How do you use the Typescript compiler's Typechecker to get the declared (alias) type, rather than the resolved type?

Question

I realize this is somewhat obscure, but maybe someone else has run into this or knows the Typescript compiler well. I am processing Typescript files using Typescript's compiler API, based on examples like this: https://github.com/Microsoft/TypeScript/wiki/Using-the-Compiler-API

Imagine I have a declared function like this in Typescript:

export type DateString = string;
export function parseDate(date: DateString): Date{
    let parsedDate = Date.parse(date);
    let retVal = new Date();
    retVal.setTime(parsedDate);
    return retVal;
}

In the examples linked above, you can see methods like this defined to extract information about symbols:

function serializeSymbol(symbol: ts.Symbol): DocEntry {
    return {
        name: symbol.getName(),
        type: checker.typeToString(checker.getTypeOfSymbolAtLocation(symbol, symbol.valueDeclaration))
    };
}

When you run checker.typeToString(checker.getTypeOfSymbolAtLocation(symbol, symbol.valueDeclaration) on the date: DateString symbol, instead of returning DateString , it returns string . In other words, you don't get the declared type alias, but the fully resolved type. In my case, I'd like to know that the type of the date field is DateString . Is there an easy way to look up a parameter's declared type rather than its resolved type?

Answer 1

Unfortunately this doesn't work because of "type interning." See here .

What works is to get the text of the typeNode . So basically get the text from the node. Here's a working example that you may be apply to your scenario:

// fileToAnalyze.ts
type DateString = string;
function myFunction(date: DateString) {
}

// main.ts
import * as ts from "typescript";
import * as path from "path";

const program = ts.createProgram([path.join(__dirname, "fileToAnalyze.ts")], { });
const file = program.getSourceFiles().filter(f => /fileToAnalyze/.test(f.fileName))[0];

const funcNode = (file.statements[1] as ts.FunctionDeclaration);
const parameterNode = funcNode.parameters[0];
console.log(parameterNode.type.getText(file)); // DateString

By the way, you might want to check out this library ts-type-info ts-simple-ast ts-morph that I've been working on in case you haven't seen it.