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Function vs. Block Level Scoping with let and const

2016-08-25 Tiffany WhiteJavaScript

This is an exposé on the difference between function and block level scope using let and const in JavaScript. The following explanation is how I’ve come to understand them so far 1.

Function or Lexical Scope

Take this code:

  function foo(a) {
   var b = 2;
   // some code
   function bar() {
   // ...
  // more code
  var c = 3;

Here we have a function foo that sits in the global scope. Nested within this scope is the function bar() and nested inside foo. Also within foo, we have the variable declaration var b = 2; and var c = 3;. Each of these belong to scope— foo to the global scope, var b, var b, and var c all scope to foo. Each of these scopes have access to functions and identifiers of the scopes outside them. None of these functions or identifiers are accessible outside of the function scope foo.

Disadvantages of Function Scopes

Hoisting by the interpreter

The first pass the interpreter does on var processes it and lifts it to the top of the scope. It then processes the function declarations as uninitialized variables. 2

An example of hoisting:

function bar() {
  if (!foo) {
    var foo = 10;
bar(); // alerts 10 instead of reference error

Having to use an IIFE 3

For beginners like me, this is confusing syntax. I see all IIFE as closures and I am sure that’s not the case. For instance:

  // all your code here
  var foo = function() {};
	window.onload = foo;
	 // ...
// foo is unreachable here (it’s undefined)

In this example, the parentheses before function turn this block of code into a function expression. The final (); calls the function immediately after it is created. This creates a block scope for foo because it is nested within the function expression and invoked immediately. The syntax is hard to understand and thus remember.

Creating Block Scopes with let and const

You can easily create block scopes with let. It is a new keyword in ES6 that makes it stylistically easier to create a block scope in some block of code (between {...}). The variable cannot be accessed in the global scope. Take this code for example:

if (a) {
  b = a * 2;
  let a = 5;

console.log(b); // Reference Error! a is not defined.

The error here is because console.log() can’t access the a variable as let scopes it to the if block.

let also signals to another developer that the identifier you’re using will be mutated at some point along the way, at least according to Eric Elliott. This is why he suggests using const for immutable identifiers.


This creates block scope but allows for the data the identifier holds to be quasi immutable. The data can be mutated. const only is immutable in the sense that it binds the data to the identifier, and thus should be used when only when you want to treat a value as non-reassignable. In this instance, const is not like CONST in Java and other languages.

You can actually make a const value immutable by adding object.freeze() to it.

So let or const?

If you took a minute to read the article I linked to by Eric Elliott, you’ll see his strong opinion in favor of const. He also stated that var is the weakest link in ES6. But Kyle Simpson had something interesting to say about it

let improves scoping options in JS, not replaces. var is still a useful signal for variables that are used throughout the function. Having both, and using both, means scoping intent is clearer to understand and maintain and enforce. That’s a big win!


I am still not sure which is the right one to use and when. I am sticking with let for now, whenever I need block scopes. It is important to note you don’t have to use block scopes all the time. A mixture of function or static scopes and block scopes are fine in the same document.

What do you think? Are you taking advantage of ES6’s let and const?

  1. StackOverflow ftw
  2. An aside: In JavaScript, the interpreter will hoist variable declarations but not initializations.
  3. Immediately Invoked Function Expression