What is the difference between null and undefined

The difference between null and undefined is: 1. Semantic meaning; 2. Usage scenarios; 3. Comparison with other values; 4. Relationship with global variables; 5. Relationship with function parameters; 6. Can Nullity check; 7. Performance considerations; 8. Performance in JSON serialization; 9. Relationship with types. Detailed introduction: 1. Semantic meaning, null usually means knowing that this variable will not have any valid object value, while undefined usually means that the variable has not been assigned a value, or the object does not have this attribute; 2. Usage scenarios, etc.

The operating system for this tutorial: Windows 10 system, DELL G3 computer.

In JavaScript, both null and undefined mean no value or no object. However, there are some important differences between them.

null is a special value that means "no value", while undefined means "the variable is not defined". That is, when a variable is declared but not assigned a value, its value is undefined.

The following are some detailed explanations about the difference between null and undefined:

1. Semantic meaning:

• null usually means that we know that this variable will not Possess any valid object value. In other words, we expect this variable to have no value.
• Undefined usually means that the variable has not been assigned a value, or the object does not have this attribute. This is often an unexpected result because we might expect an object to have a property or a variable to have a value.

2. Usage scenarios:

• When we want to clearly indicate that a variable has no value, or that the object does not have a certain attribute, we usually use null.
• We may use undefined when we check whether a variable is assigned a value, or whether an object property exists.

3. Comparison with other values:

• null and undefined are special values ​​in JavaScript and cannot be compared with any other values. For example, null == undefined actually returns true, although semantically they are different. This is because JavaScript uses a method of comparison called "type casting", which ignores type differences. To compare them accurately, you should use the === or !== operators, so that only their types and values ​​are compared.

4. Relationship with global variables:

• In JavaScript, the initial value of global variables is undefined. If you declare a global variable but do not assign a value to it, its value is undefined. However, this does not mean that a global variable can only have the value undefined, you can still assign any other value to it.

5. Relationship with function parameters:

• When the function does not pass in parameters, the parameter value inside the function is undefined. Even if you try to assign a value to this parameter (for example: let a = null;), its initial value is still undefined. This parameter only has a specific value when you pass in a specific value (for example: let a = null; a = 'hello';).

6. Nullability check:

• In some cases, you may want to check whether a value is empty (whether it is null or undefined). In this case, you can use the following method:

复制代码javascript`if (value == null) {// value is null or undefined}`

This method will also check whether a value is empty or undefined. If you just want to check whether a value is undefined, you can use the following method:

复制代码javascript`if (typeof value === 'undefined') {// value is undefined}`

7. Performance considerations:

• In some cases, using null instead of undefined may improve the performance of your code. This is because when the JavaScript engine encounters a reference with a null value, it usually skips the extra step of looking up the object chain. This avoids some potential performance overhead. However, this usually only matters when a large number of object references are involved, so for most applications this is not an important factor to consider.

8. Performance in JSON serialization:

• When using the JSON.stringify method to convert JavaScript objects into JSON strings, all undefined values ​​will be Ignored and will not be included in the generated JSON string. Null values ​​will be included. If you wish to include all undefined values ​​in the serialized result, you can use a custom serialization function to handle this situation.

9. Relationship with types:

• null and undefined are primitive types in JavaScript, not object types. This means they cannot be inherited or extended, and no methods can be called on them. Nonetheless, some built-in methods in JavaScript (such as Object.keys()) can handle these primitive types of values ​​correctly.

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