Surviving The Javascript Type System

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Maksim Ivanov
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3 min read

“Everything is crooked. Reality is poison. I want to go back, I hate this. I can’t live like this.” - Morty from the Morty’s Mindblowers episode. Also me after investigating about Javascript type system.

What Type System Does Javascript Have?

If you search internets about Javascript and its type system - you’ll find that there is some controversy. Javascript type system is often being referred as

  • Dynamic
  • Weakly typed
  • Loosely typed
  • Untyped
  • Unityped

Let’s figure out what do those terms mean

Javascript Is Dynamic

Dynamic means that value’s type is enforced, but variable’s is not, and it can represent any value at given time.

let foo = "bar";
foo = 11;
foo = true;

This Javascript code is correct and will execute with no problems.

Attention! The fact that this code is valid doesn’t mean you should do this! Do not assign values of different types to the same variable. It is super misleading and you’ll make your fellow developers super unhappy.

Also, use const. Not only it will make it easier to understand which things you are not going to mutate – it will also give a bit of performance advantage because Javascript doesn’t have to track type changes for this thing.

Javascript Is Weakly/Loosely Typed

Weakly/Loosely typed usually means that Javascript doesn’t require you to specify the kind of information will be stored in a variable in advance. And that you can assign different types of data to one variable.

Also, Javascript will try to resolve the type of a variable at run-time and will allow you to make operations with non-matching types by automatically doing type-coercion for you.

const strValue = "test"
const intValue = 2
const result = strValue + intValue // test2

It can lead to a lot of weird behaviour (I think everyone has already seen the WAT talk). We’ll get back to it later.

Javascript Is Unityped/Untyped

But how can Javascript be unityped or even untyped language if we know that it has 7 basic types (according to ECMAScript specification)?

These definitions are more common in academic circles and both mean that everything belongs to a single type.

As we are discussing Javascript type system – let’s define what does type system mean.

Here is definition from Benjamin Pierce book Types and Programming Languages

A type system is a tractable syntactic method for proving the absence of certain program behaviours by classifying phrases according to the kinds of values they compute.

Syntactic means that it is a language feature. And it basically means that a language will only generate a program when it can prove that the types align.

So in a typed language, a program might not be generated, because types might not match up because a program can contain multiple types.

Opposed to that, Javascript program will be generated regardless of kinds of data you do the computations on. For Javascript types always match up, which means that actually there is only one type.

Here we need to make a note about types and classes. Both are data structuring mechanisms that describe things with certain structure and behaviour.

The difference is that types are being mostly used at compile time to ensure program correctness. And Javascript doesn’t have the compilation stage or static check stage by default. What Javascript does at runtime can be categorized as classification and has nothing to do with language syntax.

Here are two articles about types/classes, dynamic/static languages

See you in next article where we’ll discuss what are the consequences of Javascript being so dynamic.