The compiler complains about "property 'state' does not exist on type 'Creep'''. It seems main.ts knows about the interface, but not about the implementation. Or is it the other way around?

Creep.ts

// src/Screeps/Creep.ts

Creep.prototype.role = function() {
    return this.memory.role;
};

index.d.ts

// typings/custom/screeps/index.d.ts

export interface Creep {
    role(): string;
}

main.ts

// src/main.ts

export function loop() {
    for (let name in Game.creeps) {
        if (Game.creeps.hasOwnProperty(name)) {
            console.log(Game.creeps[name].role());
        }
    }
}

tsconfig.json

// tsconfig.json

{
    "compilerOptions": {
        "module": "commonjs",
        "target": "es6"
    },
    "include": [
        "typings/**/*.d.ts",
        "src/**/*.ts"
    ]
}