Ben Ilegbodu
Ben Ilegbodu

Learning ES6: Block-level scoping with let and const

30 August 2015 • Ben Ilegbodu • Comments

block scoping

Coming off the heels of discussing arrow functions, let’s continue the Learning ES6 series talking about block-level scoping in ECMAScript 6.

TL;DR

let is the new var. ES6 provides two new ways for declaring variables: let and const. These pretty much replace the ES3 or ES5 way of declaring variables using var. By using block-level scoping, these two keywords help developers avoid common mistakes they make not because they write bad code, but because they don’t fully understand the idiosyncrasies of how JavaScript handles variables.

Let’s take a look at an example:

function simpleExample(value) {
	const constValue = value;

	if (value) {
		var varValue = value;
		let letValue = value;

		console.log('inside block', varValue, letValue);
	}

	console.log('outside block');

	// varValue is available even though it was defined
	// in if-block because it was "hoisted" to function scope
	console.log(varValue);

	try {
		// letValue is a ReferenceError because it
		// was defined w/in if-block
		console.log(letValue);
	}
	catch (e) {
		// e is a ReferenceError
		console.log('letValue not accessible', e);
	}

	// SyntaxError to try and update a variable
	// declared via const
	//constValue += 1;
}

simpleExample(2);

Variables declared via let are not available outside of the block in which they are declared. Variables declared via const also cannot be updated. You can find more examples in the block-level scoping code examples for the Learning ES6 Github repo.

You just know you’re interested, so keep on reading!

A quick look at var

Before we jump into let and const, let’s remind ourselves about how var works. In the History of ECMAScript, we learned that Brandon Eich supposedly created JavaScript in 10 days. I still find that hard to believe, but the way var declarations work in JavaScript may very well be the proof that it really was developed that quickly.

Nicholas C. Zakas explains it best in his book Understanding ECMAScript 6:

Traditionally, one of the tricky parts of JavaScript has been the way that var declarations work. In most C-based languages [such as C++, Java or C#], variables are created at the spot where the declaration occurs. In JavaScript, however, this is not the case. Variables declared using var are hoisted to the top of the function (or global scope) regardless of where the actual declaration occurs.

Most of the time we don’t run into any problems with var, but when we accidentally do, the resulting bugs can be hair-pulling:

function varExample() {
	var myVar = 7;

	console.log('myVar after declaration', myVar);

	// even though laterVar is defined later on in the function
	// it is "hoisted" to the beginning of the function &
	// initialized to undefined. In most C-style languages this would
	// be an error.
	console.log('laterVar before declaration', laterVar);

	laterVar = 10;

	// image some legitimate conditional
	if (myVar < 20) {
		// accidental redefintion of myVar results
		// in outer defined myVar being reassigned
		// to 'foo'
		var myVar = 'foo';
		var innerVar = true;

		console.log('myVar inside block', myVar);
	}

	// since this declaration was "hoisted", it's as if it's no
	// longer here but at the top of the function
	var laterVar;

	// looking at the code laterVar _should_ be undefined,
	// but it has the value 10 from earlier
	console.log('laterVar after declaration', laterVar);

	// we would expect myVar to still be 7
	// but it was redefined and overwritten
	// w/in the conditional
	console.log('myVar outside block', myVar === 7);

	// we would expect innerVar to no longer be accessible
	// since it was defined w/in the if-block, but it was
	// "hoisted" as well
	console.log('innerVar outside block', innerVar);
}
varExample();

You can imagine if your function was more complicated how you could accidentally re-declare your variables and be confused by why the function was misbehaving. This is why there are JSHint and ESLint rules that all var declarations must be at the top of the function.

let is the new var

let works similarly to var, but the variable it declares is block-scoped; it only exists within the current block.

function letExample(value) {
	if (value) {
		let letValue = value;

		console.log('inside block', letValue);

		// redeclaration of letValue would be a SyntaxError
		let letValue = 'foo';
	}

	try {
		// Accessing letValue is a ReferenceError because it
		// was defined w/in if-block
		console.log(letValue);

		// if we get here, it means that the JS engine didn't
		// throw an exception, which means that the engine
		// (or transpiled code) did not faithfully reproduce
		// how let should work
		console.log('let not faithfully handled');
	}
	catch (e) {
		// e is a ReferenceError
		console.log('letValue not accessible', e);
	}
}
letExample(2);

As you can see it’s a ReferenceError if you try to access a variable outside of the block in which it was declared. With var we would’ve received undefined. Also redeclaring a let variable is a TypeError. With a var declaration you would get no such warning. In a nutshell, let works how you probably thought var worked.

