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 valuenull
refers to an object whose value is not set. It is one of the basic types of JS and is consideredfalsy
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 === nul
l 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 value
operator 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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