var Component=(()=>{var h=Object.create;var a=Object.defineProperty;var p=Object.getOwnPropertyDescriptor;var y=Object.getOwnPropertyNames;var E=Object.getPrototypeOf,C=Object.prototype.hasOwnProperty;var m=(n,l)=>()=>(l||n((l={exports:{}}).exports,l),l.exports),A=(n,l)=>{for(var s in l)a(n,s,{get:l[s],enumerable:!0})},o=(n,l,s,c)=>{if(l&&typeof l=="object"||typeof l=="function")for(let r of y(l))!C.call(n,r)&&r!==s&&a(n,r,{get:()=>l[r],enumerable:!(c=p(l,r))||c.enumerable});return n};var D=(n,l,s)=>(s=n!=null?h(E(n)):{},o(l||!n||!n.__esModule?a(s,"default",{value:n,enumerable:!0}):s,n)),g=n=>o(a({},"__esModule",{value:!0}),n);var i=m((b,t)=>{t.exports=_jsx_runtime});var N={};A(N,{default:()=>B,frontmatter:()=>F});var e=D(i()),F={title:"TypeScript: from Stringly Typed to Strongly Typed",publishedAt:"2024-04-03T18:12:03.284Z",summary:'TypeScript is often referred to as being "strongly typed" when in reality most TypeScript codebases are actually "stringly typed". In this article we will take a closer look at the effects of overusing strings and how to make them strongly typed with nominal typing.',image:{src:"/images/blog/stringly.jpeg",alt:"A black background with three white sqiggle outlines each with decreasing opacity. Depicting strings from string theory"},tags:["typescript"],keywords:["newtype","opaque","branded","nominal"],showToc:!0};function d(n){let l=Object.assign({p:"p",code:"code",strong:"strong",a:"a",h2:"h2",span:"span",div:"div",pre:"pre",h3:"h3",em:"em",ul:"ul",li:"li"},n.components),{Callout:s}=l;return s||f("Callout",!0),(0,e.jsxs)(e.Fragment,{children:[(0,e.jsxs)(l.p,{children:['TypeScript is often referred to as being "strongly typed", usually based on the fact that it is a typed language containing primitives like ',(0,e.jsx)(l.code,{children:"string"}),", ",(0,e.jsx)(l.code,{children:"number"}),", and ",(0,e.jsx)(l.code,{children:"boolean"}),`. Here I will explain why this idea is wrong, and largely speaking why most TypeScript codebases are actually "stringly typed". We'll take a look at why it's not ideal to model all text based values as a string, examples of where this can cause problems in your domain model and functions, and finally look at our options for making our code `,(0,e.jsx)(l.strong,{children:"strongly"})," typed."]}),` `,(0,e.jsxs)(l.p,{children:[(0,e.jsx)(l.strong,{children:"TL;DR"}),` if you're already familiar with "stringly typed" codebases feel free to jump to the `,(0,e.jsx)(l.a,{href:"#solution",children:"solution"}),"."]}),` `,(0,e.jsxs)(l.h2,{id:"what-is-stringly-typed",children:[(0,e.jsx)(l.a,{className:"heading-anchor",href:"#what-is-stringly-typed",children:(0,e.jsx)(l.span,{className:"icon icon-link"})}),"What is Stringly Typed?"]}),` `,(0,e.jsx)(l.p,{children:`A string is a primitive data type used to represent text data. It's often overused for all different types of text, which can lead to "stringly typed" code \u2014 where variables are only distinguished by the values they hold rather than their type.`}),` `,(0,e.jsx)(l.p,{children:"Overusing strings in this way can lead to many issues. It can make it difficult to know the expected format or content of a variable. Is this string a URL? An Email? A Password? It's not clear from the type."}),` `,(0,e.jsx)(l.p,{children:"Let's take typing as whole to its extreme. If we're happy to store all text as strings regardless of their type. Why stop there? Why not represent all types as a byte array instead? The reason we don't do this is because we'd end up with a dynamic language, meaning everything is a runtime error rather than compile time."}),` `,(0,e.jsx)(l.p,{children:"This over-usage of the string type undermines being in a static language and having compile time checks. So how do we make a stronger string type? One that allows the compiler to distinguish between different types of textual data."}),` `,(0,e.jsxs)(l.h2,{id:"domain-modelling-with-primitive-types",children:[(0,e.jsx)(l.a,{className:"heading-anchor",href:"#domain-modelling-with-primitive-types",children:(0,e.jsx)(l.span,{className:"icon icon-link"})}),"Domain modelling with primitive types"]}),` `,(0,e.jsxs)(l.p,{children:["Let's look at an example of when you might currently use a string, by modelling a simple ",(0,e.jsx)(l.code,{children:"User"})," type."]