remove leading comma in JTD serialisation result, fixes #2001, fixes #2171, fixes #2181

lib/compile/jtd/serialize.ts

@@ -117,7 +117,7 @@ function serializeValues(cxt: SerializeCxt): void {
   gen.add(N.json, str`}`)
 }
 
-function serializeKeyValue(cxt: SerializeCxt, key: Name, schema: SchemaObject, first: Name): void {
+function serializeKeyValue(cxt: SerializeCxt, key: Name, schema: SchemaObject, first?: Name): void {
   const {gen, data} = cxt
   addComma(cxt, first)
   serializeString({...cxt, data: key})
@@ -156,20 +156,24 @@ function serializeSchemaProperties(cxt: SerializeCxt, discriminator?: string): v
   const optProps = keys(optionalProperties)
   const allProps = allProperties(props.concat(optProps))
   let first = !discriminator
+  let firstProp: Name | undefined
+
   for (const key of props) {
+    if (first) first = false
+    else gen.add(N.json, str`,`)
     serializeProperty(key, properties[key], keyValue(key))
   }
+  if (first) firstProp = gen.let("first", true)
   for (const key of optProps) {
     const value = keyValue(key)
-    gen.if(and(_`${value} !== undefined`, isOwnProperty(gen, data, key)), () =>
+    gen.if(and(_`${value} !== undefined`, isOwnProperty(gen, data, key)), () => {
+      addComma(cxt, firstProp)
       serializeProperty(key, optionalProperties[key], value)
-    )
+    })
   }
   if (schema.additionalProperties) {
     gen.forIn("key", data, (key) =>
-      gen.if(isAdditional(key, allProps), () =>
-        serializeKeyValue(cxt, key, {}, gen.let("first", first))
-      )
+      gen.if(isAdditional(key, allProps), () => serializeKeyValue(cxt, key, {}, firstProp))
     )
   }
 
@@ -190,8 +194,6 @@ function serializeSchemaProperties(cxt: SerializeCxt, discriminator?: string): v
   }
 
   function serializeProperty(key: string, propSchema: SchemaObject, value: Name): void {
-    if (first) first = false
-    else gen.add(N.json, str`,`)
     gen.add(N.json, str`${JSON.stringify(key)}:`)
     serializeCode({...cxt, schema: propSchema, data: value})
   }
@@ -251,10 +253,14 @@ function serializeEmpty({gen, data}: SerializeCxt): void {
   gen.add(N.json, _`JSON.stringify(${data})`)
 }
 
-function addComma({gen}: SerializeCxt, first: Name): void {
-  gen.if(
-    first,
-    () => gen.assign(first, false),
-    () => gen.add(N.json, str`,`)
-  )
+function addComma({gen}: SerializeCxt, first?: Name): void {
+  if (first) {
+    gen.if(
+      first,
+      () => gen.assign(first, false),
+      () => gen.add(N.json, str`,`)
+    )
+  } else {
+    gen.add(N.json, str`,`)
+  }
 }

spec/issues/2001_jtd_only_optional_properties.spec.ts

@@ -0,0 +1,24 @@
+import _Ajv from "../ajv_jtd"
+import * as assert from "assert"
+
+describe("schema with optional/additional properties only", () => {
+  const ajv = new _Ajv()
+
+  it("should correctly serialize optional properties", () => {
+    const schema = {
+      optionalProperties: {
+        prop0: {type: "uint16"},
+        prop1: {type: "uint16"},
+        prop2: {type: "uint16"},
+      },
+      additionalProperties: true,
+    }
+    const serialize = ajv.compileSerializer(schema)
+    const test = (data, json) => assert.strictEqual(serialize(data), json)
+    test({prop0: 0, prop1: 1, prop2: 2}, '{"prop0":0,"prop1":1,"prop2":2}')
+    test({prop1: 1, prop2: 2}, '{"prop1":1,"prop2":2}')
+    test({prop0: 0, prop1: 1, prop2: 2, foo: "bar"}, '{"prop0":0,"prop1":1,"prop2":2,"foo":"bar"}')
+    test({prop1: 1, prop2: 2, foo: "bar"}, '{"prop1":1,"prop2":2,"foo":"bar"}')
+    test({foo: "bar"}, '{"foo":"bar"}')
+  })
+})