Json Schema: Add support for simple defaults

common/changes/@typespec/json-schema/feature-json-schema-default_2023-11-09-18-20.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/json-schema",
+      "comment": "Add support for simple literal default on model properties",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/json-schema"
+}

packages/json-schema/src/json-schema-emitter.ts

@@ -1,5 +1,6 @@
 import {
   BooleanLiteral,
+  compilerAssert,
   emitFile,
   Enum,
   EnumMember,
@@ -19,6 +20,8 @@ import {
   getRelativePathFromDirectory,
   getSummary,
   IntrinsicType,
+  isArrayModelType,
+  isNullType,
   Model,
   ModelProperty,
   NumericLiteral,
@@ -64,7 +67,7 @@ import {
   isJsonSchemaDeclaration,
   JsonSchemaDeclaration,
 } from "./index.js";
-import { JSONSchemaEmitterOptions } from "./lib.js";
+import { JSONSchemaEmitterOptions, reportDiagnostic } from "./lib.js";
 export class JsonSchemaEmitter extends TypeEmitter<Record<string, any>, JSONSchemaEmitterOptions> {
   #seenIds = new Set();
   #typeForSourceFile = new Map<SourceFile<any>, JsonSchemaDeclaration>();
@@ -160,16 +163,66 @@ export class JsonSchemaEmitter extends TypeEmitter<Record<string, any>, JSONSche
   }
 
   modelPropertyLiteral(property: ModelProperty): EmitterOutput<object> {
-    const result = this.emitter.emitTypeReference(property.type);
+    const propertyType = this.emitter.emitTypeReference(property.type);
 
-    if (result.kind !== "code") {
+    if (propertyType.kind !== "code") {
       throw new Error("Unexpected non-code result from emit reference");
     }
 
-    const withConstraints = new ObjectBuilder(result.value);
-    this.#applyConstraints(property, withConstraints);
+    const result = new ObjectBuilder(propertyType.value);
 
-    return withConstraints;
+    if (property.default) {
+      result.default = this.#getDefaultValue(property.type, property.default);
+    }
+
+    this.#applyConstraints(property, result);
+
+    return result;
+  }
+
+  #getDefaultValue(type: Type, defaultType: Type): any {
+    const program = this.emitter.getProgram();
+
+    switch (defaultType.kind) {
+      case "String":
+        return defaultType.value;
+      case "Number":
+        return defaultType.value;
+      case "Boolean":
+        return defaultType.value;
+      case "Tuple":
+        compilerAssert(
+          type.kind === "Tuple" || (type.kind === "Model" && isArrayModelType(program, type)),
+          "setting tuple default to non-tuple value"
+        );
+
+        if (type.kind === "Tuple") {
+          return defaultType.values.map((defaultTupleValue, index) =>
+            this.#getDefaultValue(type.values[index], defaultTupleValue)
+          );
+        } else {
+          return defaultType.values.map((defaultTuplevalue) =>
+            this.#getDefaultValue(type.indexer!.value, defaultTuplevalue)
+          );
+        }
+
+      case "Intrinsic":
+        return isNullType(defaultType)
+          ? null
+          : reportDiagnostic(program, {
+              code: "invalid-default",
+              format: { type: defaultType.kind },
+              target: defaultType,
+            });
+      case "EnumMember":
+        return defaultType.value ?? defaultType.name;
+      default:
+        reportDiagnostic(program, {
+          code: "invalid-default",
+          format: { type: defaultType.kind },
+          target: defaultType,
+        });
+    }
   }
 
   booleanLiteral(boolean: BooleanLiteral): EmitterOutput<object> {

packages/json-schema/src/lib.ts

@@ -1,4 +1,4 @@
-import { createTypeSpecLibrary, JSONSchemaType } from "@typespec/compiler";
+import { createTypeSpecLibrary, JSONSchemaType, paramMessage } from "@typespec/compiler";
 
 export type FileType = "yaml" | "json";
 export type Int64Strategy = "string" | "number";
@@ -82,7 +82,14 @@ export const EmitterOptionsSchema: JSONSchemaType<JSONSchemaEmitterOptions> = {
 
 export const libDef = {
   name: "@typespec/json-schema",
-  diagnostics: {},
+  diagnostics: {
+    "invalid-default": {
+      severity: "error",
+      messages: {
+        default: paramMessage`Invalid type '${"type"}' for a default value`,
+      },
+    },
+  },
   emitter: {
     options: EmitterOptionsSchema as JSONSchemaType<JSONSchemaEmitterOptions>,
   },

packages/json-schema/test/models.test.ts

@@ -1,4 +1,4 @@
-import assert from "assert";
+import assert, { deepStrictEqual } from "assert";
 import { emitSchema } from "./utils.js";
 
 describe("emitting models", () => {
@@ -254,4 +254,41 @@ describe("emitting models", () => {
       $ref: "RecordInt32.json",
     });
   });
+
+  describe("default values", () => {
+    it("specify default value on enum property", async () => {
+      const res = await emitSchema(
+        `
+        model Foo {
+          optionalEnum?: MyEnum = MyEnum.a;
+        };
+
+        enum MyEnum {
+          a: "a-value",
+          b,
+        }
+        `
+      );
+
+      deepStrictEqual(res["Foo.json"].properties.optionalEnum, {
+        $ref: "MyEnum.json",
+        default: "a-value",
+      });
+    });
+
+    it("specify default value on string property", async () => {
+      const res = await emitSchema(
+        `
+        model Foo {
+          optional?: string = "abc";
+        }
+        `
+      );
+
+      deepStrictEqual(res["Foo.json"].properties.optional, {
+        type: "string",
+        default: "abc",
+      });
+    });
+  });
 });