When to use null and undefined
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- 2023-11-13 14:11:27756browse
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 it is necessary 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.
# Operating system for this tutorial: Windows 10 system, Dell G3 computer.
In programming, both null and undefined represent a missing value or an undefined state. Although they are somewhat similar, there are some differences in their use:
- null: null is a special keyword in the JavaScript language that represents an empty, invalid, or valueless object. You can assign null to any variable, indicating that the value of the variable is empty. Normally, null is assigned explicitly by the programmer.
For example:
let person = null; // 人员信息为空
- undefined: undefined indicates a state in which a variable is declared but has not been initialized, or a state in which an object property does not exist. When a variable is declared but not assigned a value, the default initial value is undefined. In addition, when there is no return value in the function, undefined is returned by default.
For example:
let name;
console.log(name); // 输出 undefined
function foo() {
// 没有返回值
}
console.log(foo()); // 输出 undefined
According to different usage scenarios, there are some guiding principles for choosing to use null or undefined:
- When a variable needs to be clearly indicated When empty or invalid, null can be used.
- When a variable has been declared but not assigned a value, it will be set to undefined by default.
- 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.
In general, null and undefined are used in JavaScript to indicate a missing value or an undefined state, but which one to use depends on the code logic and requirements.
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