I saw friends discussing a question, asking whether null is equal to 0.
After hearing this, write a demo and try it out.
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title></title> </head> <body> </body> <script type="text/javascript"> console.log(null > 0); // false console.log(null < 0); // false console.log(null >= 0); // true console.log(null <= 0); // true console.log(null == 0); // false console.log(null === 0); // false </script> </html>
Why are the two judgments of console.log(null = 0); true?
First, let’s take a look at ES3’s algorithm implementation of internal equality operations.
11.9.3 The Abstract Equality Comparison Algorithm The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows: 1. If Type(x) is different from Type(y), Go to step 14. 2. If Type(x) is Undefined, return true. 3. If Type(x) is Null, return true. 4. If Type(x) is not Number, go to step 11. 5. If x is NaN, return false. 6. If y is NaN, return false. 7. If x is the same number value as y, return true. 8. If x is +0 and y is -0, return true. 9. If x is -0 and y is +0, return true. 10. Return false. 11. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false. 12. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false. 13. Return true if x and y refer to the same object or if they refer to objects joined to each other (see 13.1.2). Otherwise, return false. 14. If x is null and y is undefined, return true. 15. If x is undefined and y is null, return true. 16. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y). 17. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x)== y. 18. If Type(x) is Boolean, return the result of the comparison ToNumber(x)== y. 19. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y). 20. If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y). 21. If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x)== y. 22. Return false.
Next, let’s take a look at ES3’s algorithm implementation of internal relational operations.
11.8.5 The Abstract Relational Comparison Algorithm The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that at least one operand is NaN). Such a comparison is performed as follows: 1. Call ToPrimitive(x, hint Number). 2. Call ToPrimitive(y, hint Number). 3. If Type(Result(1)) is String and Type(Result(2)) is String, go to step 16. (Note that this step differs from step 7 in the algorithm for the addition operator **+ * in using *and instead of or.) 4. Call ToNumber(Result(1)). 5. Call ToNumber(Result(2)). 6. If Result(4) is NaN, return undefined. 7. If Result(5) is NaN, return undefined. 8. If Result(4) and Result(5) are the same number value, return false. 9. If Result(4) is +0 and Result(5) is -0, return false. 10. If Result(4) is -0 and Result(5) is +0, return false. 11. If Result(4) is +∞, return false. 12. If Result(5) is +∞, return true. 13. If Result(5) is -∞, return false. 14. If Result(4) is -∞, return true. 15. If the mathematical value of Result(4) is less than the mathematical value of Result(5) — note that these mathematical values are both finite and not both zero — return true. Otherwise, return false. 16. If Result(2) is a prefix of Result(1), return false. (A string value p is a prefix of string value q if q can be the result of concatenating p and some other string*r*. Note that any string is a prefix of itself, because r may be the empty string.) 17. If Result(1) is a prefix of Result(2), return true. 18. Let k be the smallest nonnegative integer such that the character at position k within Result(1) is different from the character at position k within Result(2). (There must be such a k, for neither string is a prefix of the other.) 19. Let m be the integer that is the code point value for the character at position k within Result(1). 20. Let n be the integer that is the code point value for the character at position k within Result(2). 21. If m < n, return true. Otherwise, return false.
ES3’s “>” operator:
The Greater-than Operator ( > ) The production RelationalExpression : RelationalExpression > ShiftExpression is evaluated as follows: 1. Evaluate RelationalExpression. 2. Call GetValue(Result(1)). 3. Evaluate ShiftExpression. 4. Call GetValue(Result(3)). 5. Perform the comparison Result(4) < Result(2). 6. If Result(5) is undefined, return false. Otherwise, return Result(5).
ES3’s “>=" operator:
The Greater-than-or-equal Operator ( >= ) The production RelationalExpression : RelationalExpression >= ShiftExpression is evaluated as follows: 1. Evaluate RelationalExpression. 2. Call GetValue(Result(1)). 3. Evaluate ShiftExpression. 4. Call GetValue(Result(3)). 5. Perform the comparison Result(2) < Result(4). (see 11.8.5). 6. If Result(5) is true or undefined, return false. Otherwise, return true.
ES3’s “=” =” Operator:
The Equals Operator ( == ) The production EqualityExpression : EqualityExpression == RelationalExpression is evaluated as follows: 1. Evaluate EqualityExpression. 2. Call GetValue(Result(1)). 3. Evaluate RelationalExpression. 4. Call GetValue(Result(3)). 5. Perform the comparison Result(4) == Result(2). (see 11.9.3). 6. Return Result(5).
Content based on data
- ##Relational operators and equality operators are not of the same category.
- The design of relational operators always requires the operand to be converted into a number. However, the design of equality operators does not take this into consideration.
- The most important point is not to take the results of a > b and a == b for granted and establish a relationship with a >= b. The correct relationship that conforms to the original design idea is a > b and a >= b are a group. a == b and other equality operators are a group. For example, a === b , a != b, a !== b .
null > 0 // null 尝试转型为number , 则为0 . 所以结果为 false, null >= 0 // null 尝试转为number ,则为0 , 结果为 true. null == 0 // null在设计上，在此处不尝试转型. 所以 结果为false.
a >= b operator simply inverts the result of a 0 , undefined
Another example
function case1(a){
if(a == null){
....
