A closer look at null in JavaScript

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JavaScript has 2 types: basic type (string,booleansnumber,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 valuenull— 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 betweennullandundefinedAnd why usingnullmakes code maintenance difficult.

1. The concept of null

The JS specification explains information aboutnull:

The value nullrefers to an object whose value is not set. It is one of the basic types of JS and is considered falsyin Boolean operations.

For example, the functiongreetObject()creates an object, but it can also returnnullwhen 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 functiongreetObject()is called with parameters, the function returnsnull. Returningnullis reasonable because thewhoparameter has no value.

2. How to check for null

A good way to check for anullvalue is to use the strict equality operator:

const missingObject = null; const existingObject = { message: 'Hello!' }; missingObject === null; // => true existingObject === null; // => false

missingObject === nullresults intruebecause themissingObjectvariable contains anullvalue.

If the variable contains a non-null value (such as an object), the expressionexistObject === null evaluates tofalse.

2.1 null is a virtual value

nullandfalse,0,'',undefined,NaNare all imaginary values. If a false value is encountered in a conditional statement, JS will force the false value tofalse.

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 oftypeof {prop: 'Value'}is'object'.

Interestingly, what is the result oftype null

typeof null; // => 'object'

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

To detectnullvalues, use thetypeofoperator. As mentioned before, use the strict equality operatormyVar === null.

If we want to use thetypeofoperator to check if a variable is an object, we also need to exclude thenullvalue:

function isObject(object) { return typeof object === 'object' && object !== null; } isObject({ prop: 'Value' }); // => true isObject(15); // => false isObject(null); // => false

3. The trap of null

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

Use thegreetObject()function again and try to access themessageproperty from the returned object:

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

becausewhoThe variable is an empty string, so the function returnsnull. When accessing themessageproperty fromnull, aTypeErrorerror 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 returnnull, but this approach has shortcomings. Whennullappears in the execution stack, a check must be performed.

Try to avoid returningnull:

• return the default object instead ofnull
• throwing an error Instead of returningnull

, return to thegreetObject()function that originally returned thegreetingobject. When parameters are missing, you can return a default object instead of returningnull:

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 usingnull.

5.nullvsundefined

undefinedis the value of an uninitialized variable or object property, andundefinedis the value of an uninitialized variable or object property. The main difference between

let myVariable; myVariable; // => undefined

nullandundefinedis thatnullrepresents a missing object whileundefinedrepresents Uninitialized state.

Strict equality operatoroperator===Distinguish betweennullandundefined:

null === undefined // => false

And double equality operator==is considered to be equal tonullandundefined

null == undefined // => true

我使用双等相等运算符检查变量是否为null或undefined:

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'。

null和undefined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。

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