Distinguish between undefined, null, '', 0 and false_javascript techniques in JS

Among the various data types defined in programming languages, we will define a "null value" or "false value" for them, such as the null value of the object type, the null value of the database field in the .NET Framework DBNull, False values ​​of boolean type false and so on. There are many kinds of "null values" and "false values" in JavaScript, so what do they have in common and what are the differences?

In fact, in the title, I have listed all the "null values" and "false values" in JavaScript. Except for the two cases where the boolean value itself is true and false, the "null values" of other data types There are mainly two categories: undefined and defined. The types of these null values ​​are:

Copy code The code is as follows:

typeof(undefined) == 'undefined'
typeof(null) == 'object'
typeof("") == 'string'
typeof(0) == 'number'
typeof(false) = = 'boolean'

What these five values ​​have in common is that when making a judgment in the if statement, the false branch will be executed. Of course, in a broad sense, it means that these values ​​​​are invalid values ​​or null values ​​​​in their corresponding data types. There are also these five values ​​​​for the ! operation, and the results are all: true.

There are also differences among these values. Undefined and null are special. Although the type of null is object, null does not have any object characteristics, which means that we cannot execute null.toString(), null Default call for object instances such as .constructor. So in this sense, null and undefined have the greatest similarity. Looking at the result of null == undefined (true) will illustrate this point even more. However, similarities are similar, but there is still a difference, that is, when operating with numbers, the result of 10 null is: 10; the result of 10 undefined is: NaN.

In addition, although "", 0 and false appear as "false values" in the if statement, they are all meaningful data, but are regarded as "null values" or "false values" because: "" .toString(), (0).toString(), and false.toString() are all legal executable expressions.

In fact, among the differences mentioned above, these 5 values ​​will not cause too many problems for process flow control, so what should we distinguish between them? It should be noted that the difference between these values ​​when converted to String is relatively large. Their conversion relationship to String is:

Copy code The code is as follows:

String(undefined) -> "undefined"
String(null) -> "null"
String("") -> " "
String(0) -> "0"
String(false) -> "false"

This conversion relationship requires special attention when adding strings. Otherwise, some unexpected problems may occur. Today, I encountered a null that was converted to "null" by default, which confused me for a while.