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.

The above is the detailed content of What is the difference between null and undefined. For more information, please follow other related articles on the PHP Chinese website!