JSON to TypeScript Converter

This is a deterministic source-to-source transformation, not a guess. The JSON is parsed once with JSON.parse following RFC 8259, which defines the six JSON value types — object, array, string, number, boolean, and null. A single recursive pass then infers a TypeScript type for every node, using the rules from the TypeScript Handbook.

1. Primitives. JSON string → string, number → number (TypeScript has no int/float split, so every JSON number is number), true/false → boolean, and null → null.
2. Objects become named types. Each object is emitted as an interface (or a type alias). The name is the parent name plus the PascalCased key — a customer object under Root becomes RootCustomer — so nested types never collide.
3. Arrays become element types. Under the Union strategy every element is inferred and the results are merged: object fields are unioned, and a key present in only some elements is marked optional. An empty array has nothing to infer from, so it becomes unknown[] — never any[]. The First-element strategy instead infers from index 0 only, which is faster for lists you know are uniform.
4. Optional and nullable. A key whose value is null is handled by your Null handling setting: key?: T (optional), key: T | null (nullable), or key?: T | null(both). Keys missing from some merged siblings are always optional, per the Handbook's optional-property rule.
5. Dedup, ordering, and safety. Structurally identical objects are emitted once and reused. Declarations print children before parents so the file compiles top to bottom. Keys that are not valid identifiers are quoted ("first-name": string), and reserved root names are rejected before generation.

Because the rules are fixed, identical input and identical settings always produce byte-identical output. As a cross-check, the converter runs a structural assignability test — it confirms your original JSON value satisfies the types it just generated — and shows the result as the Accepts inputbadge. For a shape inferred straight from your value under the Union strategy this is always green; with First-element it may warn that later array elements don't match.

• RFC 8259 — The JavaScript Object Notation (JSON) Data Interchange Format
• TypeScript Handbook — Everyday Types (interfaces, unions, optional properties)
• TypeScript Handbook — Object Types (nested and optional members)

The type-mapping rules on this page were last cross-checked against the RFC 8259 value grammar and the TypeScript Handbook on 2026-06-23. The converter is deterministic, so its output can be reproduced exactly from the same input and settings.