Using mongoose promises with async/await

Question

I'm trying to get the hang of using Mongoose promises with the async/await functionality of Node.js. When my function printEmployees is called I want to save the list of employees which are queried by the orderEmployees function. While, the console.log statement inside orderEmployees returns the expected query, the console.log inside of printEmployees returns undefined, suggesting that I'm not returning the promise correctly.

I'm new to promises so entirely possible that I'm not correctly understanding the paradigm... any help is much appreciated.

  printEmployees: async(company) => {     var employees = await self.orderEmployees(company);     // SECOND CONSOLE.LOG     console.log(employees);   },    orderEmployees: (companyID) => {     User.find({company:companyID})     .exec()     .then((employees) => {       // FIRST CONSOLE.LOG       console.log(employees);       return employees;     })     .catch((err) => {       return 'error occured';     });   }, 

Answer 1

In order to make orderEmployees behave like async functions, you have to return the resulting promise. There are two rules to follow when using promises without async/await keywords:

1. A function is asynchronous if it returns a Promise
2. If you have a promise (for example returned by an async function) you must either call .then on it or return it.

When you are using async/await then you must await on promises you obtain.

This said you will notice that you do not return the promise generated inside orderEmployees. Easy to fix, but its also easy to rewrite that function to async too.

orderEmployees: (companyID) => {   return User.find({company:companyID}) // Notice the return here   .exec()   .then((employees) => {     // FIRST CONSOLE.LOG     console.log(employees);     return employees;   })   .catch((err) => {     return 'error occured';   }); }, 

or

orderEmployees: async(companyID) => {   try {     const employees = await User.find({company:companyID}).exec();     console.log(employees);     return employees;   } catch (err) {     return 'error occured';   } }, 

PS: the error handling is somewhat flawed here. We usually do not handle errors by returning an error string from a function. It is better to have the error propagate in this case, and handle it from some top-level, UI code.

Answer 2

You need to return your Promise.

• Currently, you are awaiting on a function that returns undefined.
• await only actually "waits" for the value if it's used with a Promise.
• Always keep in mind that you can only await Promises or async functions, which implicitly return a Promise¹.

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orderEmployees: (companyID) => {   return User.find({ company:companyID }).exec() } 

Also really important, you should throw instead of return in your .catch handler. Returning from within a .catch handler will cause the promise chain to trigger it's .then instead of it's .catch thus breaking the error handling chain.

Better yet, don't include .catch at all and let the the actual error bubble up the promise chain, instead of overriding it with your own non-descriptive 'error occured' message.

Error conditions should throw the error, not return it.

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¹ You can also await non-Promises, but only for values that are evaluated synchronously.