}),` `,(0,e.jsxs)(l.div,{"data-rehype-pretty-code-fragment":"",children:[(0,e.jsx)(l.pre,{"data-language":"ts","data-theme":"dark",children:(0,e.jsxs)(l.code,{"data-language":"ts","data-theme":"dark",children:[(0,e.jsx)(l.span,{"data-line":"",children:(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:"type User = {"})}),` `,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#D6DEEB"},children:"firstName"}),(0,e.jsx)(l.span,{style:{color:"#7FDBCA"},children:":"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#C5E478"},"data-highlighted-chars":"",children:"string"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:";"})]}),` `,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#D6DEEB"},children:"lastName"}),(0,e.jsx)(l.span,{style:{color:"#7FDBCA"},children:":"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#C5E478"},"data-highlighted-chars":"",children:"string"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:";"})]}),` `,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#D6DEEB"},children:"email"}),(0,e.jsx)(l.span,{style:{color:"#7FDBCA"},children:":"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#C5E478"},"data-highlighted-chars":"",children:"string"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:";"})]}),` `,(0,e.jsx)(l.span,{"data-line":"",children:(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:"};"})})]})}),(0,e.jsx)(l.pre,{"data-language":"ts","data-theme":"light",children:(0,e.jsxs)(l.code,{"data-language":"ts","data-theme":"light",children:[(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:"type"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#6F42C1"},children:"User"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:"="}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" {"})]}),` `,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#E36209"},children:"firstName"}),(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:":"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#005CC5"},"data-highlighted-chars":"",children:"string"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:";"})]}),` `,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#E36209"},children:"lastName"}),(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:":"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#005CC5"},"data-highlighted-chars":"",children:"string"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:";"})]}),` `,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#E36209"},children:"email"}),(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:":"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#005CC5"},"data-highlighted-chars":"",children:"string"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:";"})]}),` `,(0,e.jsx)(l.span,{"data-line":"",children:(0,e.jsx)(l.span,{style:{color:"#24292E"},children:"};"})})]})})]}),` `,(0,e.jsxs)(l.p,{children:["This looks typical of a TypeScript project right? Let's start trying to work with this model, we need a function that can convert a user's name to their initials. i.e. ",(0,e.jsx)(l.code,{children:"Matt Phillips"})," \u2192 ",(0,e.jsx)(l.code,{children:"MP"}),"."]}),` `,(0,e.jsxs)(l.div,{"data-rehype-pretty-code-fragment":"",children:[(0,e.jsx)(l.pre,{"data-language":"ts","data-theme":"dark",children:(0,e.jsxs)(l.code,{"data-language":"ts","data-theme":"dark",children:[(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:"const "}),(0,e.jsx)(l.span,{style:{color:"#82AAFF"},children:"getInitials"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" = "}),(0,e.jsx)(l.span,{style:{color:"#D9F5DD"},children:"("}),(0,e.jsx)(l.span,{style:{color:"#D7DBE0"},children:"firstName"}),(0,e.jsx)(l.span,{style:{color:"#7FDBCA"},children:":"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" 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`,(0,e.jsx)(l.span,{"data-line":"",children:(0,e.jsx)(l.span,{style:{color:"#24292E"},children:"};"})})]})})]}),` `,(0,e.jsxs)(l.p,{children:["Now let's use our ",(0,e.jsx)(l.code,{children:"getInitials"})," function and see why string isn't the best type option here."]