Using JS: const

daniel13rady profile image Daniel Brady Updated on ・10 min read

This post is part of my miniseries, Declaring Variables in JavaScript.

If you've already read some of the sibling posts, you can skip straight to here.


The basics: declaring variables

Let's begin at the beginning: variable declarations declare variables. This may seem obvious to many, but in practice we often confuse variables with values, and it is important, particularly for this conversation, that we are clear on the differences.

A variable is a binding between a name and a value. It's just a box, not the contents of the box, and the contents of the box may vary either in part or in whole (hence the term 'variable').

The kind of box you use, that is, the declarator you use to create a binding, defines the way it can be handled by your program. And so when it comes to the question of, "How should I declare my variables?" you can think of the answer in terms of finding a box for your data that is best suited to the way you need to manipulate it.

The specifics: declaring variables in JavaScript

At the time of this writing, JavaScript gives us these tools for declaring our variables:

  • var
  • let
  • const

πŸ’‘ The function keyword is not, in fact, a variable declarator, but it can create a bound identifier. I won't focus on it here; it's a bit special because it can only bind identifiers to values of a particular type (function objects), and so has a rather undisputed usage.

Why so many options? Well, the simple answer is that in the beginning, there was only var; but languages evolve, churn happens, and features come (but rarely go).

One of the most useful features in recent years was the addition of block scoping to the ECMAScript 2015 Language specification (a.k.a. ES6), and with it came new tools for working with the new type of scope.

In this post, we'll dive into the behavior of one of these new block-scope tools: const.

What is it?

Block scoping in JavaScript is wonderful. It gives us the ability to create scopes on-demand by "slicing up" a function into as many encapsulated bits of scope as we deem necessary, without the need for more functions.

But it would be rather useless without the ability to declare variables which exist only within these 'blocks' of scope.

Enter const.

const declarations define variables that are scoped to the running execution context's LexicalEnvironment. The variables are created when their containing Lexical Environment is instantiated but may not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created.

Source: ECMAScript 2019 Language Specification, Β§13.3.1

...creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N....

Source: ECMAScript 2019 Language Specification, Β§

It is a Syntax Error if Initializer is not present and IsConstantDeclaration of the LexicalDeclaration containing this LexicalBinding is true.

Source: ECMAScript 2019 Language Specification, Β§

Let defaultValue be the result of evaluating Initializer.

Source: ECMAScript 2019 Language Specification, Β§, run-time algorithm for "SingleNameBinding"

Okay...but what does it do?

Translation? 🀨 Let's learn by doing.

const binds a name to a value and doesn't let me bind it to anything else.

Const introduces a new variable that forbids reassignment after initialization

During compilation, that variable is

  1. scoped to the nearest enclosing lexical environment (i.e. a block, a function, or the global object) and
  2. created as immutable but not initialized during the instantiation of that scope

πŸ’‘ You might come across the term "variable hoisting" in your JS travels: it refers to this 'lifting' behavior of the JS compiler with respect to creating variables during instantiation of the scope itself. All declarators hoist their variables, but the accessibility of those variables varies with the tool you use.

At run-time, my variable is initialized and references to it can then be evaluated and manipulated.

Const variables are scoped to the nearest enclosing scope and must be given an initial value

⚠️ In JavaScript error messages, "not defined" is not the same thing as having a value of undefined πŸ˜“ The term "not defined," in this context, should really be "not declared."

A run-time reference to a variable declared with const is not valid unless it occurs after the variable declaration, with respect to the current flow of execution, not necessarily the "physical" location of the declaration in my code. For example, this is valid:

Reference to a variable in a function physically above its outer declaration with const is okay

But this will give me a run-time error:

Reference to a variable in a function before its const binding is evaluated throws a run-time error

Furthermore, additional declarations of the same name in the same scope using const or let are not allowed: the name is essentially reserved by the first declaration encountered by the compiler.

Re-declarations with const throw early errors

What is it good for?

const, like var and let, gives the ability to encapsulate, manipulate, share, and hide data in boxes within my JavaScript.

But unlike var, const restricts access to my box to the to the nearest enclosing lexical environment, not merely the closest function, and so const really shines at close-quarters data management.

In JavaScript, functions have lexical environments, but so do blocks, and this ability to reduce the scope of a variable and hide my data even from the nearest enclosing function is where the strength of const lies.

