var, let, and const in JavaScript have significant differences in scope, variable promotion, and variability. 1. var is the function scope, accessible anywhere within the function; let and const are block-level scopes, accessible only within the code block that declares them. 2. var has variable promotion and is initialized to undefined, while let and const are also promoted, but are in a "temporary dead zone". An error will be reported before the declaration. 3. var can be repeatedly declared and assigned; let cannot be repeatedly declared but can be reassigned; const cannot be repeatedly declared and cannot be reassigned (object or array content can be modified). 4. Const should be used first in actual development. If reassignment is required, use let, and try to avoid using var.

var , let and const in JavaScript seem to be used to declare variables, but their behavior and usage scenarios are actually very different. Many people will use var directly when they first start writing JS, but they encounter problems such as variable improvement and confusion in scope. Understanding the difference between these three keywords can help you write more stable and maintainable code.

1. Variable scope: block-level scope vs function scope

This is one of the most core differences between the three.

• var is function-scoped: it only works inside the current function, not inside the code block {} .
• let and const are block-scoped: as long as they are declared in {} , they can only be accessed in that code block.

For example:

 if (true) {
  var nameVar = 'Tom';
  let nameLet = 'Jerry';
}
console.log(nameVar); // Output 'Tom'
console.log(nameLet); // Error: nameLet is not defined

As can be seen from the above, the variables declared by var will be "uplifted" to the external scope, while let and const can only be used in the current block.

2. Variable hoisting behavior is different

Variable promotion is a feature of JS, that is, variable declarations will be "moved" to the top of the current scope.

• var will be promoted and initialized to undefined .
• let and const will also be promoted, but will not be initialized, entering a so-called "temporal dead zone" (TDZ), and an error will be reported when accessing before the declaration.

for example:

 console.log(a); // undefined
var a = 5;

console.log(b); // ReferenceError
let b = 10;

So if you try to use variables before let or const declaration, JS throws an error instead of returning undefined like var .

3. Whether it is possible to reassign and redeclare

This is also a factor to consider when choosing which keyword to use.

• var : You can declare it repeatedly or reassign it.
• let : The declaration cannot be repeated, but the value can be reassigned.
• const : You cannot declare repeatedly, and you cannot reassign values ??(if it is an object or array, the content can be modified).

For example:

 var x = 1;
var x = 2; // Legal let y = 1;
let y = 2; // error const z = 1;
z = 2; // Error const obj = { name: 'Alice' };
obj.name = 'Bob'; // Legal, because the content of the object is modified, not the reference address

Therefore, when a constant is needed, const is preferred, so as to avoid the problem of accidentally modifying and changing the variable.

4. How to choose in actual development?

Simply put:

• Use const first, unless you are sure that the variable will be reassigned after it is determined.
• If a variable needs to be assigned multiple times, use let .
• Try to avoid using var unless you are maintaining old projects or have special needs.

Some common usage scenarios:

• ? Constant definition: API address, configuration item, etc. → const
• ? Loop counter, status flag → let
• ? It is not recommended to use var to declare variables

Basically that's it. Although it seems like only a different way of declaring variables, in actual encoding, they affect the scope, life cycle, and maintainability of the variables. Figuring out the difference between them will allow you to write clearer and less buggy JavaScript code.

The above is the detailed content of Understanding var, let, and const in JavaScript. For more information, please follow other related articles on the PHP Chinese website!