add error helper

src/lib/error.ts

@@ -0,0 +1,59 @@
+/**
+ * An error returned by a GraphQL server.
+ */
+export interface GraphError {
+	message: string;
+	path?: string[];
+}
+
+/** RawError is an error that can be converted to a string by ErrorMessage */
+export type RawError = Error | string | GraphError[];
+
+export class ErrorMessage {
+	private _message: string;
+
+	/** converts a RawError to a string and stores it for later access */
+	constructor(raw: RawError) {
+		this._message = ErrorMessage.rawErrorToString(raw);
+	}
+
+	/** returns the stored message */
+	get message(): string {
+		return this._message;
+	}
+	/** returns the error as a string */
+	toString(): string {
+		return this._message;
+	}
+
+	/** optionally returns a new ErrorMessage only if the RawError is not empty */
+	static from(raw: RawError | null | undefined): ErrorMessage | null {
+		if (!raw) return null;
+		return new ErrorMessage(raw);
+	}
+
+	/** converts a RawError to a string */
+	static rawErrorToString(raw: RawError | null | undefined): string {
+		if (!raw) return 'No error';
+
+		let errorString: string;
+		if (typeof raw === 'string') {
+			errorString = raw;
+		} else if (raw instanceof Error) {
+			errorString = raw.message;
+		} else if (Array.isArray(raw)) {
+			errorString = raw
+				.flatMap((e) => {
+					const messageString = e.message || 'Unknown error';
+					if (e.path && e.path.length > 0) {
+						return `"${messageString}" at ${e.path.join('.')}`;
+					}
+				})
+				.join('<br />');
+		} else {
+			throw `Bad error value ${raw}`;
+		}
+
+		return errorString;
+	}
+}

src/lib/index.ts

@@ -38,3 +38,4 @@ export {
 	ToolbarGroup,
 	Toolbar
 } from './Toolbar';
+export { type GraphError, type RawError, ErrorMessage } from './error';