Transpiled let code

When let declarations are transpiled down to ES5, they are basically converted to var declarations. If you tried to transpile the code above but with let letValue = 'foo'; uncommented, both Babel and TypeScript throw compilation errors. They won’t even transpile the code because of the redeclaration.

However, accessing letValue out of block scope is a different story. The transpiled Babel code changes all of the appropriate uses of letValue to _letValue. The result is that there is still a ReferenceError when accessing letValue within the try-catch block even with the transpiled code using var declarations. Traceur unfortunately is not as robust resulting in the console.log('let not faithfully handled'); line actually being executed. I’m wondering if the try-catch is somehow throwing Traceur off. Something to keep in mind when choosing a transpiler.

Shadowing variables with let

We saw in the varExample earlier that when you redeclare a variable with var in a nested scope (such as an if-block), the variable isn’t actually redeclared. Since the variables had the same name, the second declaration just resulted in the variable’s value being reassigned. This isn’t the case with let:

function letShadowExample() {
	let x = 15;

	if (true) {
		// this x "shadows" the x defined in the outer scope.
		// this new x just exists within the scope of the
		// if-block
		let x = 21;

		// x should be 21
		console.log('x inner block', x);
	}

	// x should be 15
	console.log('x outer block', x);
}
letShadowExample();

Within the nested scope of the if-block, the let declaration of x is different than that of the outer scope. Now hopefully you wouldn’t write code like this because it’s very confusing, but at least it now works like how other major programming languages work. Both Babel and Traceur rename the nested x variable to something different so that when the code is transpiled to ES5 and using var declarations, the variables are treated differently.

Keeping things const

A const declaration works much like let except you must initialize the variable immediately with a value. And that value cannot be changed afterwards. You will get a SyntaxError if you either fail to initialize the variable at declaration or if you try to reassign its value. Let’s take a look at a quick example:

function constExample() {
	const NAME_KEY = 'name';
	const UNFROZEN_OBJ_CONST = { key: 'adam', val: 'eve' };
	const FROZEN_OBJ_CONST = Object.freeze({ key: 'jesus', val: 'paul' });

	// All const declarations must be initialized.
	// It's a SyntaxError otherwise
	const VALUE_KEY;

	// Const variables are read-only, so trying to
	// reassign is a SyntaxError too
	NAME_KEY = 'key';

	// GOTCHA: even though the object is const, you can still
	// change properties of it. It's the variable
	// that cannot be reassigned
	UNFROZEN_OBJ_CONST.key = 'moses';

	// by freezing the object, using ES5 Object.freeze
	// its properties cannot be changed.
	// in strict mode this a TypeError. In non-strict
	// mode the value silently doesn't change
	FROZEN_OBJ_CONST.val = 'peter';

	console.log('const value', NAME_KEY);
	console.log('unfrozen object', UNFROZEN_OBJ_CONST);
	console.log('frozen object', FROZEN_OBJ_CONST);
}
constExample();

As shown in the code, a variable declared via const means that it cannot be a reassigned, but it does not mean that its contents cannot be changed when it is an object. We can somewhat fix this problem (if it is one), by using the Object.freeze method we got from ES5.

