00 — Start here¶
Level: 101 · read this first
One line: A page here makes one claim about JavaScript or TypeScript and shows the program that proves it, or, while it is still a stub, says plainly that it is an outline.
This library is for someone who can already write a little code, in JavaScript or in another language, and wants to know what JavaScript and TypeScript actually do: why [10, 9, 1].sort() gives [1, 10, 9], why a resolved promise runs before a zero-millisecond timer, why a TypeScript type can say one thing while the running program does another. Each page takes one such question and answers it with a program that Node runs and CI checks.
Stubs first¶
The library was laid out whole before it was written. Every topic in the sidebar already has its page, and most of those pages are stubs: the claim, the questions the finished page will answer, the examples it will need, and links to its neighbours and to the same question in the sibling libraries. A stub says so in a box at the top. It becomes a lesson when its first example runs in CI, and the chapter tables mark each page stub or checked.
How to read a page¶
A finished page has the same shape everywhere: a one-line claim, an output block printed by the program in that page's examples/ folder, Reading the output, What to do, In other languages, and Sources. The output blocks are generated, never typed. How a page is checked walks through one: the same function in JavaScript and in TypeScript, and the call tsc refuses.
The pages in this chapter¶
| Page | Level | In one line | Status |
|---|---|---|---|
| How a page is checked | 101 | Every output block in this library was printed by a program that CI runs on two machines: Node runs a JavaScript example, tsc type-checks a TypeScript example before Node runs it with its types erased, and an example meant to fail records exactly what tsc said. |
checked |
Running the examples¶
You need Node 24 or later; Node runs .ts files itself by erasing their types. The TypeScript compiler that checks them is pinned in package.json, so install it once from the repo root:
The first command installs TypeScript into node_modules/; the second runs every example and compares its output with the recorded answer key, exactly as CI does.
Where to go next¶
- The keyword index — every keyword, operator and API, and the page that explains it.
- Running JavaScript — the first chapter: what the language is and what Node adds to it.
- TypeScript basics — where the TypeScript half begins.
- Resources — books, official documentation, and where each idea lives in the sibling libraries.