Support message testing function in throws & throwsAsync assertions

docs/03-assertions.md

@@ -172,7 +172,10 @@ Assert that an error is thrown. `fn` must be a function which should throw. The
 
 * `instanceOf`: a constructor, the thrown error must be an instance of
 * `is`: the thrown error must be strictly equal to `expectation.is`
-* `message`: either a string, which is compared against the thrown error's message, or a regular expression, which is matched against this message
+* `message`: the following types are valid: 
+  * *string* - it is compared against the thrown error's message
+  * *regular expression* - it is matched against this message
+  * *function* - it is passed the thrown error message and must return a boolean for whether the assertion passed
 * `name`: the expected `.name` value of the thrown error
 * `code`: the expected `.code` value of the thrown error
 
@@ -204,7 +207,10 @@ The thrown value *must* be an error. It is returned so you can run more assertio
 
 * `instanceOf`: a constructor, the thrown error must be an instance of
 * `is`: the thrown error must be strictly equal to `expectation.is`
-* `message`: either a string, which is compared against the thrown error's message, or a regular expression, which is matched against this message
+* `message`: the following types are valid: 
+  * *string* - it is compared against the thrown error's message
+  * *regular expression* - it is matched against this message
+  * *function* - it is passed the thrown error message and must return a boolean for whether the assertion passed
 * `name`: the expected `.name` value of the thrown error
 * `code`: the expected `.code` value of the thrown error
 

lib/assert.js

@@ -106,7 +106,12 @@ function validateExpectations(assertion, expectations, numberArgs) { // eslint-d
 			});
 		}
 
-		if (hasOwnProperty(expectations, 'message') && typeof expectations.message !== 'string' && !(expectations.message instanceof RegExp)) {
+		if (
+			hasOwnProperty(expectations, 'message')
+			&& typeof expectations.message !== 'string'
+			&& !(expectations.message instanceof RegExp)
+			&& !(typeof expectations.message === 'function')
+		) {
 			throw new AssertionError({
 				assertion,
 				message: `The \`message\` property of the second argument to \`t.${assertion}()\` must be a string or regular expression`,
@@ -230,6 +235,19 @@ function assertExpectations({assertion, actual, expectations, message, prefix, s
 		});
 	}
 
+	if (expectations.message instanceof Function && expectations.message(actual.message) === false) {
+		throw new AssertionError({
+			assertion,
+			message,
+			savedError,
+			actualStack,
+			values: [
+				formatWithLabel(`${prefix} unexpected exception:`, actual),
+				formatWithLabel('Expected message to return true:', expectations.message),
+			],
+		});
+	}
+
 	if (typeof expectations.code !== 'undefined' && actual.code !== expectations.code) {
 		throw new AssertionError({
 			assertion,

types/assertions.d.ts

@@ -12,7 +12,7 @@ export type ThrowsExpectation = {
 	is?: Error;
 
 	/** The thrown error must have a message that equals the given string, or matches the regular expression. */
-	message?: string | RegExp;
+	message?: string | RegExp | Function;
 
 	/** The thrown error must have a name that equals the given string. */
 	name?: string;