In this guide 6 sections
Choose TypeScript for any business application that will be maintained for years or touched by more than one developer. It catches many mistakes before release and documents the shape of your data. Plain JavaScript still suits small internal tools and one off scripts.
What changes when the developers change
Every business application outlives its first team. A new developer opens an invoice function and wants to know what it accepts. In plain JavaScript they read the body and guess. In TypeScript the signature says it.
Types act as documentation that cannot go stale, because the compiler checks them. A field renamed from customerId to accountId fails everywhere it was used, not only where someone remembered.
That matters most at handover, when the original author has moved on and the questions have nobody to answer them.
Which bugs are caught before release
TypeScript flags a call with the wrong argument, a property that may be undefined and a switch that forgets a status. These are the small slips that reach production as a blank screen on the dispatch page.
It does not test business rules. An invoice total calculated wrongly still compiles. You need tests and review for that, with or without types.
Types also stop at the edge of the system. Data from a form, an API or a CSV import is untrusted, so validate it at runtime with a schema library and then trust the type inside.
- Misspelled property names
- Missing null checks on optional fields
- Wrong argument order in shared helpers
- Unhandled order or ticket statuses
- API response shapes that drifted
Hiring, handover and build setup
Most developers who work on NestJS, Angular or React with Expo already write TypeScript. Asking for it widens the pool rather than narrowing it. Plain JavaScript projects often lean on one person’s habits.
The cost is a build step. You need a tsconfig, a compiler or bundler, and a strict setting that is switched on from day one. Switching strict on later means fixing a long list of errors.
A fresh project hides most of this setup in its template. A rescue of an old JavaScript codebase can migrate file by file, using allowJs.
- tsconfig with strict enabled
- Type checking run in the CI pipeline
- Lint rules against the any type
- Shared types for API payloads
Where plain JavaScript is still fine
A one page internal tool, a data cleanup script or a quick prototype does not repay the setup. If the code will be deleted next quarter, keep it simple.
Be honest about the word temporary. Scripts that touch payroll or stock figures tend to stay for years. If it writes to a production database, give it types or at least JSDoc annotations.
A middle path exists. TypeScript can check plain JavaScript files through JSDoc comments, which suits small utilities.
What to write into the vendor contract
Name the language in the scope. State that the codebase is TypeScript with strict mode on, that the build passes with no type errors, and that tests run in the pipeline before delivery.
Ask for the repository, the lockfile and the build instructions at handover. A clean build on a fresh machine, done by your own person, is the best acceptance test.
Before signing, put one question to the vendor: how many unchecked any types are in the sample code they show you? The answer tells you how they work.
Language choice worksheet
A single-sheet worksheet with five rows: expected life of the app, number of developers, data handled, handover plan and integrations. Each row has a score from plain JavaScript to TypeScript and a note. A lower block lists contract clauses: strict mode, clean build, tests and repository handover.
Open a blank worksheet to printQuestions owners ask
Does TypeScript make an app slower?
No. It compiles to JavaScript before the app runs, so the browser or server executes ordinary JavaScript. The extra work happens only at build time.
Can an existing JavaScript app move to TypeScript gradually?
Yes. Enable allowJs, rename files one at a time and begin with shared modules such as API clients and data models. The app keeps working throughout.
Is TypeScript harder for new developers?
There is a short learning period. After it, most developers find unfamiliar code easier to read, because the editor shows what each function takes and returns.
Do types replace testing?
No. Types catch shape mistakes. Tests catch wrong business rules, such as a discount applied twice. A healthy project uses both.