Javascript Series: Which one to choose: var, let, or const?

In JavaScript, there are three keywords for variable declaration: var, let, and const. While they are all used to create variables, they have important differences. Understanding these differences will help us write more readable and efficient code, and avoid unexpected errors. In this article, we will explore the var, let, and const keywords and their usage.

Var - Classic but Unsafe

The var keyword has been around since the early versions of JavaScript. It allows variable declaration without scope and has hoisting behavior - meaning variables can be used before they are declared.

console.log(name); // Output: undefined
var name = "John";
console.log(name); // Output: John

In the above example, the variable name is declared using the var keyword after the console.log statement, but it can still be accessed before being declared. However, the value of the variable will be undefined until it is assigned a value.

Another thing to note is that variables declared with var have function scope or global scope (if not declared inside a function). This can lead to variable name conflicts and make it difficult to control.

Let - Block Scope and More Limited

The let keyword was introduced in ECMAScript 6 (ES6). It limits the scope of a variable to the block in which it is declared. Importantly, let is not affected by hoisting.

console.log(name); // Error: name is not defined
let name = "John";
console.log(name); // Output: John

In the above example, when we try to access the variable name before it is declared, an error is thrown. This helps us catch and fix errors earlier.

Another advantage of let is that we can declare variables in separate blocks, keeping the variable from leaking outside.

Const - Unchanging, Immutable

The const keyword, also introduced in ES6, creates variables with immutable values. When a variable is declared with const, we cannot assign a new value to it.

const pi = 3.14;
pi = 3.14159; // Error: Assignment to constant variable.

In the above example, we cannot assign a new value to the constant pi. This ensures that the value of a const variable remains unchanged throughout the execution of the code.

An important note is that when we use const to declare an object or array, only the value of the variable is immutable. The properties or elements inside the object or array can still be modified.

When to Use Var, Let, and Const?

• Use var when you need support for variable hoisting and availability before and after its declaration.
• Use let when you only need the variable to have block scope and don’t want it to be accessed from outside.
• Use const when you need to declare an unchanging constant.

Conclusion

Understanding the differences between var, let, and const in JavaScript is crucial for writing readable, efficient code and avoiding unexpected errors. Using the appropriate keyword helps us manage variables tightly and produce higher-quality JavaScript code.