Add simple JSON schema / validation helpers

github · mbg · Apr 30, 2026 · Apr 25, 2026 · Apr 25, 2026 · Apr 25, 2026
commit 243c274daf79e2158519c66f93640001e92c4699
@@ -0,0 +1,46 @@
+import test from "ava";
+
+import { setupTests } from "../testing-utils";
+
+import * as json from ".";
+
+setupTests(test);
+
+const testSchema = {
+  requiredKey: json.string,
+};
+
+const optionalSchema = {
+  optionalKey: json.optional(json.string),
+};
+
+test("validateSchema - required properties are required", async (t) => {
+  t.false(json.validateSchema(testSchema, {}));
+  t.false(json.validateSchema(testSchema, { requiredKey: undefined }));
+  t.false(json.validateSchema(testSchema, { requiredKey: null }));
+  t.false(json.validateSchema(testSchema, { requiredKey: 0 }));
+  t.false(json.validateSchema(testSchema, { requiredKey: 123 }));
+  t.false(json.validateSchema(testSchema, { requiredKey: false }));
+  t.false(json.validateSchema(testSchema, { requiredKey: true }));
+  t.false(json.validateSchema(testSchema, { requiredKey: [] }));
+  t.false(json.validateSchema(testSchema, { requiredKey: {} }));
+  t.true(json.validateSchema(testSchema, { requiredKey: "" }));
+  t.true(json.validateSchema(testSchema, { requiredKey: "foo" }));
+});
+
+test("validateSchema - optional properties are optional", async (t) => {
+  // Optional fields may be absent
+  t.true(json.validateSchema(optionalSchema, {}));
+  t.true(json.validateSchema(optionalSchema, { optionalKey: undefined }));
+  t.true(json.validateSchema(optionalSchema, { optionalKey: null }));
+
+  // But, if present, should have the expected type
+  t.false(json.validateSchema(optionalSchema, { optionalKey: 0 }));
+  t.false(json.validateSchema(optionalSchema, { optionalKey: 123 }));
+  t.false(json.validateSchema(optionalSchema, { optionalKey: false }));
+  t.false(json.validateSchema(optionalSchema, { optionalKey: true }));
+  t.false(json.validateSchema(optionalSchema, { optionalKey: [] }));
+  t.false(json.validateSchema(optionalSchema, { optionalKey: {} }));
+  t.true(json.validateSchema(optionalSchema, { optionalKey: "" }));
+  t.true(json.validateSchema(optionalSchema, { optionalKey: "foo" }));
+});
@@ -36,3 +36,77 @@ export function isStringOrUndefined(
 ): value is string | undefined {
   return value === undefined || isString(value);
 }
+
+/**
+ * Represents a field of type `T` in a schema.
+ * Carries a validation function and flag indicating whether the field is required or not.
+ */
+export type Validator<T> = {
+  validate: (val: unknown) => val is T;
+  required: boolean;
+};
+
+/** Extracts `T` from `Validator<T>`. */
+export type UnwrapValidator<V> =
+  V extends Validator<infer A>
+    ? V["required"] extends true
+      ? A
+      : A | undefined
+    : never;
+
+/** A validator for string fields in schemas. */
+export const string = {
+  validate: isString,
+  required: true,
+} as const satisfies Validator<string>;
+
+/** Transforms a validator to be optional. */
+export function optional<T>(validator: Validator<T>) {
+  return {
+    validate: (val: unknown) => {
+      return val === undefined || val === null || validator.validate(val);
+    },
+    required: false,
+  } as const satisfies Validator<T | undefined | null>;
+}
+
+/** Represents an arbitrary object schema. */
+export type Schema = Record<string, Validator<any>>;
+
+/** Constructs an object type corresponding to a schema. */
+export type FromSchema<S extends Schema> = {
+  [K in keyof S]: UnwrapValidator<S[K]>;
+};
+
+/**
+ * Validates `obj` against `schema`.
+ *
+ * @param schema The schema to validate against.
+ * @param obj The object to validate.
+ * @returns Asserts that `obj` is of the `schema`'s type if validation is successful.
+ */
+export function validateSchema<S extends Schema>(
+  schema: S,
+  obj: UnvalidatedObject<any>,
+): obj is FromSchema<S> {
+  for (const [key, validator] of Object.entries(schema)) {
+    const hasKey = key in obj;
+
+    // If the property is required, but absent, fail.
+    if (validator.required && !hasKey) {
+      return false;
+    }
+
+    // If the property is required, but undefined or null, fail.
+    if (validator.required && (obj[key] === undefined || obj[key] === null)) {
+      return false;
+    }
+
+    // If the property is present, validate it.
+    if (hasKey && !validator.validate(obj[key])) {
+      return false;
+    }
+  }
+
+  return true;
+}