What Is TypeScript? Inside Microsoft's Typed Superset of JavaScript

A superset that adds types to JavaScript

TypeScript is a programming language that adds a syntax for types on top of JavaScript: a typed superset, developed by Microsoft, described on its official site as a strongly typed language that builds on JavaScript. Every valid JavaScript program is also a valid TypeScript program: the difference is that developers can add optional type annotations, which a compiler checks before the code runs. The main benefit, per the official site, is catching errors right in the editor, before you even run the code; the Handbook sums it up by saying the advantage is highlighting unexpected behavior, lowering the chance of bugs.

Practical example: an interface describes the shape of an object, and a union type limits the values it can hold.

interface User {
  name: string;
  id: number;
}
const user: User = { name: "Hayes", id: 0 };

type WindowState = "open" | "closed" | "minimized";

Try assigning a string to user.id instead of a number, and the compiler flags the error before the code ever runs: the same kind of check that, in PHP, only exists if type hints are explicitly added to functions.

The origins, from Microsoft to Anders Hejlsberg

TypeScript is developed by Microsoft, with Anders Hejlsberg as lead architect and a contribution from Luke Hoban. Hejlsberg had already been the lead architect of C# at Microsoft and, before that, of Turbo Pascal and Delphi. According to Wikipedia, the language was built to address JavaScript’s shortcomings for large-scale application development, both inside Microsoft and among its outside customers: managing complex JavaScript code with no static types called for dedicated tooling that JavaScript alone didn’t offer.

Year	Milestone
2012	First public release, version 0.8, October 1
2014	TypeScript 1.0, announced at Microsoft Build, April 12
2016	Angular 2.0, rewritten in TypeScript from the start, September 14
2026	TypeScript 7.0, reaches general availability July 8, with the compiler rewritten in Go

The design’s declared influences are C#, F#, Java and JavaScript itself. TypeScript 7.0, generally available since July 8, 2026, ships the first compiler rewritten in Go instead of TypeScript: per the official announcement, full builds run 8 to 12 times faster than version 6.0, with one concrete case, the Visual Studio Code project, dropping from 125.7 to 10.6 seconds — a curious technical twist, for a language born specifically to add types to JavaScript.

What the tsc compiler actually does

The official compiler, tsc, does two distinct things: type checking, verifying that the types used in the code are consistent, and transpiling (or emit), converting TypeScript code into standard, readable, non-obfuscated JavaScript, as the official Microsoft/TypeScript repository describes it. It installs with npm install -D typescript and runs with tsc file.ts. Once checked, the types get stripped out: at runtime, whether in the browser, in Node.js, Deno or Bun, only plain JavaScript runs, with no trace of the types left.

TypeScript also relies on type inference: writing let helloWorld = "Hello World", the compiler automatically recognizes that helloWorld is a string, with no explicit annotation needed. The type system is structural (also known as “duck typing”): two objects with the same shape, the same fields and the same types, count as the same type, regardless of the interface name they were declared with. It also supports narrowing: a check like typeof obj === "string" narrows the type inside that block of code, updating the analysis as it reads through the program.

	JavaScript	TypeScript
Types	Dynamic, checked only while the program runs	Static and optional, checked by the compiler before it runs
let x = "hi"; x = 5;	Valid, no error	Compile-time error
To use it	Runs natively in every browser	Needs the tsc compiler (npm install -D typescript)
What actually executes	The code you wrote	The JavaScript tsc generated, stripped of types

Why more and more frameworks choose it

GitHub’s Octoverse 2025 report notes that nearly every major frontend framework scaffolds projects in TypeScript by default, citing Next.js 15, Astro 3 and SvelteKit 2 as examples. Angular, developed by Google, explicitly recommends using TypeScript and has used it since the 2.0 rewrite on September 14, 2016: the choice dates back to that framework’s design, not a later switch, with an ecosystem that counts over 1.7 million developers.

Among the factors the Octoverse 2025 report points to for TypeScript’s growth, besides the frameworks that scaffold with it by default, is also compatibility with artificial intelligence-assisted workflows: typed systems reportedly help catch errors in AI-generated code earlier in the development pipeline, and projects that lean on AI tools benefit from stricter type contracts. The report also cites tools like Vite, ts-node and Bun as factors that lower the barrier for people approaching the language for the first time.

How widely used it is, by the numbers

In August 2025, TypeScript became, for the first time, the most used language on GitHub by monthly contributors: 2,636,006 in total, growing 66.63% year over year, overtaking Python by about 42,000 contributors. The Octoverse 2025 report calls this overtake “the most significant language shift in more than a decade.”

In the 2025 Stack Overflow Developer Survey, the metric looks different: 43.6% of developers report using it, 6th place overall, behind JavaScript, HTML/CSS, SQL, Python and Bash/Shell. Among professional developers who use AI tools, that share climbs to 51.4%, the highest of any subgroup the survey measured: a sign of correlation between static typing and AI-assisted workflows, more than a single cause the survey itself claims to establish.

Not all smooth sailing, the learning curve

The critiques exist too. On freeCodeCamp, an article aimed at developers weighing whether to learn TypeScript notes that using it “isn’t as simple as opening the browser console and writing code”: it takes a compiler, a build system and a configuration file, tsconfig.json, which can produce confusing setup errors for beginners. Coming from JavaScript, the same article notes, writing type annotations can feel like “extra code” that takes extra time, and the initial phase slows development down. Once the project is set up, though, the same account describes a workflow that goes back to feeling smooth: the cost sits mostly in the initial adoption, not in day-to-day use once things are running. Anyone curious about the kind of step-by-step reasoning that goes into writing a correct program, types included, can compare it with the logic of any algorithm: that, too, comes down to defining precise rules in advance so the result stays predictable.