JavaScript Var、Let、Const: 主な違いと最適な使用法

導入

JavaScript では、変数はコード全体でデータを保存および操作できるようにする基本的な構成要素です。ユーザー入力を追跡する場合でも、状態を管理する場合でも、単に後で使用するために値を保持する場合でも、変数は JavaScript アプリケーションに不可欠です。 JavaScript が進化するにつれて、これらの変数を定義する方法も進化しました。

現在、JavaScript で変数を宣言するには、var、let、const という 3 つの主な方法があります。これらのキーワードはそれぞれ異なる動作を提供するため、クリーンで効率的でバグのないコードを作成するには、それぞれをいつ使用するかを理解することが重要です。

このブログでは、JavaScript の var、let、const の違いを調べ、その使用法を比較し、それぞれのベスト プラクティスを示す実用的なコード例を提供します。最後には、ニーズに合った適切な変数宣言を選択する方法を明確に理解し、より良い JavaScript コードを作成するのに役立ちます。

var について理解する

var は JavaScript で変数を宣言するための元の方法であり、長年にわたって定番でした。ただし、JavaScript が進化するにつれて、var の制限と問題により、ES6 (ECMAScript 2015) では let と const が導入されました。
var の主な特徴は、関数スコープであることです。つまり、var が宣言されている関数内でのみアクセス可能です。関数の外で宣言するとグローバル変数になります。この関数のスコープは、let および const によって提供されるブロックのスコープとは異なります。

var のもう 1 つの重要な機能は、実行中に変数宣言がスコープの先頭に移動されるホイスティングです。これにより、var 変数を宣言する前に参照できますが、その値は代入が行われるまで未定義になります。ホイスティングは便利ですが、特に大規模なコードベースでは、混乱や微妙なバグを引き起こすことがよくあります。

吊り上げ例：

console.log(x); // Outputs: undefined
var x = 5;
console.log(x); // Outputs: 5

この例では、x が宣言される前にログに記録されていますが、コードはエラーをスローしません。代わりに、巻き上げにより unknown が出力されます。 JavaScript は、var x 宣言がスコープの先頭に移動されたかのようにコードを処理します。

var に関する問題: 偶発的なグローバルと再宣言

var でよくある落とし穴の 1 つは、グローバル変数が誤って作成されてしまうことです。関数内で var を使用するのを忘れると、JavaScript によってグローバル変数が作成され、予期しない動作が発生する可能性があります。

function setValue() {
    value = 10; // Accidentally creates a global variable
}
setValue();
console.log(value); // Outputs: 10 (global variable created unintentionally)

もう 1 つの問題は、var が同じスコープ内での再宣言を許可していることです。これにより、追跡が困難なバグが発生する可能性があります。

var x = 10;
var x = 20;
console.log(x); // Outputs: 20

ここでは、変数 x が再宣言されて新しい値が割り当てられ、警告なしに前の値が上書きされる可能性があります。

var を使用する場合

最新の JavaScript let と var と const では、var は一般に推奨されず、より適切なスコープを提供し、多くの一般的な問題を防ぐ let と const が優先されます。ただし、var は、リファクタリングがオプションではない従来のコードベース、または関数レベルのスコープが明示的に必要な特定のシナリオでは引き続き適用できる可能性があります。

理解しましょう

let は、ES6 (ECMAScript 2015) で導入されたブロック スコープの変数宣言です。関数スコープの var とは異なり、let はループや if ステートメント内など、それが定義されているブロックに限定されます。このブロックのスコープ設定は、変数へのアクセスを必要な特定のブロックに制限することで、エラーを防止し、コードをより予測しやすくします。

関数スコープとブロック スコープの主な違いは、関数スコープの変数 (var) は宣言されている関数全体を通じてアクセスできるのに対し、ブロック スコープの変数 (let) は特定のブロック内でのみアクセスできることです。ループまたは条件文として定義されます。 let のこの動作は、変数が意図されたスコープ外で意図せずアクセスされることによって発生する問題を回避するのに役立ちます。

ループ内のレットの例:

for (let i = 0; i < 3; i++) {
    console.log(i); // Outputs 0, 1, 2
}
console.log(i); // ReferenceError: i is not defined

この例では、ブロックスコープのため、i はループ内でのみアクセス可能です。

var との比較:

if (true) {
    var x = 10;
    let y = 20;
}
console.log(x); // Outputs 10 (function-scoped)
console.log(y); // ReferenceError: y is not defined (block-scoped)

ここで、x は var の関数スコープのため if ブロックの外でアクセスできますが、y は let's ブロックのスコープのためブロックの外ではアクセスできません。

Understanding const

const is another block-scoped variable declaration introduced in ES6, similar to let. However, const is used to declare variables that are intended to remain constant throughout the program. The key difference between const and let is immutability: once a const variable is assigned a value, it cannot be reassigned. This makes const ideal for values that should not change, ensuring that your code is more predictable and less prone to errors.

