JavaScript’s let vs var: Key Differences and Usage Guidelines

JavaScript’s let and var are both used for variable declaration, but with significant differences. var has function scope and can be redeclared, while let has block scope and cannot be redeclared in the same scope. Modern JavaScript recommends using let and const over var for more predictable behavior, but var may still be useful in certain legacy code scenarios or when you specifically need function-scoped variables.

Contents

• Scope Differences: Function Scope vs Block Scope
• Hoisting Behavior: How var and let are handled differently
• Variable Redeclaration: Rules and limitations
• Temporal Dead Zone: Understanding let’s unique behavior
• When to Use let: Best practices and use cases
• When to Use var: Legacy scenarios and specific needs
• Migration Strategy: Updating older code to modern practices

Scope Differences: Function Scope vs Block Scope

The most fundamental difference between let and var lies in their scoping rules.

var has function scope:

• Variables declared with var are accessible throughout the entire function in which they are declared
• They are not limited to the block (defined by curly braces {}) where they are declared
• This can lead to unexpected behavior in complex functions with multiple blocks

function example() {
  if (true) {
    var x = 10;
  }
  console.log(x); // Output: 10 (accessible outside the if block)
}

let has block scope:

• Variables declared with let are only accessible within the block where they are declared
• This includes any nested blocks within their declaration context
• This provides better encapsulation and reduces the risk of unintended side effects

function example() {
  if (true) {
    let y = 20;
  }
  console.log(y); // ReferenceError: y is not defined
}

Block scope applies to all code blocks, including:

• Function blocks
• if/else statements
• switch statements
• for/while loops
• try/catch blocks

This difference becomes particularly important in loops:

// Using var in a loop
for (var i = 0; i < 3; i++) {
  // Each iteration uses the same i
}
console.log(i); // Output: 3 (i is still accessible outside the loop)

// Using let in a loop
for (let j = 0; j < 3; j++) {
  // Each iteration has its own j
}
console.log(j); // ReferenceError: j is not defined

Hoisting Behavior: How var and let are handled differently

Another critical difference is how these declarations are processed during the compilation phase, a concept known as hoisting.

var hoisting:

• Variables declared with var are “hoisted” to the top of their function scope
• This means you can access the variable before its declaration, though the value will be undefined
• The declaration is moved up, but the assignment stays in place

console.log(a); // Output: undefined
var a = 5;
console.log(a); // Output: 5

// Internally, JavaScript treats this as:
var a;
console.log(a); // undefined
a = 5;
console.log(a); // 5

let hoisting:

• Variables declared with let are also hoisted, but with a different behavior
• They are placed in a “Temporal Dead Zone” (TDZ) from the start of the block until the declaration is executed
• Accessing a let variable before its declaration results in a ReferenceError

console.log(b); // ReferenceError: Cannot access 'b' before initialization
let b = 10;
console.log(b); // Output: 10

This TDZ behavior with let helps prevent common programming errors by making variable usage more predictable.

Variable Redeclaration: Rules and limitations

Redeclaration behavior differs significantly between var and let.

var redeclaration:

• Variables declared with var can be redeclared in the same scope without error
• This can lead to unintended variable shadowing and hard-to-debug issues

var x = 10;
var x = 20; // Perfectly valid
console.log(x); // Output: 20

function test() {
  var y = 30;
  var y = 40; // Also valid within function scope
  console.log(y); // Output: 40
}

let redeclaration:

• Variables declared with let cannot be redeclared in the same scope
• This prevents accidental shadowing and makes the code more robust

let z = 50;
let z = 60; // SyntaxError: Identifier 'z' has already been declared

However, let variables can have the same name as var variables in different scopes:

var name = "Global";
if (true) {
  let name = "Local"; // Different scope, so this is fine
  console.log(name); // Output: "Local"
}
console.log(name); // Output: "Global"

Temporal Dead Zone: Understanding let’s unique behavior

The Temporal Dead Zone (TDZ) is a unique characteristic of let (and const) declarations that doesn’t apply to var.

What is the TDZ?

• The TDZ is the period from the start of a variable’s scope until the point where it’s declared
• During this time, the variable exists in a state where it’s declared but not yet initialized
• Accessing the variable during this period results in a ReferenceError

{
  // TDZ starts here for variable 'x'
  console.log(x); // ReferenceError: x is not defined
  
  let x = 10; // TDZ ends here
  console.log(x); // Output: 10
}

TDZ with function parameters:

• The TDZ concept also applies to function parameters when using default values

function example(param = param) { // ReferenceError: Cannot access 'param' before initialization
  console.log(param);
}

TDZ benefits:

• Prevents accessing variables before they’re initialized
• Makes code more predictable and less error-prone
• Helps developers catch potential issues early in the development process

When to Use let: Best practices and use cases

In modern JavaScript development, let is generally preferred over var in most scenarios.

