Let vs Var in JavaScript: How They Differ and When to Use Each

As you write JavaScript, binding values to variables is essential. You‘ll often reach for either let or var to accomplish this. But beyond both declaring variables, they have important differences in how they are scoped, hoisted, and reused.

Let‘s quickly summarize these behaviors in a comparison table before diving into the details:

Scope refers to where variables are accessible. Hoisting determines availability before declaration. Redeclarations allow reusing the same variable names.

As we‘ll see, how let and var differ here leads to substantially different outcomes when writing JavaScript functions and scripts.

A Brief History of Let and Var

To understand the motivations behind let, we should revisit the early days of the JavaScript language you may be learning…

When Brendan Eich first created JavaScript in 1995, var declarations were the only way to define variables. Back then, variables could either have:

• Function scope: When declared inside a function
• Global scope: When declared outside functions

This posed challenges as JS apps grew larger in scale:

• Accidentally declaring globals that collided with other scripts
• No easy way to limit vars to specific blocks of code
• Closure function issues since state was shared
• Allowing access to vars before initialization

To address this, ECMAScript 2015 introduced block scoping with the let keyword. For the first time, developers could restrict variables explicitly to the block, statement, or expression they were declared in.

"Block scoping is one of the most important changes I want to get into JavaScript"

Brendan Eich, Inventor of JavaScript

The initial motivation for let was fixing weaknesses around global namespaces, closures, and variable visibility with var. Let introduced fundamentally different behavior that encouraged writing safer and more modular JavaScript.

Today, let is the preferred way to declare variables in modern JS, largely replacing var usage in newer code. However, you‘ll still frequently encounter var declarations in legacy code.

Alright, now that we‘ve covered some historical context, let‘s move onto…

Key Behavioral Differences Between Let and Var

While let and var both declare variables in JavaScript, how they differ can profoundly impact your code…

Difference #1: Block Scope vs. Function Scope

The first vital difference is in their lexical scoping rules.

Consider this code block:

function myFunction() {

  if (true) {
    let a = 5; 
  }

  // let a not accessible here
  console.log(a);
}

• let variables are block scoped

  • They only exist within the block they are defined in { }

• var variables are function scoped

  • Available throughout the entire function

So in the code above, a only exists within the if block. Trying to reference it afterwards throws a ReferenceError as that binding no longer exists.

Conversely, variables declared with var anywhere inside a function are visible throughout the entire function body.

Block scope vs function scope has critical implications on where variables are accessible in our code.

Difference #2: Hoisting Behavior

The next vital difference between let and var is hoisting behavior during compilation.

But what is hoisting exactly?

Hoisting in JavaScript refers to variable declarations being moved conceptually to the top of their enclosing scope during compilation.

This means variables can be referenced in code before they are declared, just as if they were declared at the scope‘s top. But the specific hoisting rules differ between our variable types…

Consider this code:

console.log(x); // undefined

var x = 10; 
console.log(x); // 10 

console.log(y); // Throws ReferenceError

let y = 20;

Here‘s what happens step-by-step:

1. var x declaration is hoisted to the top and initialized as undefined
2. First console.log(x) returns undefined rather than throwing an error
3. x is later defined, re-assigning it from undefined to 10

In contrast, let variables are NOT initialized during the hoisting phase. They remain uninitialized until the line where the let statement is executed.

So console.log(y) tries accessing this uninitialized temporal dead zone variable and throws an error. This visibility restriction prevents subtle bugs due to using vars before properly defining them.

Difference #3: Redeclaration Rules

Next, the two declaration types differ in whether they allow variables to be re-declared within the same scope.

var allows multiple variables with the same name in the same scope:

var x = 10;
var x = 20; // Allowed!

Here, the existing x binding is simply replaced in the environment record.

By contrast, let follows strict mode and will NOT tolerate re-declarations:

let y = 10; 
let y = 30; // Uncaught SyntaxError: Identifier ‘y‘ has already been declared

Attempting to re-declare a let variable in the same block is invalid syntax and will break your script.

