The difference between null and NULL in C language is: null is a macro definition in C language, usually used to represent a null pointer, which can be used to initialize pointer variables, or to determine whether the pointer is null in a conditional statement; NULL is a macro definition in C language A predefined constant in , usually used to represent a null value, used to represent a null pointer, null pointer array or null structure pointer.

In JavaScript, both undefined and null represent the concept of "nothing": 1. undefined represents an uninitialized variable or a non-existent property. When a variable is declared but no value is assigned to it, the value of the variable is undefined , when accessing properties that do not exist in the object, the returned value is also undefined; 2. null represents an empty object reference. In some cases, the object reference can be set to null to release the memory it occupies.

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. Nullability 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.

Usage: 1. Initialize the reference type variable to null, indicating that the variable does not currently point to any object; 2. Set the reference type variable to null, which can release the memory space of the object referenced by the variable and help the garbage collector to recover This object; 3. Use null to check whether a reference is empty. You can avoid the occurrence of NullPointerException by judging whether the reference is null. 4. Use null in conditional judgment to judge whether a reference is empty.

Both null and undefined indicate a lack of value or an undefined state. Depending on the usage scenario, there are some guiding principles for choosing to use null or undefined: 1. When you need to clearly indicate that a variable is empty or invalid, you can use null; 2. When a variable has been declared but not yet assigned a value, it will be set to undefined by default; 3. When you need to check whether a variable is empty or undefined, use the strict equality operator "===" to determine whether the variable is null or undefined. .

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Basic table Before developing the table component, first think about what style of API to use. Because the author uses element in production work, the styles of the previous components are similar to element, but this time I do not plan to use the element style. , I plan to change it and display it directly: We expect users to use it like this: constdataList=[{id:1,name:'"JavaEE Enterprise Application Practice"',author:'dev1ce',price:'10.22',desc:&# 3

During the use of MySQL, dates are generally stored in datetime, timestamp and other formats. However, sometimes due to special needs or historical reasons, the date is stored in varchar format. So how should we process the date data in varchar format? ? Use function: STR_TO_DATE(str,format) The STR_TO_DATE(str,format) function is the inverse function of the DATE_FORMAT() function. It takes a string str and a format string format. STR_TO_DATE() returns a DATETIME value if the format string contains date and time components, or if the word