Entering the Temporal Dead Zone

The temporal dead zone (TDZ) is just a fancy term used for the time period where code execution is in the scope of a variable declared by let or const, but before it is actually declared. The variable is in scope, but not yet initialized. Accessing an uninitialized variable is a ReferenceError. Let’s take a look at some example code:

{
	// Uninitialized “binding” for `disciple` variable is created
	// upon entering scope. TDZ for `disciple` variable begins

	// accessing a variable in TDZ either to get or set
	// is a ReferenceError
	disciple = matthew;
	console.log(disciple);

	// TDZ ends at declaration and `disciple` is initialized
	// w/ `undefined` value
	let disciple;

	console.log(disciple); // undefined

	disciple = thomas;
	console.log(disciple); // ‘thomas’
}

So why is it called the temporal dead zone? It’s because the dead zone is based on the period of code execution time versus where the code actually resides:

function temporalDeadZoneExample() {
	// TDZ for `value` begins

	const func = function() {
		// Even though this function is defined _before_
		// `value` in the code, it's not called until after
		// `value` is declared, so accessing it is OK.
		console.log('value is: ', value);
	}

	// TDZ for `value` continues. Accessing `value`
	// here would be a ReferenceError. Calling `func`
	// here would cause a ReferenceError.

	// TDZ ends with declaration of `value`
	let value = 'foo';

	// no longer in TDZ when calling function so now
	// any access of `value` is ok
	func();
}
temporalDeadZoneExample();

Variables declared by var don’t have a TDZ because the variables are “hoisted” to the beginning of functions. Therefore they are always declared as well as initialized with a value of undefined.

let and loops

Unbeknownst to most JavaScript developers, the iteration variable declared with var within the head of for-loops (such as for (var i = 0; i < 5; i++)) is available outside of the for-loop. Because of block-level scoping, with let this is no longer the case.

function simpleLoopExample () {
	for (var i = 0; i < 5; i++) {
		console.log('i=', i);
	}
	for (let j = 0; j < 5; j++) {
		console.log('j=', j);
	}

	// i is accessible outside of the for loop
	// and has the value 5
	console.log('after i=', i);

	// j is not accessible outside of the for loop
	// and is a ReferenceError
	console.log('after j=', j);
}
simpleLoopExample();

In practice, this typically is not a problem because we would rarely try to access a loop iteration variable outside of a for-loop. However, this issue can crop up when newbie JavaScript developers create callback functions within loops.

function callbackLoopVarExample() {
	var $body = $('body');

	for (var i = 0; i < 5; i++) {
		// create 5 buttons with the index in the name
		var $button = $('<button>var ' + i + '</button>');

		// wire click handler w/ callback using arrow function!
		$button.click(
			// BUG! When button is clicked, the value of `i` is 5!
			() => console.log('var button ' + i + ' clicked!')
		);

		// add button to the body
		$body.append($button);
	}
}
callbackLoopVarExample();

For those not too familiar with JavaScript development, it may not be immediately apparent why the console.log message always has 'var button 5 clicked!'. Because the i variable is “hoisted” to the top of the function it still has a value after the for loop has ended. That value is 5, which is what caused the termination of the loop. And since i is scoped to the whole function, all of the callback functions are bound to the same i, resulting in them all displaying 'var button 5 clicked!'.

The ES3/ES5 way of solving this problem was to use a separate named function or an IIFE that would create a new scope for the iteration variable such that each callback function would be bound to its own version. Here’s an example:

function callbackLoopNamedFunctionExample() {
	var $body = $('body');

	// Create a named function passing in the loop iteration variable
	// which creates a unique scope for each iteration so
	// that the callback function binds to its own variable.
	var loop = function(index) {
		// create 5 buttons with the index in the name
		var $button = $('<button>function ' + index + '</button>');

		// wire click handler w/ callback using arrow function!
		$button.click(
			// Fixed! `index` is unique per iteration
			() => console.log('function button ' + index + ' clicked!')
		);

		// add button to the body
		$body.append($button);
	}

	for (var i = 0; i < 5; i++) {
		loop(i);
	}
}
callbackLoopNamedFunctionExample();

Now when we click each button, the appropriate message is displayed. This problem could have also been solved by having an IIFE defined within the for-loop in much the way our loop function variable was defined. The need for this sort of workaround goes away when declaring the iteration variable via let:

function callbackLoopLetExample() {
	let $body = $('body');

	for (let i = 0; i < 5; i++) {
		// create 5 buttons with the index in the name
		let $button = $('<button>let ' + i + '</button>');

		// wire click handler w/ callback using arrow function!
		$button.click(
			// Fixed! `i` is a different variable declaration for
			// each iteration of the loop as one would expect!
			() => console.log('let button ' + i + ' clicked!')
		);

		// add button to the body
		$body.append($button);
	}
}
callbackLoopLetExample();

The key here is using let for the iteration variable. The i variable is now a new declaration for each iteration of the loop, resulting in the callback function having its own i variable. Once again, things work with let as we would’ve expected them with var.

