The difference between const and var in js

In JavaScript, const and var are keywords used to declare variables. They have the following differences in scope, variability and initial value: Scope: const is only within the block-level scope in which it is declared. is valid, whereas var is valid throughout the function scope. Mutability: Once declared, a const variable cannot be changed, whereas a var variable can be reassigned. Initial value: const variables must be initialized when declared, while var variables do not need to be initialized, and the default value is undefined.

The difference between const and var in JavaScript

Start

const and var are two keywords used to declare variables in JavaScript. They have different behaviors regarding scope, mutability, and initialization.

Scope

• var: Valid throughout the entire function scope.
• const: is only valid within the block-level scope in which it is declared.

Variability

• var: Variable values ​​can be reassigned.
• const: Once the variable value is declared, it cannot be changed.

Initial value

• var: Variables may not be initialized, and the default value is undefined.
• const: Must be initialized when declared.

Other Differences

In addition to these main differences, there are some other differences between const and var:

• hoisting: var variables are hoisted to the top of the function, while const variables are not.
• Block-level scope: const uses block-level scope, but var does not.
• Temporary dead zone: Accessing a const variable before it is declared causes a temporary dead zone, but not a var variable.

When to use const and var

• Use const to declare values ​​that will not change, such as constants and enumerations Lift.
• Use var to declare values ​​that need to be changed within the function scope.

Example

<code class="javascript">// 使用 var 声明变量
var name = "John";
name = "Mary"; // 重新赋值

// 使用 const 声明常量
const PI = 3.14;
PI = 3.15; // 错误：常量不可变</code>

Conclusion

const and var are two different ways to declare variables in JavaScript Keywords, which have clear differences in scope, mutability, and initial values. Understanding these differences is important for writing robust and maintainable JavaScript code.

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