Default isMultiple to empty array

index.d.ts

@@ -125,9 +125,13 @@ declare namespace meow {
 		readonly inferType?: boolean;
 
 		/**
-		Value of `boolean` flags not defined in `argv`. If set to `undefined` the flags not defined in `argv` will be excluded from the result. The `default` value set in `boolean` flags take precedence over `booleanDefault`.
+		Value of `boolean` flags not defined in `argv`.
 
-		__Caution: Explicitly specifying undefined for `booleanDefault` has different meaning from omitting key itself.__
+		If set to `undefined`, the flags not defined in `argv` will be excluded from the result. The `default` value set in `boolean` flags take precedence over `booleanDefault`.
+
+		_Note: If used in conjunction with `isMultiple`, the default flag value is set to `[]`._
+
+		__Caution: Explicitly specifying `undefined` for `booleanDefault` has different meaning from omitting key itself.__
 
 		@example
 		```

index.js

@@ -68,6 +68,7 @@ const buildParserFlags = ({flags, booleanDefault}) =>
 
 		if (flag.isMultiple) {
 			flag.type = flag.type ? `${flag.type}-array` : 'array';
+			flag.default = flag.default || [];
 			delete flag.isMultiple;
 		}
 

readme.md

@@ -239,9 +239,14 @@ Type: `boolean | null | undefined`\
 Default: `false`
 
 Value of `boolean` flags not defined in `argv`.
-If set to `undefined` the flags not defined in `argv` will be excluded from the result.
+
+If set to `undefined`, the flags not defined in `argv` will be excluded from the result.
 The `default` value set in `boolean` flags take precedence over `booleanDefault`.
 
+_Note: If used in conjunction with `isMultiple`, the default flag value is set to `[]`._
+
+__Caution: Explicitly specifying `undefined` for `booleanDefault` has different meaning from omitting key itself.__
+
 Example:
 
 ```js

test/test.js

@@ -312,6 +312,20 @@ test('supports `number` flag type - throws on incorrect default value', t => {
 	});
 });
 
+test('isMultiple - unset flag returns empty array', t => {
+	t.deepEqual(meow({
+		argv: [],
+		flags: {
+			foo: {
+				type: 'string',
+				isMultiple: true
+			}
+		}
+	}).flags, {
+		foo: []
+	});
+});
+
 test('isMultiple - flag set once returns array', t => {
 	t.deepEqual(meow({
 		argv: ['--foo=bar'],
@@ -408,25 +422,6 @@ test('isMultiple - boolean flag is false by default', t => {
 	});
 });
 
-test('isMultiple - flag with `booleanDefault: undefined` => filter out unset boolean args', t => {
-	t.deepEqual(meow({
-		argv: ['--foo'],
-		booleanDefault: undefined,
-		flags: {
-			foo: {
-				type: 'boolean',
-				isMultiple: true
-			},
-			bar: {
-				type: 'boolean',
-				isMultiple: true
-			}
-		}
-	}).flags, {
-		foo: [true]
-	});
-});
-
 test('isMultiple - number flag', t => {
 	t.deepEqual(meow({
 		argv: ['--foo=1.3', '--foo=-1'],
@@ -510,17 +505,6 @@ test('isMultiple - handles multi-word flag name', t => {
 	});
 });
 
-test('isMultiple - handles non-set flags correctly', t => {
-	t.deepEqual(meow({
-		argv: [],
-		flags: {
-			foo: {
-				isMultiple: true
-			}
-		}
-	}).flags, {});
-});
-
 if (NODE_MAJOR_VERSION >= 14) {
 	test('supports es modules', async t => {
 		try {