What is null in javascript

null in javascript is a special value provided by JS, indicating the lack of an object; in javascript, you can use the strict equality operator to check the null value, and its check statement is such as "missingObject === null;".

The operating environment of this article: Windows 7 system, JavaScript version 1.8.5, DELL G3 computer.

What is null in javascript?

Everything about null in JavaScript

JavaScript has 2 types: basic types (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 the object cannot be created. In this case, JS provides a special value null — indicating that the object is missing.

let myObject = null

1. The concept of null

The value null refers specifically to the object whose value is not set. It is one of the basic types of JS and is used in Boolean operations. Think it's falsy.

For example, the function greetObject() creates an object, but 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, the function greetObject() is called without 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

# The result of ##missingObject === null is 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 virtual 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 What is the result of typeof null

type null

typeof null; // => 'object'

Why is

'object', typeef null is object was a bug in early JS implementations.

To use the

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

If we want to use the

typeof operator to check whether the 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. So when you want to get attributes from an object, you should pay attention to judge that the object may be
null, otherwise an error may occur.

[Recommended study: "

Javascript Basics Tutorial"]

4. Alternative method to null

When the object cannot be constructed When, our usual approach is to return

null, but this approach has disadvantages. When null appears in the execution stack, a check must be performed.

Try to avoid returning

null :

Return the default object instead of
• null
Throw an error instead Instead of returning
• null

5. null vs undefined

undefined is a variable that has not been initialized or The main difference between the value null
and undefined is that null represents a missing object , and undefined represents the uninitialized state.

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