access to the location in the memory, which means that the memory space of the object cannot be directly manipulated. When you operate on an object, you are actually operating on a reference to the object rather than the actual object. For this purpose, values ​​of reference types are accessed by reference.

Copy variable value

  • [Copy variable value] Copying the value of a basic type variable will create a new value on the variable object, and then copy the value to The location where the new variable is assigned. Operations on any variable value do not affect each other.

  • [Copy reference pointer] Copying the value of a reference type variable will also copy the value stored in the variable object into the space allocated by the new variable. The difference is this The copy of the value is actually a pointer to an object stored in the heap. After copying, both variables will actually refer to the same object. Therefore, changing any variable will affect another variable.

Parameter passing

The parameters of all functions in ECMAScript are passed by value. There are two ways to access variables: by value and by reference, while parameters can only be passed by value.

  • Basic type parameter passing: What is passed to the function is a copy of the value, and modifications to it in the function are not visible externally.

var a = 1;
var b = 2;
function change(a, b) {
    var c = a;
    a = b;
    b = c;
    console.log(a);    //2
    console.log(b);    //1
}
change(a, b);
console.log(a);    //1
console.log(b);    //2
  • Reference type parameter passing: What is passed to the function is a reference to the value. The modification of its properties in the function is externally visible, but it is overwritten with a new reference. Then it is not visible externally

var a = [1, 2, 3];
var b = [5, 6];
function change(a,b) {
  a[0] = 4;    //对其属性的修改外部可见 
  var c = a;
  a = b;      //用新引用覆盖
  b = c;
  console.log(a);  //"5,6"        
  console.log(b);  //"4,2,3"
}
change(a,b);
console.log(a);    //"4,2,3"
console.log(b);    //"5,6"

 a and b are variables in the change function. When calling the function, the references of a and b are assigned to these two variables, but they cannot Change a, b in the global. Because overwriting with a new reference is not visible to the outside, because the function only gets the reference and has no power to change the reference.

var a = [1, 2, 3];
var b = [5, 6];
function change() {
  var c = a;
  a[0] = 4;    //对其属性的修改外部可见 
  a = b;      //用新引用覆盖
  b = c;
}
change(a,b);
console.log(a);  //"5,6" 
console.log(b);  //"4,2,3"

Because js does not have a block-level scope, it cannot find variable a in change, and b will consciously go to the upper layer to find it, so a and b here are references to global variables.


☞☞☞Deep dive into the JavaScript series☜☜☜

  ECMAScript variables may contain values ​​of two different data types: basic type values ​​and reference type values. Primitive type values ​​refer to simple data segments, while reference type values ​​refer to objects that may be composed of multiple values.

5 basic data types: Undefined, Null, Boolean, Number and String. These 5 basic data types are accessed by value because the actual value stored in the variable can be manipulated. ES6 has one more Symbol type.

 The value of a reference type is an object stored in memory. JavaScript does not allow direct access to the location in the memory, which means that the memory space of the object cannot be directly manipulated. When you manipulate an object, you are actually manipulating a reference to the object rather than the actual object. For this purpose, values ​​of reference types are accessed by reference.

Copy variable value

  • [Copy variable value] Copying the value of a basic type variable will create a new value on the variable object, and then copy the value to The location where the new variable is assigned. Operations on any variable value do not affect each other.

  • [Copy reference pointer] Copying the value of a reference type variable will also copy the value stored in the variable object into the space allocated by the new variable. The difference is this The copy of the value is actually a pointer to an object stored in the heap. After copying, both variables will actually refer to the same object. Therefore, changing any variable will affect another variable.

Parameter passing

The parameters of all functions in ECMAScript are passed by value. There are two ways to access variables: by value and by reference, while parameters can only be passed by value.
  Basic type parameter passing: What is passed to the function is a copy of the value, and its modification in the function is not visible to the outside.

var a = 1;
var b = 2;
function change(a, b) {
    var c = a;
    a = b;
    b = c;
    console.log(a);    //2
    console.log(b);    //1
}
change(a, b);
console.log(a);    //1
console.log(b);    //2
  • Reference type parameter passing: What is passed to the function is a reference to the value. The modification of its properties in the function is visible externally, but overwriting it with a new reference is externally visible. Invisible

var a = [1, 2, 3];
var b = [5, 6];
function change(a,b) {
  a[0] = 4;    //对其属性的修改外部可见 
  var c = a;
  a = b;      //用新引用覆盖
  b = c;
  console.log(a);  //"5,6"        
  console.log(b);  //"4,2,3"
}
change(a,b);
console.log(a);    //"4,2,3"
console.log(b);    //"5,6"

  a, b are variables in the change function. When calling the function, the references of a and b are assigned to these two variables, but they cannot change the global of a,b. Because overwriting with a new reference is not visible to the outside, because the function only gets the reference and has no power to change the reference.

var a = [1, 2, 3];
var b = [5, 6];
function change() {
  var c = a;
  a[0] = 4;    //对其属性的修改外部可见 
  a = b;      //用新引用覆盖
  b = c;
}
change(a,b);
console.log(a);  //"5,6" 
console.log(b);  //"4,2,3"

Because js does not have a block-level scope, it cannot find variable a in change, and b will consciously go to the upper layer to find it, so a and b here are references to global variables.

