TypeScript

With typescript: true the parser reads TypeScript. The tree has TypeScript nodes such as TSTypeAnnotation and TSInterfaceDeclaration, in the TS-ESTree shape.

const { node } = new Source('let n: number = 1', { typescript: true }).parse();
node.body[0].declarations[0].id.typeAnnotation.type; // 'TSTypeAnnotation'

Erase the types

typescript: 'erase' reads TypeScript and returns JavaScript, with the types removed, in the same pass. Positions are positions in the text you gave it.

const source = new Source('let n: number = 1', { typescript: 'erase' });
const { node, typescript } = source.parse();
node.body[0].declarations[0].id.typeAnnotation; // undefined
typescript;                                     // []

Some TypeScript syntax does something at runtime: enums, namespaces that hold values, parameter properties, export =, import =, decorators and accessor fields. Removing those would change the program, so they stay in the tree, and the answer's typescript lists each one by type and span. An empty list means the tree is plain JavaScript.

Decorators

Decorators are read when typescript is on, in every placement either decorator dialect allows; which dialect a project uses is a compiler option, so its placement rules are the type checker's.