promise catch not working

Question

employeeData.js

function getById(id) {
    return dbPromise.one(); // using pg-promise
}

employeeService.js

function getInfoById(id) {
    return employeeData.getXYZ(id); // purposefully calling a function that does not exist in the employeeData module
}

employeeApi.js // Express route function

const getInfoById = (req, res) {
    employeeService.getInfoById(123)
    .then(response => {
         res.json(response);
    })
    .catch(err => {
         res.json(err); // this is not being triggered
    });
}

In the employeeService above I purposefully mis-typed the function name and I think it should throw an undefined error for calling a function getXYZ that does not exist in the employeeData.js. However, the catch block in employeeApi.js is not called.

Can you suggest what I might be missing here?

FYI, `function getInfoById() {....}` in employeeService.js needs the parentheses in its definition that I show here that your code in your question does not show. — jfriend00, May 22 '18 at 05:09

Bergi · Accepted Answer · 2018-05-21T21:36:36.010

I think it should throw an undefined error for calling a function that does not exist

Yes, it does that. It throws an exception, it does not return a promise. As such, there is no promise object on which the then(…).catch(…) methods could be invoked.

However, the catch block is not called.

There is no catch block, it's just a method call - which doesn't happen because there's an exception. To catch it, you would need an actual try/catch block.

But that would look weird, and that's the reason why promise-returning functions should never throw. You could wrap the code with the mistake in a new Promise executor which would catch them, but the simpler choice is to use async/await syntax which guarantees to always return a promise (and reject it in case of an exception in the function body).

async function getInfoById {
    return employeeData.getXYZ(123);
}

promise catch not working

1 Answers1