The above is the detailed content of Js parameter passing and variable copying. For more information, please follow other related articles on the PHP Chinese website!

Statement
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Python vs. JavaScript: Use Cases and Applications ComparedPython vs. JavaScript: Use Cases and Applications ComparedApr 21, 2025 am 12:01 AM

Python is more suitable for data science and automation, while JavaScript is more suitable for front-end and full-stack development. 1. Python performs well in data science and machine learning, using libraries such as NumPy and Pandas for data processing and modeling. 2. Python is concise and efficient in automation and scripting. 3. JavaScript is indispensable in front-end development and is used to build dynamic web pages and single-page applications. 4. JavaScript plays a role in back-end development through Node.js and supports full-stack development.

The Role of C/C   in JavaScript Interpreters and CompilersThe Role of C/C in JavaScript Interpreters and CompilersApr 20, 2025 am 12:01 AM

C and C play a vital role in the JavaScript engine, mainly used to implement interpreters and JIT compilers. 1) C is used to parse JavaScript source code and generate an abstract syntax tree. 2) C is responsible for generating and executing bytecode. 3) C implements the JIT compiler, optimizes and compiles hot-spot code at runtime, and significantly improves the execution efficiency of JavaScript.

JavaScript in Action: Real-World Examples and ProjectsJavaScript in Action: Real-World Examples and ProjectsApr 19, 2025 am 12:13 AM

JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.

JavaScript and the Web: Core Functionality and Use CasesJavaScript and the Web: Core Functionality and Use CasesApr 18, 2025 am 12:19 AM

The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.

Understanding the JavaScript Engine: Implementation DetailsUnderstanding the JavaScript Engine: Implementation DetailsApr 17, 2025 am 12:05 AM

Understanding how JavaScript engine works internally is important to developers because it helps write more efficient code and understand performance bottlenecks and optimization strategies. 1) The engine's workflow includes three stages: parsing, compiling and execution; 2) During the execution process, the engine will perform dynamic optimization, such as inline cache and hidden classes; 3) Best practices include avoiding global variables, optimizing loops, using const and lets, and avoiding excessive use of closures.

Python vs. JavaScript: The Learning Curve and Ease of UsePython vs. JavaScript: The Learning Curve and Ease of UseApr 16, 2025 am 12:12 AM

Python is more suitable for beginners, with a smooth learning curve and concise syntax; JavaScript is suitable for front-end development, with a steep learning curve and flexible syntax. 1. Python syntax is intuitive and suitable for data science and back-end development. 2. JavaScript is flexible and widely used in front-end and server-side programming.

Python vs. JavaScript: Community, Libraries, and ResourcesPython vs. JavaScript: Community, Libraries, and ResourcesApr 15, 2025 am 12:16 AM

Python and JavaScript have their own advantages and disadvantages in terms of community, libraries and resources. 1) The Python community is friendly and suitable for beginners, but the front-end development resources are not as rich as JavaScript. 2) Python is powerful in data science and machine learning libraries, while JavaScript is better in front-end development libraries and frameworks. 3) Both have rich learning resources, but Python is suitable for starting with official documents, while JavaScript is better with MDNWebDocs. The choice should be based on project needs and personal interests.

From C/C   to JavaScript: How It All WorksFrom C/C to JavaScript: How It All WorksApr 14, 2025 am 12:05 AM

The shift from C/C to JavaScript requires adapting to dynamic typing, garbage collection and asynchronous programming. 1) C/C is a statically typed language that requires manual memory management, while JavaScript is dynamically typed and garbage collection is automatically processed. 2) C/C needs to be compiled into machine code, while JavaScript is an interpreted language. 3) JavaScript introduces concepts such as closures, prototype chains and Promise, which enhances flexibility and asynchronous programming capabilities.

See all articles

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Tools

MinGW - Minimalist GNU for Windows

MinGW - Minimalist GNU for Windows

This project is in the process of being migrated to osdn.net/projects/mingw, you can continue to follow us there. MinGW: A native Windows port of the GNU Compiler Collection (GCC), freely distributable import libraries and header files for building native Windows applications; includes extensions to the MSVC runtime to support C99 functionality. All MinGW software can run on 64-bit Windows platforms.

SublimeText3 English version

SublimeText3 English version

Recommended: Win version, supports code prompts!

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

VSCode Windows 64-bit Download

VSCode Windows 64-bit Download

A free and powerful IDE editor launched by Microsoft

DVWA

DVWA

Damn Vulnerable Web App (DVWA) is a PHP/MySQL web application that is very vulnerable. Its main goals are to be an aid for security professionals to test their skills and tools in a legal environment, to help web developers better understand the process of securing web applications, and to help teachers/students teach/learn in a classroom environment Web application security. The goal of DVWA is to practice some of the most common web vulnerabilities through a simple and straightforward interface, with varying degrees of difficulty. Please note that this software