[Compiler API] function which check if a class implements an interface

[ajaxlab]

Search Terms

Compiler API, InterfaceDeclaration, ClassDeclaration, Checker

Suggestion

A function to check whether a ClassDeclaration implements an InterfaceDeclaration

Use Cases

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

Examples

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

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[ajafff]

I don't think this should be part of the public API. The more API surface there is, the more the TS team has to maintain.

The current API provides everything you need to write your own utility function for this check (untested, but should work):

function classImplementsInterface(classDeclaration: ts.ClassDeclaration, interfaceDeclaration: ts.InterfaceDeclaration, checker: ts.TypeChecker) {
    const heritageClause = classDeclaration.heritageClauses?.find((clause) => clause.token === ts.SyntaxKind.ImplementsKeyword);
    if (!heritageClause) {
        return false;
    }
    const interfaceSymbol = checker.getSymbolAtLocation(interfaceDeclaration.name);
    if (!interfaceSymbol) {
        return false;
    }
    return heritageClause.types.some((baseType) => {
        let symbol = checker.getSymbolAtLocation(baseType);
        if (symbol && symbol.flags & ts.SymbolFlags.Alias) {
            symbol = checker.getAliasedSymbol(symbol);
        }
        return symbol === interfaceSymbol;
    });
}

If you need help to understand what it does, just ask here.

If I'm in the need for such functionality, I typically stuff it in my utility library tsutils which already contains a whole lot of useful (and mostly undocumented) functions.

[ajaxlab]

@ajafff Wow, 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

[ajaxlab]

@ajafff I tested the above code. It works well, but I fixed a part.

getSymbolAtLocation(baseType) to getSymbolAtLocation(baseType.expression)

function classImplementsInterface(classDeclaration: ts.ClassDeclaration, interfaceDeclaration: ts.InterfaceDeclaration, checker: ts.TypeChecker) {
    const heritageClause = classDeclaration.heritageClauses?.find((clause) => clause.token === ts.SyntaxKind.ImplementsKeyword);
    if (!heritageClause) {
        return false;
    }
    const interfaceSymbol = checker.getSymbolAtLocation(interfaceDeclaration.name);
    if (!interfaceSymbol) {
        return false;
    }
    return heritageClause.types.some((baseType) => {
        let symbol = checker.getSymbolAtLocation(baseType.expression);
        if (symbol && symbol.flags & ts.SymbolFlags.Alias) {
            symbol = checker.getAliasedSymbol(symbol);
        }
        return symbol === interfaceSymbol;
    });
}

I hope I did right :D
Thanks!