Puppeteer: Throwing custom error message within a javascript function being called using await in an async function

Question

I am using puppeteer to build a small app to automatically fill forms online. I am using Gmail as an example to test the functionality without clicking the submit button.

I open the Gmail URL and locate the username input field by using a function called findInputFieldByName. The code works fine, webpage opens and I can see puppeteer printing [email protected] in the username field.

The code is:

const puppeteer = require('puppeteer');

const gmailUrl='https://accounts.google.com/signin/v2/identifier?continue=https%3A%2F%2Fmail.google.com%2Fmail%2F&service=mail&sacu=1&rip=1&flowName=GlifWebSignIn&flowEntry=ServiceLogin';
const userName='[email protected]';
const findInputFieldByName = (page,inputName) => {
    return page.$('input[name="'+inputName+'"]');
}

(async () => {
    try {
    const Browser = await puppeteer.launch({headless:false,
                                        defaultViewport:{width:600,height:800}
                                        });
    const page=await Browser.newPage();

    page.on('load', () => console.log('Page loaded ' + page.url()));
    await page.goto(gmailUrl);

    const userNameElement = await findInputFieldByName(page,'identifier').catch(error => {
       console.log('The following error occurred: ' + error);
       });
    await userNameElement.type(userName,{delay:50});
    } catch(e) {console.log('main program error:' + e);
    }

})();

As you can see the code is looking for an input field whose name is identifier in the returned HTML. However, if I were to use a different name such as identifier1, which is not on the Gmail Login page, I want the code to throw a custom error message saying "Input field name identifier1 not found" and the code should stop.

If I change

const userNameElement = await findInputFieldByName(page,'identifier').catch(error => {
           console.log('The following error occurred: ' + error);
           });

to

const userNameElement = await findInputFieldByName(page,'identifier1').catch(error => {
           console.log('The following error occurred: ' + error);
           });

I get the following error:

main program error:TypeError: Cannot read property 'type' of null

I understand that this is because the function findInputFieldByName has not failed. It just returned a null object because there is no input field with the name identifier1. The code actually failed when it tried to type the email address.

However, my question is how can I throw an error from within the findInputFieldByName function so that it stops the at the const userNameElement = await findInputFieldByName.. execution?

I have tried to use promises within the findInputFiledByName function as:

const findInputFieldByName = (page,inputName) => {
    let promise=new Promise(function (resolve,reject){
    var elm= page.$('input[name="'+inputName+'"]');
    if (elm.length>0) {
        resolve(elm);
    } else {
        reject(Error('Could not find input name ' + inputName));
    }
});
}

but I am getting the following message:

main program error:TypeError: Cannot read property 'catch' of undefined (node:12332) UnhandledPromiseRejectionWarning: Error: Could not find input name identifier1

at C:\Users\test1.js:11:10

at new Promise (< anonymous >)

at findInputFieldByName (C:\Users\test1.js:6:14)

at C:\Users\test1.js:26:32

at < anonymous >

at process._tickCallback (internal/process/next_tick.js:188:7) (node:12332) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1) (node:12332) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

How should I handle the custom errors? I am new to Javascript to any help is much appreciated.enter code here

Md. Abu Taher · Accepted Answer · 2018-09-15 16:39:56Z

3

First of all, return the promise instead of assigning it to a variable.

const findInputFieldByName = (page, inputName) => {
  return new Promise(((resolve, reject) => { // <-- Return it

After that, you can throw error if need.

await findInputFieldByName(page,'identifier').catch(error => {
    throw ('The following error occurred: ' + error) // <-- pass the error to main program
});

answered Sep 15, 2018 at 16:39

Md. Abu Taher

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