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This article brings you relevant knowledge about javascript. Null is a primitive type, which means it intentionally does not contain any object value. Let’s take a look at everything about null in JavaScript: What does it mean, how to detect it, the difference between null and undefined, and why extensive use of null can cause code maintenance difficulties, etc. I hope it will be helpful to everyone.
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JavaScript
There are two types: primitive types (strings, booleans, numbers, symbols
) and objects
Objects are a complex data structure. The simplest JavaScript
objects are ordinary objects - a collection of keys and related values
let myObject = { name: 'Eric Cartman' };
But in many cases it is not possible to create an object. In this case, JavaScript
provides a special value null
- indicating that the object is missing
let myObject = null;
In this article, you will learn about ## Everything about null
in #JavaScript: what it means, how to detect it, the difference between
null and
undefined, and why # is used a lot ##null
Will cause difficulty in code maintenance, etc.1. The concept of null
is described in the specificationnull
null is a primitive type that intentionally does not contain any object value
If you see
null (assigned to a variable or returned by a function), then at that location it should have is an object, but for some reason, an object was not createdFor example, the function
creates an object, but it can also returnnull## when the object cannot be created. #:
function greetObject(who) { if (!who) { return null; } return { message: `Hello, ${who}!` }; } greetObject('Eric'); // => { message: 'Hello, Eric!' } greetObject(); // => null
When a string parameter is passed in to the above function, as expected, the function returns an object
{ message: 'Hello, Eric!' }
However, when no parameters are passed in the function, the function returns
null
null because the
who parameter has no value, resulting in the
greeting object being unable to be created
1.1 A more appropriate metaphor for null
null value
Check
null
const missingObject = null; const existingObject = { message: 'Hello!' }; missingObject === null; // => true existingObject === null; // => false
missingObject === null The result is
true
missingObject variable contains a
null value. If the variable contains a non-null value, such as an object, the expression
existingObject === null evaluates to
false2.1 null is a false value
null
undefined,
NaN are all false values. If they are encountered in a conditional statement , then
JavaScript will coerce them to
false
Boolean(null); // => false if (null) { console.log('null is truthy'); } else { console.log('null is falsy'); // logs 'null is falsy' }
2.2 typeof null
typeof value type operators can determine The type of value. For example, type 15 is typeof { prop: 'Value' } is equal to
object.
Interesting Yes,
null What is the result of value type
typeof null; // => 'object'
objectis treated as? It turns out that
typoef null
object. This is a bug in the early
JavaScript implementation.
Do not use The
typeof operator detects
null
myVar === null if you want To use
typeof to check whether a variable is an object, you must exclude the
null
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 usually appears unexpectedly in situations where you expect to use an object. Then, if you try to extract a property from JavaScript will throw an error
Let's use the
greetObject() function again and try to access the
message
let who = ''; greetObject(who).message; // throws "TypeError: greetObject() is null"
because the
who variable is a empty string, so the function returns
null. When accessing the message property from
null, a TypeError error will be thrown
You can pass Use the optional chaining operator to handle
null
let who = ''; greetObject(who)?.message ?? 'Hello, Stranger!'; // => 'Hello, Stranger!'or use the two alternatives described in the next section.
当你不能构造一个对象时,很容易返回 null
。但这种做法也有缺点
一旦 null
出现在执行堆栈中,你总是必须检查它
我们尽量避免返回 null
:
null
null
让我们回忆一下 greetObject()
函数返回 greeting
对象
当缺少参数时,可以返回一个默认对象,而不是返回 null
function greetObject(who) { if (!who) { who = 'Stranger'; } return { message: `Hello, ${who}!` }; } greetObject('Eric'); // => { message: 'Hello, Eric!' } greetObject(); // => { message: 'Hello, Stranger!' }
或者抛出一个错误
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
这些实践可以让你完全避免处理 null
undefined
就是未初始化的变量或对象属性的值
例如,如果在声明变量时没有赋初值,那么访问该变量的结果为 undefined
let myVariable; myVariable; // => undefined
null
和 undefined
之间的主要区别是,null
表示丢失了的对象,而 undefined
表示未初始化的状态
严格相等运算符 === 区分 null
和 undefined
null === undefined; // => false
而松散相等运算符 ==
则认为 null
和 undefined
相等
null == undefined; // => true
我使用松散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
null
是 JavaScript
中的一个特殊值,表示丢失的对象
严格相等运算符判断变量是否为空: variable === null
。
typoef
操作符用于确定变量的类型(number, string, boolean
)。但是,typeof
在 null
情况下会引起误解: typeof null
结果为 object
null
和 undefined
在某种程度上是等价的,但null
表示一个丢失的对象,而 undefined
表示未初始化状态
尽可能避免返回 null
或将变量设置为 null
。因为这种做法会导致 null
值的扩展和需要对 null
的验证。相反,尝试使用具有默认属性的对象,或者甚至抛出错误会是更好的实践
你会使用什么方法来检查 null
?
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