Pass an undefined title argument to macro-title functions

Previously, AVA would pass the empty string. By passing an undefined value, the title function can use argument defaults which is more natural.

This is a breaking change for users who were concatenating `title`, under the assumption it was an empty string.

docs/recipes/typescript.md

@@ -66,7 +66,7 @@ import test, {Macro} from 'ava';
 const macro: Macro = (t, input: string, expected: number) => {
 	t.is(eval(input), expected);
 };
-macro.title = (providedTitle: string, input: string, expected: number) => `${providedTitle} ${input} = ${expected}`.trim();
+macro.title = (providedTitle = '', input: string, expected: number) => `${providedTitle} ${input} = ${expected}`.trim();
 
 test(macro, '2 + 2', 4);
 test(macro, '2 * 3', 6);

index.d.ts

@@ -390,13 +390,13 @@ export interface Macro<Context = {}> {
 	 * Implement this function to generate a test (or hook) title whenever this macro is used. `providedTitle` contains
 	 * the title provided when the test or hook was declared. Also receives the remaining test arguments.
 	 */
-	title?: (providedTitle: string, ...args: Array<any>) => string;
+	title?: (providedTitle: string | undefined, ...args: Array<any>) => string;
 }
 
 /** A reusable test or hook implementation, for tests & hooks declared with the `.cb` modifier. */
 export interface CbMacro<Context = {}> {
 	(t: CbExecutionContext<Context>, ...args: Array<any>): ImplementationResult;
-	title?: (providedTitle: string, ...args: Array<any>) => string;
+	title?: (providedTitle: string | undefined, ...args: Array<any>) => string;
 }
 
 export interface TestInterface<Context = {}> {

index.js.flow

@@ -403,13 +403,13 @@ export interface Macro<Context = {}> {
 	 * Implement this function to generate a test (or hook) title whenever this macro is used. `providedTitle` contains
 	 * the title provided when the test or hook was declared. Also receives the remaining test arguments.
 	 */
-	title?: (providedTitle: string, ...args: Array<any>) => string;
+	title?: (providedTitle: string | void, ...args: Array<any>) => string;
 }
 
 /** A reusable test or hook implementation, for tests & hooks declared with the `.cb` modifier. */
 export interface CbMacro<Context = {}> {
 	(t: CbExecutionContext<Context>, ...args: Array<any>): ImplementationResult;
-	title?: (providedTitle: string, ...args: Array<any>) => string;
+	title?: (providedTitle: string | void, ...args: Array<any>) => string;
 }
 
 export interface TestInterface<Context = {}> {

lib/runner.js

@@ -55,7 +55,7 @@ class Runner extends Emittery {
 
 			const specifiedTitle = typeof args[0] === 'string' ?
 				args.shift() :
-				'';
+				undefined;
 			const implementations = Array.isArray(args[0]) ?
 				args.shift() :
 				args.splice(0, 1);
@@ -65,7 +65,7 @@ class Runner extends Emittery {
 					throw new TypeError('`todo` tests are not allowed to have an implementation. Use `test.skip()` for tests with an implementation.');
 				}
 
-				if (specifiedTitle === '') {
+				if (specifiedTitle === undefined || specifiedTitle === '') {
 					throw new TypeError('`todo` tests require a title');
 				}
 
@@ -97,14 +97,14 @@ class Runner extends Emittery {
 
 				for (const implementation of implementations) {
 					let title = implementation.title ?
-						implementation.title.apply(implementation, [specifiedTitle].concat(args)) :
+						implementation.title(specifiedTitle, ...args) :
 						specifiedTitle;
 
-					if (typeof title !== 'string') {
+					if (title !== undefined && typeof title !== 'string') {
 						throw new TypeError('Test & hook titles must be strings');
 					}
 
-					if (title === '') {
+					if (title === undefined || title === '') {
 						if (metadata.type === 'test') {
 							throw new TypeError('Tests must have a title');
 						} else if (metadata.always) {

readme.md

@@ -691,14 +691,14 @@ function macro(t, input, expected) {
 	t.is(eval(input), expected);
 }
 
-macro.title = (providedTitle, input, expected) => `${providedTitle} ${input} = ${expected}`.trim();
+macro.title = (providedTitle = '', input, expected) => `${providedTitle} ${input} = ${expected}`.trim();
 
 test(macro, '2 + 2', 4);
 test(macro, '2 * 3', 6);
 test('providedTitle', macro, '3 * 3', 9);
 ```
 
-The `providedTitle` argument defaults to an empty string if the user does not supply a string title. This allows for easy concatenation without having to worry about `null` / `undefined`. It is worth remembering that the empty string is considered a falsy value, so you can still use `if(providedTitle) {...}`.
+The `providedTitle` argument defaults to `undefined` if the user does not supply a string title. This means you can use a parameter assignment to set the default value. The example above uses the empty string as the default.
 
 You can also pass arrays of macro functions:
 

test/runner.js

@@ -611,7 +611,7 @@ test('macros: Customize test names attaching a `title` function', t => {
 		avaT.pass();
 	}
 
-	macroFn.title = (title, firstArg) => (title || 'default') + firstArg;
+	macroFn.title = (title = 'default', firstArg) => title + firstArg;
 
 	return promiseEnd(new Runner(), runner => {
 		runner.on('stateChange', evt => {