JSON to TypeScript

What it does

The type system is the thing being written to, so the hard parts are the ones TypeScript makes hard: a field observed once is not a field that is always there, an array holding a number and a string is a union rather than one type, and an object whose keys vary needs an index signature rather than a property list.

Walk the sample and emit interfaces. Report, per field: whether it is emitted optional, whether its type is a union, and whether an index signature was needed. Never emit `any` silently.

Why it works this way

A field present in the sample is not a field always present, so fields observed once are emitted with `?`. That is TypeScript's own way of saying it, which is why this decision belongs on this page rather than on the schema page.

An array holding a number and a string becomes `(number | string)[]`, not `any[]`. A union is what the sample actually showed, and it keeps the compiler checking the members.

When a nested object has a varying key set the generator emits an index signature and says so. `unknown` is the value type it prefers, because `any` switches checking off for every member rather than for the one that is uncertain.

null is a member of the type, not an absence. `string | null` and `string?` are different claims and the sample decides which one it supports.

Examples

{"a": 1, "b": null}

interface Root {
  a: number;
  b: null;
}

{"a": [1, "x"]}

interface Root {
  a: (number | string)[];
}

{"a": {"k1": 1, "k2": 2}}

interface Root {
  a: { [key: string]: number };
}

Inputs and outputs

• JSON sample
• root type name
• optional marking: observed once / never / always
• index signatures: on / off

• TypeScript source
• counts: fields marked optional, unions emitted, index signatures added

Edge cases

• an array of mixed types — union
• an empty array — the element type is unknowable, reported rather than guessed
• a key that is not a valid identifier — quoted
• null alongside a type
• an object whose keys are all distinct and non-patterned — no index signature
• a field whose only value was a number in one object and a string in another

Sources

• TypeScript handbook: Everyday Types, Object Types, Index Signatures