However, it’s important to understand that const enforces immutability on the variable binding, not the value itself. This means that while you cannot reassign a const variable, if the value is an object or array, the contents of that object or array can still be modified.

Example with Primitive Values

const myNumber = 10;
myNumber = 20; // Error: Assignment to constant variable.

In this example, trying to reassign the value of myNumber results in an error because const does not allow reassignment.

Example with Objects/Arrays

const myArray = [1, 2, 3];
myArray.push(4); // Allowed
console.log(myArray); // Output: [1, 2, 3, 4]

const myObject = { name: "John" };
myObject.name = "Doe"; // Allowed
console.log(myObject); // Output: { name: "Doe" }

Here, even though the myArray and myObject variables are declared with const, their contents can be modified. The const keyword only ensures that the variable itself cannot be reassigned, not that the data inside the object or array is immutable.

When to Use const

Best practices in modern JavaScript suggest using const by default for most variables. This approach helps prevent unintended variable reassignment and makes your code more reliable. You should only use let when you know that a variable's value will need to be reassigned. By adhering to this principle, you can reduce bugs and improve the overall quality of your code.

Comparing var, let, and const

Key Differences:

Feature	var	let	const
Scope	Function-scoped	Block-scoped	Block-scoped
Hoisting	Hoisted (initialized as undefined)	Hoisted (but not initialized)	Hoisted (but not initialized)
Re-declaration	Allowed within the same scope	Not allowed in the same scope	Not allowed in the same scope
Immutability	Mutable	Mutable	Immutable binding, but mutable contents for objects/arrays

---

Code Examples

Example of Scope:

function scopeTest() {
    if (true) {
        var a = 1;
        let b = 2;
        const c = 3;
    }
    console.log(a); // Outputs 1 (function-scoped)
    console.log(b); // ReferenceError: b is not defined (block-scoped)
    console.log(c); // ReferenceError: c is not defined (block-scoped)
}
scopeTest();

In this example, var is function-scoped, so a is accessible outside the if block. However, let and const are block-scoped, so b and c are not accessible outside the block they were defined in.

Example of Hoisting:

console.log(varVar); // Outputs undefined
console.log(letVar); // ReferenceError: Cannot access 'letVar' before initialization
console.log(constVar); // ReferenceError: Cannot access 'constVar' before initialization

var varVar = "var";
let letVar = "let";
const constVar = "const";

Here, var is hoisted and initialized as undefined, so it can be referenced before its declaration without causing an error. However, let and const are hoisted but not initialized, resulting in a ReferenceError if accessed before their declarations.

Example of Re-declaration

var x = 10;
var x = 20; // No error, x is now 20

let y = 10;
let y = 20; // Error: Identifier 'y' has already been declared

const z = 10;
const z = 20; // Error: Identifier 'z' has already been declared

With var, re-declaring the same variable is allowed, and the value is updated. However, let and const do not allow re-declaration within the same scope, leading to an error if you try to do so.

Example of Immutability:

const myArray = [1, 2, 3];
myArray.push(4); // Allowed
console.log(myArray); // Output: [1, 2, 3, 4]

myArray = [4, 5, 6]; // Error: Assignment to constant variable

In this case, const prevents reassignment of the variable myArray, which would result in an error. However, the contents of the array can still be modified, such as adding a new element.

Best Practices

In modern JavaScript, the consensus among developers is to use const and let in place of var to ensure code that is more predictable, maintainable, and less prone to bugs. Here are some best practices to follow:

1. Use const by Default Whenever possible, use const to declare variables. Since const ensures that the variable cannot be reassigned, it makes your code easier to understand and prevents accidental modifications. By defaulting to const, you signal to other developers (and yourself) that the value should remain constant throughout the code's execution.
2. Use let Only When Reassignment is Necessary If you know that a variable's value will need to change, use let. let allows for reassignment while still providing the benefits of block-scoping, which helps avoid issues that can arise from variables leaking out of their intended scope.
3. Avoid var in Modern JavaScript In modern JavaScript, it’s best to avoid using var altogether. var's function-scoping, hoisting, and the ability to be redeclared can lead to unpredictable behavior, especially in larger codebases. The only time you might need to use var is when maintaining or working with legacy code that relies on it.