}),` `,(0,e.jsxs)(l.div,{"data-rehype-pretty-code-fragment":"",children:[(0,e.jsx)(l.pre,{"data-language":"ts","data-theme":"dark",children:(0,e.jsxs)(l.code,{"data-language":"ts","data-theme":"dark",children:[(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:"const "}),(0,e.jsx)(l.span,{style:{color:"#82AAFF"},children:"user"}),(0,e.jsx)(l.span,{style:{color:"#7FDBCA"},children:":"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#FFCB8B"},children:"User"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" = {"})]}),` `,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" firstName: 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We could instead pass the whole ",(0,e.jsx)(l.code,{children:"User"})," to ",(0,e.jsx)(l.code,{children:"getInitials"})," so that it can index on the ",(0,e.jsx)(l.code,{children:"firstName"})," and ",(0,e.jsx)(l.code,{children:"lastName"})," keys. But now we are exposing all properties of a user to a function that only cares about the first and last names. If we want to write a unit test for this function, we need to generate unnecessary data that's never used."]}),` `,(0,e.jsx)(l.p,{children:"This solution also ignores the fact that TypeScript is happy to compile this code because it thinks that a firstName is equal to a lastName (or even an email!). This means the compiler can never catch accidental usage of a value in the wrong context."}),` `,(0,e.jsxs)(l.h2,{id:"firstname--lastname",children:[(0,e.jsx)(l.a,{className:"heading-anchor",href:"#firstname--lastname",children:(0,e.jsx)(l.span,{className:"icon icon-link"})}),"FirstName !== LastName"]}),` `,(0,e.jsx)(l.p,{children:"Not all text based data should be the same type, and in this case a user's first name is not the same type as their last name. How do we make it so that TypeScript treats the first and last name differently? 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They are a way to assign a different name to a type but it is still the underlying type."}),` `,(0,e.jsxs)(l.h3,{id:"class--object-boxing",children:[(0,e.jsx)(l.a,{className:"heading-anchor",href:"#class--object-boxing",children:(0,e.jsx)(l.span,{className:"icon icon-link"})}),"Class / object boxing"]}),` `,(0,e.jsxs)(l.p,{children:["What if we wrap a string inside of a ",(0,e.jsx)(l.code,{children:"class"})," / ",(0,e.jsx)(l.code,{children:"object"}),"?"]}),` `,(0,e.jsxs)(l.div,{"data-rehype-pretty-code-fragment":"",children:[(0,e.jsx)(l.pre,{"data-language":"ts","data-theme":"dark",children:(0,e.jsxs)(l.code,{"data-language":"ts","data-theme":"dark",children:[(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:"class "}),(0,e.jsx)(l.span,{style:{color:"#FFCB8B"},children:"FirstName"}),(0,e.jsx)(l.span,{style:{color:"#C792EA"},children:" {"})]}),` 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`,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:"class"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#6F42C1"},children:"LastName"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" {"})]}),` `,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:"constructor"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:"("}),(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:"public"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#E36209"},children:"value"}),(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:":"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#005CC5"},children:"string"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:") {}"})]}),` `,(0,e.jsx)(l.span,{"data-line":"",children:(0,e.jsx)(l.span,{style:{color:"#24292E"},children:"}"})}),` `,(0,e.jsx)(l.span,{"data-line":"",children:" "}),` `,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:"type"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#6F42C1"},children:"Test"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#D73A49"},children:"="}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:" "}),(0,e.jsx)(l.span,{style:{color:"#6F42C1"},children:"Equals"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:"<"}),(0,e.jsx)(l.span,{style:{color:"#6F42C1"},children:"FirstName"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:", "}),(0,e.jsx)(l.span,{style:{color:"#6F42C1"},children:"LastName"}),(0,e.jsx)(l.span,{style:{color:"#24292E"},children:">;"})]}),` `,(0,e.jsxs)(l.span,{"data-line":"",children:[(0,e.jsx)(l.span,{style:{color:"#6A737D"},children:"// "}),(0,e.jsx)(l.span,{style:{color:"#6A737D"},"data-highlighted-chars":"",children:"^ true"})]})]})})]}),` `,(0,e.jsxs)(l.p,{children:["With TypeScript being ",(0,e.jsx)(l.em,{children:"structurally typed"}),", ",(0,e.jsx)(l.code,{children:"FirstName"})," and ",(0,e.jsx)(l.code,{children:"LastName"})," are still equal in shape. We could change their structure to not be identical. But this doesn't prevent us from re-creating the same structure elsewhere, bringing us back to the original problem."]