search
HomeWeb Front-endJS TutorialA closer look at null in JavaScript

A closer look at null in JavaScript

Nov 10, 2020 pm 05:45 PM
javascriptnull

A closer look at null in JavaScript

Recommended tutorial: "JavaScript Video Tutorial"

JavaScript has 2 types: basic type (string, booleans number, symbol) and objects.

Objects are complex data structures. The simplest objects in JS are ordinary objects: a set of keys and associated values:

let myObject = {
  name: '前端小智'
}

But in some cases, objects cannot be created. In this case, JS provides a special value null — indicating that the object is missing.

let myObject = null

In this article, we will learn everything about null in JavaScript: what it means, how to detect it, the difference between null and undefined And why using null makes code maintenance difficult.

1. The concept of null

The JS specification explains information about null:

The value null refers to an object whose value is not set. It is one of the basic types of JS and is considered falsy in Boolean operations.

For example, the function greetObject() creates an object, but it can also return null when the object cannot be created:

function greetObject(who) {
  if (!who) {
    return null;
  }
  return { message: `Hello, ${who}!` };
}

greetObject('Eric'); // => { message: 'Hello, Eric!' }
greetObject();       // => null

However, if When the function greetObject() is called with parameters, the function returns null. Returning null is reasonable because the who parameter has no value.

2. How to check for null

A good way to check for a null value is to use the strict equality operator:

const missingObject = null;
const existingObject = { message: 'Hello!' };

missingObject  === null; // => true
existingObject === null; // => false

missingObject === null results in true because the missingObject variable contains a null value.

If the variable contains a non-null value (such as an object), the expression existObject === null evaluates to false.

2.1 null is a virtual value

null and false, 0, '', undefined, NaN are all imaginary values. If a false value is encountered in a conditional statement, JS will force the false value to false.

Boolean(null); // => false

if (null) {
  console.log('null is truthy')
} else {
  console.log('null is falsy')
}

2.2 typeof null

typeof valueoperator determines the type of value. For example, typeof 15 is 'number', and the calculation result of typeof {prop: 'Value'} is 'object'.

Interestingly, what is the result of type null

typeof null; // => 'object'

Why is 'object', typoef null is object is a bug in early JS implementations.

To detect null values, use the typeof operator. As mentioned before, use the strict equality operator myVar === null.

If we want to use the typeof operator to check if a variable is an object, we also need to exclude the null value:

function isObject(object) {
  return typeof object === 'object' && object !== null;
}

isObject({ prop: 'Value' }); // => true
isObject(15);                // => false
isObject(null);              // => false

3. The trap of null

null often appears unexpectedly when we think the variable is an object. Then, if you extract the property from null, JS will throw an error.

Use the greetObject() function again and try to access the message property from the returned object:

let who = '';

greetObject(who).message; 
// throws "TypeError: greetObject() is null"

because who The variable is an empty string, so the function returns null. When accessing the message property from null, a TypeError error is raised.

Can be handled by using optional chaining with null mergingnull:

let who = ''

greetObject(who)?.message ?? 'Hello, Stranger!'
// => 'Hello, Stranger!'

4. Alternatives to null

When the object cannot be constructed, our usual approach is to return null, but this approach has shortcomings. When null appears in the execution stack, a check must be performed.

Try to avoid returning null:

  • return the default object instead of null
  • throwing an error Instead of returning null

, return to the greetObject() function that originally returned the greeting object. When parameters are missing, you can return a default object instead of returning null:

function greetObject(who) {
  if (!who) {
    who = 'Stranger';
  }
  return { message: `Hello, ${who}!` };
}

greetObject('Eric'); // => { message: 'Hello, Eric!' }
greetObject();       // => { message: 'Hello, Stranger!' }

or throw an error:

function greetObject(who) {
  if (!who) {
    throw new Error('"who" argument is missing');
  }
  return { message: `Hello, ${who}!` };
}

greetObject('Eric'); // => { message: 'Hello, Eric!' }
greetObject();       // => throws an error

These two approaches can avoid using null.

5. null vs undefined

undefined is the value of an uninitialized variable or object property, and undefined is the value of an uninitialized variable or object property. The main difference between

let myVariable;

myVariable; // => undefined

null and undefined is that null represents a missing object while undefined represents Uninitialized state.

Strict equality operator operator===Distinguish between null and undefined:

null === undefined // => false

And double equality operator== is considered to be equal to null and undefined

null == undefined // => true

我使用双等相等运算符检查变量是否为nullundefined:

function isEmpty(value) {
  return value == null;
}

isEmpty(42);                // => false
isEmpty({ prop: 'Value' }); // => false
isEmpty(null);              // => true
isEmpty(undefined);         // => true

6. 总结

null是JavaScript中的一个特殊值,表示丢失的对象,严格相等运算符确定变量是否为空:variable === null

typoef运算符对于确定变量的类型(number, string, boolean)很有用。 但是,如果为null,则typeof会产生误导:typeof null的值为'object'

nullundefined在某种程度上是等价的,但null表示缺少对象,而undefined未初始化状态。

更多编程相关知识,请访问:编程视频课程!!

The above is the detailed content of A closer look at null in JavaScript. For more information, please follow other related articles on the PHP Chinese website!

Statement
This article is reproduced at:segmentfault. If there is any infringement, please contact admin@php.cn delete
From Websites to Apps: The Diverse Applications of JavaScriptFrom Websites to Apps: The Diverse Applications of JavaScriptApr 22, 2025 am 12:02 AM

JavaScript is widely used in websites, mobile applications, desktop applications and server-side programming. 1) In website development, JavaScript operates DOM together with HTML and CSS to achieve dynamic effects and supports frameworks such as jQuery and React. 2) Through ReactNative and Ionic, JavaScript is used to develop cross-platform mobile applications. 3) The Electron framework enables JavaScript to build desktop applications. 4) Node.js allows JavaScript to run on the server side and supports high concurrent requests.

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.

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

MantisBT

MantisBT

Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.

Dreamweaver Mac version

Dreamweaver Mac version

Visual web development tools

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

PhpStorm Mac version

PhpStorm Mac version

The latest (2018.2.1) professional PHP integrated development tool

WebStorm Mac version

WebStorm Mac version

Useful JavaScript development tools