4. Sample Refactor: Converting var to let and const
   Here’s a simple example of refactoring older JavaScript code that uses var to a more modern approach with let and const.

   Before Refactoring:
   

   function calculateTotal(prices) {
   var total = 0;
   for (var i = 0; i < prices.length; i++) {
       var price = prices[i];
       total += price;
   }
   var discount = 0.1;
   var finalTotal = total - (total * discount);
   return finalTotal;
   }
   

   After Refactoring:
   

   function calculateTotal(prices) {
   let total = 0;
   for (let i = 0; i < prices.length; i++) {
       const price = prices[i]; // price doesn't change within the loop
       total += price;
   }
   const discount = 0.1; // discount remains constant
   const finalTotal = total - (total * discount); // finalTotal doesn't change after calculation
   return finalTotal;
   }
   

   In the refactored version, total is declared with let since its value changes throughout the function. price, discount, and finalTotal are declared with const because their values are not reassigned after their initial assignment. This refactoring makes the function more robust and easier to reason about, reducing the likelihood of accidental errors.

Common Pitfalls and How to Avoid Them

When working with var, let, and const, developers often encounter common pitfalls that can lead to bugs or unexpected behavior. Understanding these pitfalls and knowing how to avoid them is crucial for writing clean, reliable code.

Accidental Global Variables with var

One of the most common mistakes with var is accidentally creating global variables. This happens when a var declaration is omitted inside a function or block, causing the variable to be attached to the global object.

function calculate() {
    total = 100; // No var/let/const declaration, creates a global variable
}
calculate();
console.log(total); // Outputs 100, but total is now global!

How to Avoid:
Always use let or const to declare variables. This ensures that the variable is scoped to the block or function in which it is defined, preventing unintended global variables.

Hoisting Confusion with var

var is hoisted to the top of its scope, but only the declaration is hoisted, not the assignment. This can lead to confusing behavior if you try to use the variable before it is assigned.

console.log(name); // Outputs undefined
var name = "Alice";

How to Avoid:
Use let or const, which are also hoisted but not initialized. This prevents variables from being accessed before they are defined, reducing the chance of errors.

Re-declaration with var

var allows for re-declaration within the same scope, which can lead to unexpected overwrites and bugs, especially in larger functions.

var count = 10;
var count = 20; // No error, but original value is lost

How to Avoid:
Avoid using var. Use let or const instead, which do not allow re-declaration within the same scope. This ensures that variable names are unique and helps prevent accidental overwrites.

Misunderstanding const with Objects and Arrays

Many developers assume that const makes the entire object or array immutable, but in reality, it only prevents reassignment of the variable. The contents of the object or array can still be modified.

const person = { name: "Alice" };
person.name = "Bob"; // Allowed, object properties can be modified

person = { name: "Charlie" }; // Error: Assignment to constant variable

How to Avoid: Understand that const applies to the variable binding, not the value itself. If you need a truly immutable object or array, consider using methods like Object.freeze() or libraries that enforce immutability.

Scope Misconceptions with let and const

Developers may incorrectly assume that variables declared with let or const are accessible outside of the block they were defined in, similar to var.

if (true) {
    let x = 10;
}
console.log(x); // ReferenceError: x is not defined

Always be aware of the block scope when using let and const. If you need a variable to be accessible in a wider scope, declare it outside the block.

By understanding these common pitfalls and using var, let, and const appropriately, you can avoid many of the issues that commonly arise in JavaScript development. This leads to cleaner, more maintainable, and less error-prone code.

Conclusion

In this blog, we've explored the key differences between var, let, and const—the three primary ways to define variables in JavaScript. We've seen how var is function-scoped and hoisted, but its quirks can lead to unintended behavior. On the other hand, let and const, introduced in ES6, offer block-scoping and greater predictability, making them the preferred choices for modern JavaScript development.

For further reading and to deepen your understanding of JavaScript variables, check out the following resources:

MDN Web Docs: var

MDN Web Docs: let

MDN Web Docs: const

Understanding when and how to use var, let, and const is crucial for writing clean, efficient, and bug-free code. By defaulting to const, using let only when necessary, and avoiding var in new code, you can avoid many common pitfalls and improve the maintainability of your projects.

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