}),` `,(0,e.jsxs)(l.p,{children:["This encoding also comes with the problem of adding a small overhead of creating an object anywhere we want a string. Any code using this object now needs to call ",(0,e.jsx)(l.code,{children:".value"})," to access the string."]}),` `,(0,e.jsxs)(s,{tag:"info",title:"What is structural typing?",children:[(0,e.jsx)(l.p,{children:"Structural typing is a way of relating types based on their properties. This means that TypeScript only takes into account the properties of the type for equality."}),(0,e.jsx)(l.p,{children:"For example: if the type is shaped like a pet, then it's a pet. If a cat has at least all properties of a pet, then it's also a pet."})]}),` `,(0,e.jsxs)(l.h3,{id:"solution",children:[(0,e.jsx)(l.a,{className:"heading-anchor",href:"#solution",children:(0,e.jsx)(l.span,{className:"icon icon-link"})}),"Solution"]}),` `,(0,e.jsxs)(l.p,{children:["Given TypeScript is structurally typed we need a way to make it support ",(0,e.jsx)(l.em,{children:"nominal types"}),". How do we make it so that our firstName and lastName have different nominal types? If we look at other languages we will see techniques for this exact problem. The solution has different names depending on the language. To name a couple: Haskell's ",(0,e.jsx)(l.code,{children:"newtype"})," or Flow's ",(0,e.jsx)(l.code,{children:"opaque type"}),". These are nominal type aliases that do not allow access to their underlying type."]}),` `,(0,e.jsxs)(s,{tag:"info",title:"What is nominal typing?",children:[(0,e.jsx)(l.p,{children:"Within a type system it's how types are uniquely related based on their declaration and / or name. This means that even if two types have the exact same properties, they'd be considered different."}),(0,e.jsx)(l.p,{children:`For example: if the nominal type is shaped like a pet, then it is a pet. If a cat has at least all properties of a pet, then it is not a pet but only a cat.`})]}),` `,(0,e.jsx)(l.p,{children:"Ideally we want the best of both worlds so that we can:"}),` `,(0,e.jsxs)(l.ul,{children:[` `,(0,e.jsx)(l.li,{children:"Operate over our nominal types as if they were the underlying type to access properties on them"}),` `,(0,e.jsx)(l.li,{children:"Get compile time errors that verify nominal types of the same underlying type to be different."}),` `]}),` `,(0,e.jsxs)(l.p,{children:["The solution is to brand our primitives using an intersection (",(0,e.jsx)(l.code,{children:"&"}),") with a ",(0,e.jsx)(l.a,{href:"https://www.typescriptlang.org/docs/handbook/symbols.html#unique-symbol",children:(0,e.jsx)(l.code,{children:"unique symbol"})}),". In TypeScript each unique symbol has a completely separate identity. 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`,(0,e.jsxs)(l.h2,{id:"closing-thoughts",children:[(0,e.jsx)(l.a,{className:"heading-anchor",href:"#closing-thoughts",children:(0,e.jsx)(l.span,{className:"icon icon-link"})}),"Closing thoughts"]}),` `,(0,e.jsx)(l.p,{children:"In this article we've examined what a string is, why it's not the best type for your domain model and, how we can make our code strongly typed with nominals. They're easier to understand, serving as documentation for other developers on the expected use of each variable. They will help catch accidental bugs at compile time."}),` `,(0,e.jsx)(l.p,{children:"It's worth reiterating that this article can be applied to all primitives. I have focussed on strings because they are most commonly overused."}),` `,(0,e.jsx)(l.p,{children:"Of course, nominals are not a silver bullet either. As with everything there isn't a right and wrong way to do things, and using nominal values is no different. You might not need to use a nominal value everywhere that you may use a primitive. With that said at the very least I would recommend using them for your core domain models."}),` `,(0,e.jsx)(l.p,{children:"In the next article we'll build on top of this to make our types even stronger using refinement types."})]})}function u(n={}){let{wrapper:l}=n.components||{};return l?(0,e.jsx)(l,Object.assign({},n,{children:(0,e.jsx)(d,n)})):d(n)}var B=u;function f(n,l){throw new Error("Expected "+(l?"component":"object")+" `"+n+"` to be defined: you likely forgot to import, pass, or provide it.")}return g(N);})(); ;return Component;