The difference between undefined and null in js

The difference between undefined and null in JavaScript: undefined means that the variable is not declared or assigned a value, and null means that the variable is clearly empty. undefined is a primitive type and null is an object type. undefined evaluates to false when compared to any value, and null evaluates to true when compared to itself. Use undefined when the variable has not been declared or assigned a value, and use null to explicitly indicate that the value is null.

The difference between undefined and null in JavaScript

In JavaScript, undefined and null are both special values, but there is a key difference between them.

undefined

• When a variable is not declared or assigned a value, its value is undefined.
• undefined Indicates that the variable does not exist or has not been initialized.

null

• null is a special value that clearly indicates that the value of the variable is empty.
• It is not a primitive type in JavaScript, but an object type.

Main difference

• Semantics: undefined means the variable does not exist, while null means that the variable exists but its value is null.
• Type: undefined is a primitive type, while null is an object type (special case).
• Comparison: undefined is false when compared to any value, but null when compared to itself true, compared with other values ​​are false.

When to use undefined and null

##Use
• undefined : When the variable has not been declared or initialized.
Use
• null: when it is explicitly necessary to indicate that a value is null. For example, when representing an empty field in the database as null.

Example

let myVariable1; // 未声明，值为 undefined

let myVariable2 = null; // 明确赋值为 null

In the first example, the value of

myVariable1

is undefined because it Has not been declared or assigned a value. In the second example, the value of

myVariable2

is null because it was explicitly assigned the value null.

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