fix: coercion should skip rendering altogether

This helps avoiding useless allocations. It also causes required
fields checks to be skipped entirely as those are done at rendering.

see: fastify#670

Author: rhighs

index.js

@@ -559,23 +559,13 @@ function buildObject (context, location) {
 
   let functionCode = `
   `
-  let checkNullableCode = `
-  `
 
   const nullable = schema.nullable === true
-  if (!nullable) {
-    checkNullableCode = `
-      if (obj === null) {
-        obj = {}
-      }
-    `
-  }
-
   functionCode += `
     // ${schemaRef}
     function ${functionName} (input) {
-      let obj = ${toJSON('input')}
-      ${checkNullableCode}
+      ${!nullable ? 'if (input === null) return \'{}\'' : ''}
+      const obj = ${toJSON('input')}
 
       ${buildInnerObject(context, location)}
     }
@@ -609,25 +599,14 @@ function buildArray (context, location) {
     schemaRef = schemaRef.replace(context.rootSchemaId, '')
   }
 
-  let checkNullableCode = `
-  `
-
   let functionCode = `
     function ${functionName} (obj) {
       // ${schemaRef}
   `
 
   const nullable = schema.nullable === true
-  if (!nullable) {
-    checkNullableCode = `
-      if (obj === null) {
-        obj = []
-      }
-    `
-  }
-
   functionCode += `
-    ${checkNullableCode}
+    ${!nullable ? 'if (obj === null) return \'[]\'' : ''}
     if (!Array.isArray(obj)) {
       throw new TypeError(\`The value of '${schemaRef}' does not match schema definition.\`)
     }

test/toJSON.test.js

@@ -177,7 +177,7 @@ test('on non nullable null object it should coerce to {}', (t) => {
   t.equal(result, '{}')
 })
 
-test('on non-nullable null object with required fields it should throw complaining missing required fields', (t) => {
+test('on non-nullable null object it should skip rendering, skipping required fields checks', (t) => {
   t.plan(1)
 
   const stringify = build({
@@ -198,11 +198,6 @@ test('on non-nullable null object with required fields it should throw complaini
     required: ['product']
   })
 
-  try {
-    stringify(null)
-    t.fail('stringify should throw for missing required fields')
-  } catch (err) {
-    const message = err.message
-    t.equal(message, '"product" is required!')
-  }
+  const result = stringify(null)
+  t.equal(result, '{}')
 })