Resolve AST Type Node Structure –– TypeScript Compiler API

Question

Let's say we have the following code:

interface X<Y = any> {
  y: Y;
}

interface Z extends X<"why?"> {
  abc: "ABC";
}

/**
 *
 * Structurally, the `Z` type is...
 *
 * {
 *   y: "why?";
 *   abc: "ABC";
 * }
 *
 */

Is there any built-in mechanism for resolving the final type from a series of intertwined type and/or interface definitions? I can't find anything obvious from playing around with a typechecker and Type nodes.

Any advice would be greatly appreciated!

I'm not sure what your use case is, what exactly do you need to do? The full shape of the object is known at compile time, there's nothing special you have to do for that. If you're talking about runtime, that's a whole different (and much harder) question — bugs
– bugs, Commented Aug 3, 2020 at 20:09
@bugs that's incorrect: I do not want the shape at runtime. I want it at compile time. But I'd rather not traverse and stitch it together myself, given that the TS team has likely already implemented this (for the sake of structural type checking of course). — Harry Solovay
– Harry Solovay, Commented Aug 3, 2020 at 20:27

David Sherret · Accepted Answer · 2020-08-03 22:51:17Z

2

There's no public API for structural typing (see the Type Relationship API issue).

That said, you can get all the property names with types of the interface by doing the following:

const interfaceZDecl = sourceFile.statements[1] as ts.InterfaceDeclaration;
const type = checker.getTypeAtLocation(interfaceZDecl.name);

for (const prop of type.getProperties()) {
    console.log(`Name: ${prop.getName()}`);
    const propType = checker.getTypeOfSymbolAtLocation(prop, prop.valueDeclaration);
    console.log(`Type: ${checker.typeToString(propType)}`);
    console.log("---");
}

Outputs:

Name: abc
Type: "ABC"
---
Name: y
Type: "why?"
---

answered Aug 3, 2020 at 22:51

David Sherret

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