scenario vars

vars.ts

export interface Vars<T> {
  /**
   * Returns a step that assigns the result of step to key k.
   *
   * Always returns void, so if the step returned a promise set() won't block.
   *
   * You can:
   *
   * // will NOT block on stepThatCompletesEventually return value
   * set('value',stepThatCompletesEventually),

   * doSomeOtherStuffThatCausesThePromiseToResolve(),

   * // will block on stepThatCompletesEventually return value
   * get('value'),
   *
   * Values of keys are immutable.
   */
  set<C, K extends keyof T>(k: K, step: Step<C, T[K]>): Step<C, void>;

  /**
   * Returns a step that gets the value of key k, waiting for it to resolve
   * if it is a promise.
   *
   * Throws exception if k has not been set.
   */
  get<C, K extends keyof T>(k: K): Step<C, Promise<T[K]>>;
}

export function vars<T, C = any>(): Vars<T> {
  const values: Partial<T> = {};
  return {
    set: <C, K extends keyof T>(k: K, step: Step<C, T[K]>): Step<C, void> => {
      return (c: C): void => {
        if (k in values)
          throw new Error(`'${k} already set to ${JSON.stringify(values[k])}'`);
        values[k] = execute(c, step);
        return
      }
    },
    get: <C, K extends keyof T>(k: K): Step<C, Promise<T[K]>> => {
      return async (c: C): Promise<T[K]> => {
        if (!(k in values))
          throw new Error(`'${k} has not been set`);
        return await values[k] as T[K];
      }
    }
  }
}

Also good for storing values that aren't known at runtime:

interface UserArgs {
  name: string;
}
/**
* user that's been created in the database
**/
interface User extends UserArgs {
  id: number; // auto-increment
}
function createUser(args:UserArgs) : Step<Promise<User>> {
   return (context:ScenarioContext) : Promise<User> {
     return context.createUser(args);
  }
}
function createNote(text:string, user:Promise<User>) : Step<Promise<Note>> {
   return async (context:ScenarioContext) : Promise<Note> {
     return context.createNote({text, userId: (await user).id});
  }
}

const {get,set} = vars<{user:Promise<User>}>();
scenario(context,
  set('user', createUser({name:"bob"})), // we don't know User.id until the step is complete
  createNote("buy some milk", get('user')) // retrieves the User created in the previous step
)