Understanding let and const in ES6
The introduction of `let` and `const`for variable declarations has significantly changed how developers write JavaScript code. These keywords provide block scope variables and constants, respectively, bringing JavaScript closer in line with other high-level programming languages. This post will explore the features of let and `const`, their advantages, and how to use them effectively.
What is `let`?
The `let`statement declares a block-scoped local variable, optionally initializing it to a value. Unlike variables declared with `var`, which are function-scoped, `let` variables are scoped to the nearest enclosing block.
function varTest() {
var x = 31;
if (true) {
var x = 71; // Same variable!
console.log(x); // 71
}
console.log(x); // 71
}
function letTest() {
let x = 31;
if (true) {
let x = 71; // Different variable
console.log(x); // 71
}
console.log(x); // 31
}
Advantages of let
- Block Scope: `
let`provides block-level scoping, reducing errors and confusion that arise from the function-scoped nature of `var`. - No Hoisting: Variables declared with
letare not hoisted to the top of the block, which means they do not exist until the declaration is evaluated. - Reduces Global Pollution: Since `
let`is block-scoped, it helps in minimizing global variables and thus reduces the chance of variable collisions.
What is `const`?
The `const` statement declares a block-scoped variable that cannot be reassigned after it is declared. `const` is ideal for defining constants, providing an assurance that their values will not change through reassignment.
Example of const:
const PI = 3.14;
PI = 3.14159; // TypeError: Assignment to constant variable.
const settings = {
url: 'http://api.example.com'
};
settings.url = 'http://api.example2.com'; // This is allowed.
settings = {}; // TypeError: Assignment to constant variable.
Advantages of `const`
- Immutability: While `
const`itself does not make objects immutable, it prevents reassignment of the variable identifier, which adds a layer of protection over values meant to stay constant. - Block Scope: Similar to
let,constis also block-scoped. - Readability and Maintenance: Using
constmakes it clear to other developers that the variable should not change, improving the readability and maintainability of the code.
Recommendations: When to Use let and `const`
- Use `
const`: When you declare a variable that you don’t plan to reassign. This includes objects that might have their properties modified but not reassigned entirely. - Use
let: When you need to reassign variables, such as in a loop or to swap values.
By using let and const, developers can write more predictable and bug-free JavaScript, adhering to modern programming practices. As best practices, aim to use const by default and let when reassignment is needed. This approach leads to safer and cleaner code.
