TypeScript Compiler API function which can check if a class implements an interface

Question

I wanted to check whether a ClassDeclaration of a file a.ts implements an InterfaceDeclaration from a file b.ts using Compiler API. But I couldn't find a method or a function for it.

function isClassImplementInterface(
  ts.ClassDeclaration: classDeclaration,
  ts.InterfaceDeclaration: interfaceDeclaration
): boolean {
  // return true if classDeclaration implements interfaceDeclaration correctly
}

Is there any function for it out of Compiler API?

You can use checker.isTypeAssignableTo after you get the types of the class and the interface declaration. — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented Dec 29, 2019 at 12:19
@TitianCernicova-Dragomir that's internal API so it can't be used. See the GitHub issue here. — David Sherret
– David Sherret, Commented Dec 29, 2019 at 17:27
@DavidSherret Wops, my bad, thought that ways part of the public api — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented Dec 29, 2019 at 18:18

David Sherret · Accepted Answer · 2019-12-29 21:07:47Z

4

To check if a class directly implements a certain interface you can look at the types of the implements heritage clause.

For example:

function doesClassDirectlyImplementInterface(
    classDec: ts.ClassDeclaration,
    interfaceDec: ts.InterfaceDeclaration,
    typeChecker: ts.TypeChecker
) {
    const implementsClause = classDec.heritageClauses
        ?.find(c => c.token === ts.SyntaxKind.ImplementsKeyword);

    for (const clauseTypeNode of implementsClause?.types ?? []) {
        const clauseType = typeChecker.getTypeAtLocation(clauseTypeNode);
        if (clauseType.getSymbol()?.declarations.some(d => d === interfaceDec))
            return true;
    }

    return false;
}

You may want to expand on that to also check if the class declaration has a base class and then check that class' heritage clauses as well.

edited Dec 29, 2019 at 21:07

answered Dec 29, 2019 at 17:26

David Sherret

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1 Comment

ajaxlab Over a year ago

Thanks a lot, the above code will be a great help. I'll try to make a utility code based on your guide. Thanks ! :D

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