What is the difference between let and var in Javascript?

The difference between let and var is the scope in which the variables are available.

• let declares a block-scoped variable. They are accessible only within the block (enclosed by curly braces) in which they are defined.
• var declares function scoped or globally-scoped variable. This means they are accessible throughout the entire function in which they are defined or globally if defined outside of any function.

But what does that mean? Let's look at the examples below to understand the differences better.

let vs var example

if (true) {
    var varVariable = 'I am var';
    let letVariable = 'I am let';
}
console.log(varVariable); // Outputs "I am var"
console.log(letVariable); // Throws "ReferenceError: letVariable is not defined"

varVariable is globally available, whereas letVariable is only available within the block {}.

What is scope in JavaScript?

Scope in JavaScript refers to the context in which variables are defined and accessed. It determines where a variable can be used and where it is not accessible. Scopes can be global, function, or block-based, with variables having different levels of visibility within these scopes. The concept of scope helps manage the visibility and lifetime of variables, preventing unintended interactions and conflicts between different parts of the code.

Blocks only scope let and const declarations, but not var declarations.

{
  var myVar = 1;
}
console.log(myVar); // 1

{
  let myLet = 1;
}
console.log(myLet); // ReferenceError: myLet is not defined

What is hoisting in JavaScript?

Hoisting in JavaScript is a behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase before the code is executed. This means that you can use variables and functions before they are actually declared in your code.

However, only the declarations are hoisted, not the initializations. This can sometimes lead to unexpected behavior and bugs if not understood correctly. It's important to be aware of hoisting to write more predictable and maintainable code.

var hoisting

// using variable before it was declared
console.log(test);   // undefined
var test;

As you can see in the example above, the test variable is defined after console.log(test), but the no reference error is thrown. This is because var declaration is initialized with undefined before its actual declaration, potentially leading to unexpected behavior.

let hoisting

// using variable before it was declared
console.log(test);   // ReferenceError: Cannot access 'test' before initialization
let test;

we'll get

ReferenceError: Cannot access 'test' before initialization

Variable declaration

Variables declared with let cannot be re-declared within the same scope, helping catch errors where you might accidentally declare the same variable name more than once.

{
  let myLet = 1;
  // do some work
  let myLet = 2; // SyntaxError: Identifier 'myLet' has already been declared
}

where compared to var

{
  var myVar = 1;
  // do some work
  var myVar = 2; // No problemo
}

Is it better to use let instead of var?

Yes, in most modern JavaScript development scenarios, it is generally recommended to use let instead of var.

Here are some reasons why let is preferred over var:

• Block Scoping: Variables declared with let have block scope. This helps prevent unintended variable leakage and makes the code more predictable and maintainable.
• No Hoisting Issues: Variables declared with let are hoisted to the top of their block, but they are not initialized. This prevents the "hoisting" behavior of var, where the variable is initialized with undefined before its actual declaration, potentially leading to unexpected behavior.
• Avoiding Global Namespace Pollution: Variables declared with var in the global scope become properties of the global object (e.g., window in browsers), which can lead to accidental global namespace pollution and potential conflicts with other variables.
• Redeclaration Prevention: Variables declared with let cannot be re-declared within the same scope, helping catch errors where you might accidentally declare the same variable name more than once.
• More Predictable Scoping: The use of let provides clearer scoping rules, which helps in understanding where a variable is accessible and where it's not. This contributes to code that is easier to read and maintain.

It's also a good practice to limit the scope of variables to the smallest context necessary for better code organization and to avoid potential issues. This helps in reducing the potential for bugs and makes it easier to reason about your code.

Conclusion

The choice between let and var in JavaScript revolves around the principles of modern coding practices and scoping clarity.

While both are used for variable declaration, let is favored for its block-scoping nature, making variables confined to specific blocks of code.

This improves predictability and reduces unintended variable leaks. 

By using let, developers can ensure cleaner, more maintainable code with fewer chances of naming conflicts and unexpected behaviors. Embracing let aligns with the evolution of JavaScript and best practices for coding clarity and predictability.

However, there are still cases where var might be useful, especially if you're dealing with older codebases or specific scenarios where the hoisting behavior is desired.