How to get console.log output in Terminal via Headless Chrome Runtime.evaluate

Question

I am following this issue post here:

https://github.com/cyrus-and/chrome-remote-interface/issues/105

But I cannot seem to get console.log output in the Mac Terminal. It's probably inside the Chrome Devtools window which I don't see.

So how do I get console.log output in Mac Terminal via Runtime.evaluate expression?

My code below:

const chromeLauncher = require('chrome-launcher');
const CDP = require('chrome-remote-interface');
const file = require('fs');

(async function() {
  async function launchChrome() {
    return await chromeLauncher.launch({
      chromeFlags: [
        '--headless',
        '--disable-gpu'
      ]
    });
  }
  const chrome = await launchChrome();
  const protocol = await CDP({
    port: chrome.port
  });

  const {
    DOM,
    Network,
    Page,
    Emulation,
    Runtime
  } = protocol;

  await Promise.all([Network.enable(), Page.enable(), Runtime.enable(), DOM.enable()]);

  Page.navigate({url: 'https://www.chromestatus.com/'});

  Page.loadEventFired(async () => {
    const result = await Runtime.evaluate({expression: 'console.log(\'aaa\')'});

    protocol.close();
    chrome.kill(); 
  });

})();

Answer 1

I've been doing some research on this; below are some of my findings:

When evaluating:

const result = await Runtime.evaluate({ expression: 'console.log("aaa")' });

Result is always:

{ result: { type: 'undefined' } }

However, the following expression:

const result = await Runtime.evaluate({expression: 'window.location.toString()'});

Returns:

{ result: { type: 'string', value: 'https://www.chromestatus.com/features' } }

Now if I evaluate the function without invoking it:

const result = await Runtime.evaluate({ expression: 'console.log' });

Result is set to:

{ result: { type: 'function', className: 'Function', description: 'function log() { [native code] }', objectId: '{"injectedScriptId":2,"id":1}' } }

So I did some more digging, and found that every time console.log is invoked when evaluated, the messageAdded event on the Console object is always fired.

So I went ahead and enabled the Console object and added a listener to the messageAdded event, and now I can successfully capture the console entry as expected.

This is what I did:

const chromeLauncher = require('chrome-launcher');
const CDP = require('chrome-remote-interface');
const file = require('fs');

(async function() {
  async function launchChrome() {
    return await chromeLauncher.launch({
      chromeFlags: [
        '--headless',
        '--disable-gpu'
      ]
    });
  }
  const chrome = await launchChrome();
  const protocol = await CDP({
    port: chrome.port
  });

  const {
    DOM,
    Network,
    Page,
    Emulation,
    Runtime,
    Console
  } = protocol;

  await Promise.all([Network.enable(), Page.enable(), DOM.enable(), Runtime.enable(), Console.enable()]);

  await Page.navigate({url: 'https://www.chromestatus.com/'});

  // REMARKS: messageAdded is fired every time a new console message is added
  Console.messageAdded((result) => {
    console.log(result);
  });

  Page.loadEventFired(async () => {
    const result = await Runtime.evaluate({ expression: 'console.log("aaa")' });

    // REMARKS: When evaluating console.log, result.result.value is undefined.
    console.log(result);

    protocol.close();
    chrome.kill(); 
  });

})();

/* Output from listening on messageAdded:
{ message: 
   { source: 'console-api',
     level: 'log',
     text: 'aaa',
     url: '',
     line: 1,
     column: 9 } }
*/

I got the details from Chrome DevTools Protocol Viewer - Console

From the documentation:

Console.messageAdded

Issued when new console message is added.

Hope this helps!