feat: support output: null

README.md

@@ -61,8 +61,10 @@ run('rule-name', rule, {
     {
       input: 'let foo = 1',
       output(output) {
+        expect(output.slice(0, 3)).toBe('let')
         expect(output)
           .toMatchInlineSnapshot(`"const foo = 1;"`)
+        // Any custom assertion...
       },
     },
   ],
@@ -82,8 +84,10 @@ run('rule-name', rule, {
     {
       input: 'let foo = 1',
       errors(errors) {
+        expect(errors).toHaveLength(1)
         expect(errors.map(e => e.messageId))
           .toMatchInlineSnapshot(`["error-message-id"]`)
+        // Any custom assertion...
       },
     },
   ],

src/index.ts

@@ -124,9 +124,11 @@ export function createRuleTester(options: RuleTesterOptions): RuleTester {
     }
     result.messages = messages
 
-    if (_case.output) {
-      if (typeof _case.output === 'function')
-        _case.output(result.output!)
+    if (_case.output !== undefined) {
+      if (_case.output === null) // null means the output should be the same as the input
+        expect(result.output, 'output').toBe(_case.code)
+      else if (typeof _case.output === 'function') // custom assertion
+        _case.output(result.output!, _case.code)
       else
         expect(result.output, 'output').toBe(_case.output)
     }

src/types.ts

@@ -16,8 +16,18 @@ export interface ValidTestCaseBase extends Linter.FlatConfig {
 }
 
 export interface InvalidTestCaseBase extends ValidTestCaseBase {
+  /**
+   * Expected errors.
+   * If a number is provided, it asserts that the number of errors is equal to the number provided.
+   * If an array of strings is provided, it asserts that the error messageIds are equal to the array provided.
+   * If an array of objects is provided, it asserts that the errors are partially equal to the objects provided.
+   */
   errors?: number | (string | Partial<Linter.LintMessage>)[] | ((errors: Linter.LintMessage[]) => void)
-  output?: string | ((output: string) => void)
+  /**
+   * Assert if output is expected.
+   * Pass `null` to assert that the output is the same as the input.
+   */
+  output?: string | null | ((output: string, input: string) => void)
 }
 
 export interface NormalizedTestCase extends InvalidTestCaseBase {