Use TS Compiler API to Determine if Interface property signature allows undefined (eg prop1?: string;)

Question

I am using the TypeScript Compiler API to harvest Interface details so I can create database tables. Works well but I would like to identify whether fields are nullable, or in TS jargon, have a type guard like string|undefined.

Eg

export default interface IAccount{
name: string;
description?: string;

}

I would like to be able to identify property "description" as string|undefined and hence, "nullable". I can access the node text via the compiler API and find "?" but is there a better way?

Answer 1

The InterfaceDeclaration node contains node.members (PropertySignature nodes) and if such property signature contains a questionToken key, it is nullable:

const str = `
interface IAccount {
  name: string;
  description?: string;
}
`;
const ast = ts.createSourceFile('repl.ts', str, ts.ScriptTarget.Latest, true /*setParentNodes*/);
const IAccount = ast.statements[0];
for (const member of IAccount.members) {
    const name = member.name.getText();
    let nullable = 'not nullable';
    if (member.questionToken !== undefined) {
        nullable = 'nullable';
    }
    console.log(`${name} is ${nullable}`);
}

Output:

name is not nullable
description is nullable