This constraint helps avoid accidental variable shadowing that can introduce subtle, hard-to-catch bugs during execution.

Difference #4: Global Window Attachment

Finally, an impactful difference to note when declaring variables:

• var declarations attach to the global window object as properties

    var x = 10;
  
    console.log(window.x) // 10

• let variables do NOT attach as window properties

    let y = 20;
  
    console.log(window.y); // undefined

So var accidentally pollutes the global namespace which can cause naming collisions. Let avoids this.

Combined, these nuances fundamentally impact how we write JavaScript…

Why Prefer Let Over Var

Given the above differences, why is let considered superior to var in most cases?

Several reasons why let encourages writing safer and more maintainable code:

It Reduces Bugs Through Stricter Rules

Let eliminates bugs by promoting:

• Local variable scope

  No accidental globals that could collide with other scripts

• No use before declaration errors

  Cannot access uninitialized variables unlike var

• No accidental closure state sharing

  Unique variable binding per closure loop iteration

• Simpler code organization

  Declare variables closer to usage for improved readability

Let forces following stricter rules that surface issues immediately rather than failing silently.

It Allows Writing More Modular Code

The block scoping introduced with let:

• Enables cleanly limiting variables to narrower scopes
• Discourages use of global variables
• Makes code more self-contained and reusable

This modular code tendency improves maintainability and simplifies reasoning about our programs.

It Aligns With Modern JavaScript Environments

Tools and standards embrace let scoping:

• Prettier and linters prefer let by default
• Compatible across all modern browsers
• More future-proof as global scope pollution falls out of favor

Overall, let usage leads to safer code, catches more errors during development, and embraces widespread best practices.

This is why you should default to using let over var for declaring variables in modern JavaScript.

However, var may suit situations where you explicitly require function-scoped variables not limited to a block, or need compatibility with older JS environments lacking let support.

Now that we‘ve thoroughly compared let and var, let‘s answer common questions that often arise…

Frequently Asked Questions

Here are answers to some frequent reader questions about using let and var declarations:

When Should I Use Let vs Var?

Use let anytime you need your variable scoped locally to a block rather than accessible throughout an entire function.

Great candidates for let are temporary variables, loop iterators, and bindings you only need access to for a short segment of code.

Use var when you specifically want access to your variable across an entire function body without block limitations. But restrict it only to cases where function scope is needed rather than defaulting to var everywhere.

Can I Redeclare Variables?

No, let does not permit re-declaring the same variable name within the same block scope. Doing so raises a syntax error.

However, var allows re-declaring variables with the same name in the same scope. Rather than throwing an error, this simply replaces the prior binding with the new assignment.

Is There a Performance Difference?

No material difference exists between let and var when it comes to JavaScript engine performance optimizations in modern runtimes.

Certain micro-performance tweaks were discussed early on such as slightly slower initial variable binding resolution for let. But engines now treat both similarly speed-wise.

Optimize first for readable and maintainable code before worrying about micro performance differences between variable types. Computers are fast – your time is precious!

How Do Closures Differ?

Closures have access to their outer scope variables. The key behavioral difference arises for loops contained within an outer closure binding function:

• var shares the same variable instance across all loop iterations
• let binds a new variable instance per iteration

This means closures created inside let loops maintain the value fixed at their time of declaration rather than grabbing the latest shared state. Used properly, let avoids closure issues due to unintended variable sharing across environments.

Key Takeaways

Let‘s recap the key points on let and var:

• let is the modern standard for declaring local block scoped variables
• var is function scoped rather than limited to a block
• let prevents redeclarations and enforces one binding per scope
• let throws errors eagerly when accessing uninitialized variables
• let helps avoid closure pitfalls due to per-iteration binding

Internalizing these behavioral differences will allow you to write cleaner JavaScript code.

• Use let as your default for tidy block scoped variables
• Leverage var when you specifically need function-wide visibility

I hope this guide has shed light on these foundational concepts for declaring variables within your scripts! Please leave any questions in the comments below.