22 — TypeScript basics

One line: TypeScript is checked by one program and run by another: tsc reads the types and reports errors, Node deletes the types and runs what is left, and nothing connects the two unless you run both.

This chapter sets up the two-step model the later TypeScript chapters rely on. It starts with what TypeScript adds and what erasure leaves behind, then shows the two commands: Node, which strips types and runs, and tsc, which checks and writes nothing. Next come the syntax Node cannot strip, the tsconfig.json options this library uses, and the eight checks behind strict. It ends with inference, which decides how many annotations you write, and with TypeScript 7, the Go port of the compiler that checks every example here.

The pages

Page	Level	In one line	Status
What TypeScript adds	101	TypeScript is JavaScript plus a checker that runs before the program does: tsc rejects user.fristName and suggests firstName, while Node, which only deletes type syntax and runs the rest, prints undefined.	stub
Types are erased	101	Node erases types by overwriting each annotation with spaces, so area.toString() shows blanks where : Point and : number stood, and an interface used as a value, p instanceof Point, fails at run time with a ReferenceError.	stub
Running TypeScript	101	node server.ts blanks out the types and runs, so a file that assigns "8080" to a number prints 80801 and exits 0; the check is a separate command, tsc --noEmit, which reports TS2322, writes nothing and exits 1.	stub
Erasable syntax	201	Node runs TypeScript by overwriting type syntax with spaces and stops with ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX at enum, parameter properties, a namespace holding values, import x = require() and the old <number>x assertion; erasableSyntaxOnly makes tsc report each as TS1294.	stub
tsc and tsconfig.json	201	tsc with no file names checks the project tsconfig.json describes; tsc app.ts beside that file stops with TS5112, because naming files means ignoring the config, and TypeScript 7 makes you say so with --ignoreConfig.	stub
strict	201	In TypeScript 7 strict is on unless you turn it off, and it stands for eight checks: a file that breaks each once gets eight errors with no flag given and none under --strict false, while --alwaysStrict false is now error TS5108.	stub
Annotations and inference	101	Inference types most variables unaided: let count = 1 is number, const one = 1 is the literal type 1, and a map callback's parameter is typed from the array; a declared function's parameters are not, and strict reports them as TS7006.	stub
TypeScript 7	101	TypeScript 7 is the compiler ported to Go: bin/tsc in the npm package is a two-line Node script that starts a native executable which go version -m reports as built by go1.26.4 from microsoft/typescript-go, and require("typescript") now returns only a version.	stub

Boundaries

The type syntax itself, from literal types to unions and object shapes, is the next chapter, Everyday types; how .ts files import each other and find @types packages belongs to the chapter on declaration files and module resolution.