How to write unit tests for ParserErrorMessageProvider?

[durianwaffle]

Hi guys,

I wrote a customized ParserErrorMessagePrvider and want to write unit tests for it. But I find that the message returned from buildMismatchTokenMessage is always undefined.
Why is it?

describe('CustomParserErrorMessageProvider', () => {
    let provider: CustomParserErrorMessageProvider;

    beforeEach(() => {
        provider = new CustomParserErrorMessageProvider();
    });

    it('should return correct message for buildMismatchTokenMessage', () => {
        const options = {
            expected: { name: 'ExpectedToken' } as TokenType,
            actual: { tokenType: { name: 'ActualToken' } } as IToken,
            previous: {} as IToken,
            ruleName: 'someRule'
        };
        const message = provider.buildMismatchTokenMessage(options);
        expect(message).toBe('Error: Expect ExpectedToken');
    });
}

[msujew]

Hey @durianwaffle,

can you share your CustomParserErrorMessageProvider implementation? That behavior sounds very weird.

[durianwaffle]

Okay,

export class CustomParserErrorMessageProvider extends LangiumParserErrorMessageProvider {

    override buildMismatchTokenMessage(options: {
        expected: TokenType
        actual: IToken
        previous: IToken
        ruleName: string
    }): string {
        // Custom implementation
        return `${i18next.t('ERRORKEY', { what: options.expected.name })}`;
    }

    override buildNotAllInputParsedMessage(options: {
        firstRedundant: IToken
        ruleName: string
    }): string {
        return ``;
    }

    override buildNoViableAltMessage(options: {
        expectedPathsPerAlt: TokenType[][][]
        actual: IToken[]
        previous: IToken
        customUserDescription: string
        ruleName: string
    }): string {
        return ``;
    }

    override buildEarlyExitMessage(options: {
        expectedIterationPaths: TokenType[][]
        actual: IToken[]
        previous: IToken
        customUserDescription: string
        ruleName: string
    }): string {
        return ``;
    }
}

[msujew]

I'm a bit confused. Does buildMismatchTokenMessage return the string "undefined" or does it return undefined (i.e. the missing JS element)? If it returns the string, there might be an error in your localization function, i.e. the call of i18next.t.

Generally, you might want to debug the test to see what goes wrong.

[durianwaffle]

Hi @msujew , it returns undefined.

[msujew]

How it is possible to return undefined if it uses the `` strings? It should always return a string. No idea what's wrong there.