With const, unlike let and var, my box is initialized with a value and can never be re-assigned, making it a great tool to employ in an immutable approach to state management. The string "const" in English is highly associated with the word constant, and so const in JavaScript helps communicate to my reader that neither the meaning of this variable nor its value will ever change.

Const is better for communicating bindings that keep their meanings and values throughout my program

And since functions inherit the environment of their parents thanks to closure, a function nested within such a block can access the const (and var and let) bindings of their parent scopes, but not vice-versa.

Showcasing the behavior of const with respect to nested scopes

When should I use something else?

Sometimes, I want a box that will hold different things as my program executes, like as a counter or a flag. const forbids re-assignments, and so it will not work for this use case. I must use var or let instead.

Const cannot be used when re-assignment is needed

Sometimes, I need to manage state that is accessible across an entire function of decent size, not just a short block of code. Since const scopes my data to the nearest lexical environment, it will work for this purpose, but it communicates the wrong thing to my readers and so it's not the best tool for this job. In this situation, var is better.

Var is better that const for function-wide state management

Sometimes, I want a name that always means exactly one thing, but whose bound value may evolve throughout my program. Since const prevents re-assignments but doesn't care about changes to inherently mutable values, it will work for this purpose, but it communicates the wrong thing to my readers.

Something that changes is not constant, and the strong association of const to the word constant makes it misleading in this context. For this situation, I prefer employing var or let in combination with SCREAMING_SNAKE_CASE to communicate to readers that I intend the meaning to remain constant, but the exact value may vary.

Var and let are better for values that may change throughout your program

Using const inappropriately can hurt the readability and maintainability of my code because I'm communicating the wrong thing and not encapsulating my data as well as I could be.

To learn how to communicate better in my code, I dove into the other tools available and wrote about what I found:

So when should I use it?

The reason to use const isn't to declare constant values, it's to declare constant bindings.

Sometimes, I want to give a name that never changes meaning, to a value that never changes. No single construct in JavaScript can enforce this. The best I can do is communicate my intent clearly and leverage the tools available on a case-by-case basis.

In these situations I prefer using const in combination with SCREAMING_SNAKE_CASE for communicating, "This is a constant, and will never change in meaning or value over the course of this block." I find the association with the word constant overpowers everything else about const, and so I don't tend to use it for any other purpose.

The block could be something like an if statement, for loop, or even an anonymous block; one of the main values of const is in keeping variables close to where they are used without exposing them to the wider world of the enclosing function.

If a function definition is particularly short, say only two or three lines long, and my other criteria hold true, I may prefer to use a const, but the value of such a short-lived const is highly context-specific. In this case the value of const over var and let is entirely in what it communicates to my readers: this variable is short-lived and it never changes, you can forget about it soon and be at peace 😌.

Every tool has its use. Some can make your code clearer to humans or clearer to machines, and some can strike a bit of balance between both.

"Good enough to work" should not be "good enough for you." Hold yourself to a higher standard: learn a little about a lot and a lot about a little, so that when the time comes to do something, you've got a fair idea of how to do it well.

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Daniel Brady


Success is something you tell your friends about, but failure is what you learn from.


Editor guide

Regarding this statement I made:

Sometimes, I want to give a name that never changes meaning, to a value that never changes. No single construct in JavaScript can enforce this.

Someone offline (to DEV, anyway) gave me a really good "But what about this..." example, so I wanted to share it and elaborate a bit on why what my statement still holds.

Basically, they wanted to know why this simple declaration doesn't provide complete immutability:

const x = 42;

My answer is that, at least according to my own understanding of JavaScript, it does. However, it is not because of const alone. This declaration creates a box with contents that can be neither replaced nor modified because of two things:

  1. const prevents re-assignments
  2. numbers are immutable value types

If you replaced the initial value 42 with an object (arrays are a kind of JS object), then all of a sudden you've got a mutable value in an immutable variable, and the contents of your const box can be modified because the value itself can be modified:

/* Totally valid but very confusing so I don't like using `const` for this πŸ™‚*/
const config = { some_setting: 42 }; // immutable variable
config.some_setting = "cat"; // mutable value

This is what I meant when I said that no single construct in JavaScript can enforce complete immutability. Hope this helps!