}
}
function case2(a){
if(a == undefined){
...
}
}
// 上面两组完全等价, 这就是一种不明确表述.
// 我们永远不知道代码编写者的目的到底是同时匹配null 和 undefined还是只匹配其中某一个
function case3(a){
if(a === null || a === undefined){
...
}
}
// case3 才是最好的表述. 我们明确知道代码编写者的意图.
// 即使很多人可能认为这个代码很愚蠢. 但我坚定的认为这才是好代码.The above is the detailed content of In javascript, discussion on whether null is equal to 0. For more information, please follow other related articles on the PHP Chinese website!
JavaScript Engines: Comparing ImplementationsApr 13, 2025 am 12:05 AMDifferent JavaScript engines have different effects when parsing and executing JavaScript code, because the implementation principles and optimization strategies of each engine differ. 1. Lexical analysis: convert source code into lexical unit. 2. Grammar analysis: Generate an abstract syntax tree. 3. Optimization and compilation: Generate machine code through the JIT compiler. 4. Execute: Run the machine code. V8 engine optimizes through instant compilation and hidden class, SpiderMonkey uses a type inference system, resulting in different performance performance on the same code.
Beyond the Browser: JavaScript in the Real WorldApr 12, 2025 am 12:06 AMJavaScript's applications in the real world include server-side programming, mobile application development and Internet of Things control: 1. Server-side programming is realized through Node.js, suitable for high concurrent request processing. 2. Mobile application development is carried out through ReactNative and supports cross-platform deployment. 3. Used for IoT device control through Johnny-Five library, suitable for hardware interaction.
Building a Multi-Tenant SaaS Application with Next.js (Backend Integration)Apr 11, 2025 am 08:23 AMI built a functional multi-tenant SaaS application (an EdTech app) with your everyday tech tool and you can do the same. First, what’s a multi-tenant SaaS application? Multi-tenant SaaS applications let you serve multiple customers from a sing
How to Build a Multi-Tenant SaaS Application with Next.js (Frontend Integration)Apr 11, 2025 am 08:22 AMThis article demonstrates frontend integration with a backend secured by Permit, building a functional EdTech SaaS application using Next.js. The frontend fetches user permissions to control UI visibility and ensures API requests adhere to role-base
JavaScript: Exploring the Versatility of a Web LanguageApr 11, 2025 am 12:01 AMJavaScript is the core language of modern web development and is widely used for its diversity and flexibility. 1) Front-end development: build dynamic web pages and single-page applications through DOM operations and modern frameworks (such as React, Vue.js, Angular). 2) Server-side development: Node.js uses a non-blocking I/O model to handle high concurrency and real-time applications. 3) Mobile and desktop application development: cross-platform development is realized through ReactNative and Electron to improve development efficiency.
The Evolution of JavaScript: Current Trends and Future ProspectsApr 10, 2025 am 09:33 AMThe latest trends in JavaScript include the rise of TypeScript, the popularity of modern frameworks and libraries, and the application of WebAssembly. Future prospects cover more powerful type systems, the development of server-side JavaScript, the expansion of artificial intelligence and machine learning, and the potential of IoT and edge computing.
Demystifying JavaScript: What It Does and Why It MattersApr 09, 2025 am 12:07 AMJavaScript is the cornerstone of modern web development, and its main functions include event-driven programming, dynamic content generation and asynchronous programming. 1) Event-driven programming allows web pages to change dynamically according to user operations. 2) Dynamic content generation allows page content to be adjusted according to conditions. 3) Asynchronous programming ensures that the user interface is not blocked. JavaScript is widely used in web interaction, single-page application and server-side development, greatly improving the flexibility of user experience and cross-platform development.
Is Python or JavaScript better?Apr 06, 2025 am 12:14 AMPython is more suitable for data science and machine learning, while JavaScript is more suitable for front-end and full-stack development. 1. Python is known for its concise syntax and rich library ecosystem, and is suitable for data analysis and web development. 2. JavaScript is the core of front-end development. Node.js supports server-side programming and is suitable for full-stack development.
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
AI Hentai Generator
Generate AI Hentai for free.
Hot Article
Hot Tools
SublimeText3 Chinese version
Chinese version, very easy to use
mPDF
mPDF is a PHP library that can generate PDF files from UTF-8 encoded HTML. The original author, Ian Back, wrote mPDF to output PDF files "on the fly" from his website and handle different languages. It is slower than original scripts like HTML2FPDF and produces larger files when using Unicode fonts, but supports CSS styles etc. and has a lot of enhancements. Supports almost all languages, including RTL (Arabic and Hebrew) and CJK (Chinese, Japanese and Korean). Supports nested block-level elements (such as P, DIV),
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
Dreamweaver Mac version
Visual web development tools
SecLists
SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.
