Add error.originalMessage property (#373)

index.d.ts

@@ -302,6 +302,13 @@ declare namespace execa {
 		The error message.
 		*/
 		message: string;
+
+		/**
+		Original error message. This is `undefined` unless the child process exited due to an `error` event or a timeout.
+
+		The `message` property contains both the `originalMessage` and some additional information added by Execa.
+		*/
+		originalMessage?: string;
 	}
 
 	interface ExecaError<StdoutErrorType = string>

index.test-d.ts

@@ -40,6 +40,7 @@ try {
 	expectType<boolean>(execaError.isCanceled);
 	expectType<boolean>(execaError.killed);
 	expectType<string | undefined>(execaError.signal);
+	expectType<string | undefined>(execaError.originalMessage);
 }
 
 try {
@@ -69,6 +70,7 @@ try {
 	expectError(execaError.isCanceled);
 	expectType<boolean>(execaError.killed);
 	expectType<string | undefined>(execaError.signal);
+	expectType<string | undefined>(execaError.originalMessage);
 }
 
 execa('unicorns', {cleanup: false});

lib/error.js

@@ -53,6 +53,7 @@ const makeError = ({
 	const message = `Command ${prefix}: ${command}`;
 
 	if (error instanceof Error) {
+		error.originalMessage = error.message;
 		error.message = `${message}\n${error.message}`;
 	} else {
 		error = new Error(message);

readme.md

@@ -277,6 +277,14 @@ Type: `string | undefined`
 
 The signal that was used to terminate the process.
 
+#### originalMessage
+
+Type: `string | undefined`
+
+Original error message. This is `undefined` unless the child process exited due to an `error` event or a timeout.
+
+The `message` property contains both the `originalMessage` and some additional information added by Execa.
+
 ### options
 
 Type: `object`

test/error.js

@@ -163,3 +163,8 @@ errorMessage.title = (message, expected) => `error.message matches: ${expected}`
 
 test(errorMessage, /Command failed with exit code 2.*: exit 2 foo bar/, 2, 'foo', 'bar');
 test(errorMessage, /Command failed with exit code 3.*: exit 3 baz quz/, 3, 'baz', 'quz');
+
+test('Original error message is kept', async t => {
+	const {originalMessage} = await t.throwsAsync(execa('wrong command'));
+	t.is(originalMessage, 'spawn wrong command ENOENT');
+});