Use let when:

• You need a variable that can be reassigned
• You want block scope for better encapsulation
• You’re working with loops where you need a fresh variable for each iteration
• You want to prevent accidental redeclaration of variables
• You’re following modern JavaScript best practices (ES6+)

Common let use cases:

1. Loop counters:

for (let i = 0; i < array.length; i++) {
  // Each iteration has its own i
  processElement(array[i]);
}

2. Variables in conditional blocks:

if (condition) {
  let result = calculate();
  console.log(result);
}
// result is not accessible here

3. Variables with limited scope:

function processData(data) {
  let processed = transform(data);
  return processed;
}
// processed is not accessible outside the function

4. When working with asynchronous code:

function fetchData() {
  let response;
  fetch(url)
    .then(res => {
      response = res; // Safe to use the same variable
    });
  return response;
}

Avoid let when:

• You specifically need function scope (though this is rare)
• You’re working with very old codebases that haven’t been updated to ES6
• You need to declare a variable that should be accessible to the entire function

When to Use var: Legacy scenarios and specific needs

While modern JavaScript generally favors let and const, there are still scenarios where var might be appropriate.

Use var when:

• You’re maintaining legacy code that uses var extensively
• You need a variable to be accessible throughout the entire function
• You’re working with code that needs to run in very old environments (pre-ES6)
• You intentionally want to allow redeclaration of variables in the same scope
• You’re dealing with event handlers in certain legacy patterns

Common var use cases:

1. Legacy code maintenance:

// In older codebases, you might encounter this pattern
function legacyFunction() {
  var config = getConfig();
  if (config.enabled) {
    var items = getItems();
    for (var i = 0; i < items.length; i++) {
      // Here, i is accessible throughout the function
      processItem(items[i]);
    }
    console.log(i); // Accessible outside the loop
  }
  console.log(config); // Accessible throughout the function
}

2. Event handlers with shared state:

// In certain legacy patterns, var provides function-level access
function setupCounter() {
  var count = 0;
  button.addEventListener('click', function() {
    count++;
    console.log(count);
  });
  // Other event handlers can also access count
}

3. Code that needs to run in very old environments:

// When targeting browsers that don't support ES6
var polyfillNeeded = detectOldBrowser();
if (polyfillNeeded) {
  loadPolyfills();
}

Avoid var when:

• Writing new code in modern JavaScript environments
• Working with block-scoped variables
• Trying to prevent variable hoisting issues
• Following modern JavaScript best practices and style guides

Migration Strategy: Updating older code to modern practices

If you’re working with code that uses var and want to modernize it, here’s a systematic approach.

Step-by-step migration:

1. Audit your codebase:

   • Identify all uses of var
   • Determine which ones can be safely converted to let or const
   • Look for patterns where var’s function scope is actually needed

2. Replace var with let or const:

   • Use const for variables that won’t be reassigned
   • Use let for variables that need to be reassigned
   • Be mindful of scope changes that might affect behavior

3. Address scope-related issues:

   • Fix any ReferenceErrors that occur due to block scope
   • Adjust code that relies on variables being accessible outside their original block

4. Test thoroughly:

   • Run all tests to ensure behavior hasn’t changed unexpectedly
   • Check for edge cases that might be affected by scoping changes

Migration example:

Before:

function calculateTotal(items) {
  var total = 0;
  for (var i = 0; i < items.length; i++) {
    var item = items[i];
    if (item.taxable) {
      total += item.price * 1.1;
    } else {
      total += item.price;
    }
  }
  return total;
  // i and item are still accessible here (though not used)
}

After:

function calculateTotal(items) {
  const total = 0; // Can be const if not reassigned
  for (let i = 0; i < items.length; i++) {
    const item = items[i]; // const since item isn't reassigned
    if (item.taxable) {
      total += item.price * 1.1;
    } else {
      total += item.price;
    }
  }
  return total;
}

Tools to help migration:

• ESLint with rules to detect and fix var usage
• Babel for transpiling modern JavaScript to older environments
• Code analysis tools to identify potential scope issues

Conclusion

Summary of key differences:

• var has function scope, let has block scope
• var is hoisted with initial value undefined, let is hoisted but in Temporal Dead Zone
• var can be redeclared, let cannot be redeclared in the same scope
• Modern JavaScript favors let and const over var for better predictability

Recommendations:

• Use let for variables that need to be reassigned and benefit from block scope
• Use const for variables that won’t be reassigned (prefer const over let when possible)
• Reserve var for legacy code maintenance or specific edge cases where function scope is needed
• When migrating from var to let/const, pay special attention to scope changes

Final thoughts:
The evolution from var to let represents JavaScript’s growth toward more robust scoping rules. While var has its place in maintaining legacy code, modern development practices strongly favor let and const for their predictable behavior and better error prevention. Understanding these differences will help you write cleaner, more maintainable JavaScript code regardless of which environments you’re targeting.