Node.js code to extract the keys of a TypeScript type programatically

extractTypeKeys.js

const ts = require("typescript")

/**
 * Extract the keys of a type defined in a .d.ts file.
 * @param filename - name of the .d.ts file
 * @param namespace - namespace where the type is defined
 * @param typeName - name of the type
 * @param options - options to pass to the TypeScript parser
 * @returns an array of keys, expressed as strings
 */
function extractTypeKeys (filename, namespace, typeName, options = {}) {
    // reference documentation: https://github.com/Microsoft/TypeScript/wiki/Using-the-Compiler-API
    const program = ts.createProgram([filename], options)
    const sourceFile = program.getSourceFile(filename)
    const symbols = program.getTypeChecker().getRootSymbols(sourceFile)
    const envDef = symbols[0].locals.get(namespace).exports.get(typeName)
    return envDef.declarations[0].type.members.map((envVar) => envVar.name.escapedText)
}

// example
const keys = extractTypeKeys("./sample-env-vars.d.ts", "MyApp", "RequiredEnvVars")

sample-env-vars.d.ts

declare namespace MyApp {
    type RequiredEnvVars = {
        NODE_ENV: "development" | "production" | "test",
    }
}