Detailed explanation of primitive data types Null and Undefined in JavaScript_javascript techniques

When discussing primitive data types in JavaScript, most people know the basics, from String, Number to Boolean. These primitive types are fairly simple and behave like common sense. However, this article will focus more on the unique primitive data types Null and Undefined, and what makes them so similar yet paradoxical.

1. Understanding Null and Undefined
In JavaScript, null is a literal and a keyword in the language, used to represent unrecognized object values. In other words, this is used to mean "no value". Although similar, undefined actually represents non-existence of a value. They are completely immutable, have no properties and methods, and cannot assign values ​​to their properties. In fact, trying to access or define a property will raise a TypeError. As their name implies, they are completely invalid values.

Boolean values ​​represented by no value are false, which means they will evaluate to false in a conditional context, such as an if statement. Use the equality operator (==) to compare these two values ​​with other false values, they are not equal except themselves:

null == 0; // false
undefined == ""; // false
null == false; // false
undefined == false; // false
null == undefined; // true

However, and other similarities, null and undefined are not equivalent. Each is a unique member of its own type, undefined is the Undefined type and null is the Null type. Compare these two values ​​using the equality operator (===), which requires that both types and values ​​are equal, as shown below:

undefined === null; // false

This is an important distinction that serves different purposes and reasons. To distinguish between these two values, you can think of undefined as representing an unexpected no value and null as representing an expected no value.
2. Generate Undefined
There are many ways to generate code with an undefined value. It is usually encountered when trying to access a value that does not exist. In this case, in a dynamic and weakly typed language like JavaScript, an undefined value will be returned by default instead of rising to an error.
Any variable declared without an initial value will have a default value of undefined:

 var foo; // 默认值为 undefined

When trying to access a non-existent object property or array item, return an undefined value:

var array = [1, 2, 3];
var foo = array.foo; // foo 属性不存在, 返回 undefined
var item = array[5]; // 数组中没有索引为5的项，返回 undefined

If the return statement of the function is omitted, undefined is returned:

var value = (function(){})(); // 返回 undefined

Values ​​not provided when the function is called will result in undefined parameter values:

(function(undefined){
  // 参数是 undefined
})();

The void operator can also return an undefined value. Libraries like Underscore use it as a defensive type check because it is immutable and can be relied upon in any context to return undefined:

function isUndefined(obj){
  return obj === void 0;
}

Finally, undefined is a predefined global variable (unlike the null keyword) initialized to the undefined value:

'undefined' in window; // true

In ECMAScript 5, this variable is read-only, which was not the case before.

3. Use cases of Null
The use case of null is the main aspect that makes it different, because unlike undefined, null is considered to be more useful. This is why the typeof operator returns "object" when applied to a null value. The original rationale was, and still is, that a null reference is usually used as a null object is expected, just like a placeholder. This behavior of typeof has been confirmed as a bug, and although a fix was proposed, this has been left unchanged for backward compatibility purposes.
This is why the JavaScript environment never sets a value to null; it must be done programmatically. As the documentation MDN says:
In the API, null is often expected where objects are retrieved, but there is no related object.
This applies to the DOM, which is language independent and outside the scope of the ECMAScript specification. Because it's an external API, trying to get an element that doesn't exist returns a null value, not undefined.
Generally speaking, if you need to assign an immutable value to a variable or property, pass it to a function, or return null from a function, null is almost always the best choice. In short, JavaScript uses undefined and programmers should use null.
Another possible use case for null, which is also considered good practice, is to explicitly designate a variable as invalid (object= null) when a reference is no longer required. By assigning a null value, the reference is effectively cleared, and assuming the object is not referenced by other code, garbage collection is specified to ensure that the memory is reclaimed.
4. Dig deeper
What makes null and undefined like black holes is not just their behavior, but the way they are handled internally within the JavaScript environment. They often don't seem to have the same associated characteristics as other native or built-in objects.
In ES5, the Object.prototype.toString method has become the de facto type checking standard, which is proven to be consistent with null and undefined:

Object.prototype.toString.call(null); // [object Null]
Object.prototype.toString.call(undefined); // [object Undefined]

However, the Object.prototype.toString method does not actually retrieve the internal [[Class]] property of null or the public constructor of undefined. According to the documentation, the following steps occur in the called process:

If the value is undefined, return "[object Undefined]".

• If this value is null, return "[object Null]".
• Let O be the result value of calling ToObject while passing this value as a parameter.
• Let class be the value of O’s internal property [[Class]].
• The returned result is the string value of the result of concatenating the three strings "[object", class, and "]".
  

This method performs a simple string return value if it detects null or undefined and other object uniform functions. This is common throughout the specification, as most methods contain a simple catch and return when null and undefined values ​​are encountered. In fact, there is no indication that they contain internal properties associated with any native objects. It’s like they’re not objects. I'm curious to know what would happen if an explicit solution actually existed inside JavaScript's native environment? Maybe someone more familiar with an implementation can get involved.

Conclusion
However unusual these native objects may be, understand the difference between null and undefined, and their distinct roles in JavaScript's language foundation. It may not make your application breakout, but in general it will only prove to be beneficial in the long term during development and debugging.

The above is an introduction to the primitive data types Null and Undefined in JavaScript. I hope it will be helpful to everyone's learning.