One thing to note is that both Babel & Traceur, when they notice this issue, use the named function approach when transpiling the ES6 code down to ES5. This means that if you have a bug in your code, the structure of the transpiled code will look dramatically different than that of your ES6 code. As long as you have a source map in your transpiled code and your ES6 code is also accessible, any line numbers provided by the engine should point you back to the write place in your ES6 code.

Final note on loops. Variables declared by let work the same way with for-in loops as well. ECMAScript 6 added a new type of loop, the for-of loop that works with iterators (also added w/ ES6), but we’ll talk about those in a later article.

Working with parameters

Declaring a variable with let with the same name as a function parameter is a TypeError:

function sellFruits(fruits) {
	let fruits = [];
}

However, if that let declaration happens within a nested scope (such as an if-block), then the variable will be shadowed:

function sellFruits(fruits) {
	// create a simple code block
	{
		// this let declaration of `fruits` shadows the
		// `fruits` parameter
		let fruits = [];

		console.log(values); // []
	}

	// `fruits` here is the parameter value
	console.log(fruits);
}

And like all of the examples prior, var does not act this way. When a parameter is redeclared using var with the same, whether at the top-level function scope or within a nested block, nothing happens. Due to var declarations being functioned scoped, it’s as if those declarations weren’t even there because the parameter has already declared the variable in the scope.

function sellFruits(fruits) {
	// this declaration does nothing
	var fruits;

	// create simple code block
	{
		// the declaration does nothing, but the assignment
		// does assign the parameter value to []
		var fruits = [];

		console.log(fruits); // []
	}

	// `fruits` here is still [] from the assignment
	// in the block
	console.log(fruits);
}

var vs let vs const

Now that we know how let and const work, when should we use them in place of var? Here are some suggestions:

  • Use const for variables you want to be immutable. This works best for primitive values (like Number, String, Boolean, etc). You can use const for objects, but you should probably use Object.freeze in concert to make the object truly immutable. You could use const for a mutable object, but that defeats the “spirit” of const.
  • Use let for the mutable variables (i.e. everything else)
  • The only time you may need to still use var is for objects in the global scope, particularly ES3- and ES5-style namespaces or modules. Ideally you would convert those to ES6-style modules, but for backwards compatibility you may still need to use var.
  • Do not mix and match let and var in a file. Be consistent, otherwise it’ll lead to even more confusion.
  • Do not do a global search and replace of var for let. You may have code that is unintentionally relying on the quirkiness of var. You should do the conversion manually one file at a time.

JavaScript engine support

According to the ECMAScript 6 compatibility table, the following JavaScript engines support let and const:

  • Babel
  • Traceur
  • TypeScript
  • Edge
  • Chrome (with experimental flag enabled and in strict mode)
  • Firefox (code blocks must be wrapped in <script type="application/javascript;version=1.7"> tag)
  • Opera (in strict mode)
  • Node 5 (in strict mode)

The most notable missing engine is Safari. Of course IE 11 and lower do not support any ES6 features. In order to have the engine faithfully support let and const, particularly the temporal dead zone, you should ensure your scripts are running in strict mode.

Additional resources

You can check out the Learning ES6 examples page for the Learning ES6 Github repo where you will find all of the code used in this article running natively in the browser (for those that support let and const). There are also examples running through Babel and Traceur transpilation.

Other super helpful resources:

Coming up next…

We will be continuing the Learning ES6 series by looking at the fun new destructuring techniques introduced with ES6. Until then…