Set Custom Error Message for Assert (Node.js)

Question

I'm lost in the Node documentation and I'm having difficulties figuring out how I could create a custom (or modify the existing) error handling for all of my assert statements, without having to include individual messages with each assertion.

const assert = require('assert');

describe('Test 1', function(){
  describe('Checks State', function(){
    it('will fail', function(){
        assert.strictEqual(true, false);
    });
  });
});

As expected, the previous code would just generate something like:

1) "Test 1 Checks State will fail"
true === false

I'm running with WebDriverIO and my goal is to include the browser.sessionId in the error message, without having to manually fill in the third (message) parameter on each test.

assert.strictEqual(true, false, browser.sessionId);

It would be ideal if I could generate an error message like:

1) "Test 1 Checks State will fail"
abc012-efg345-hij678-klm901
true !== false

I apologize, I know I should include "what I have done so far" -- but everything I've done so far has had no impact. And again, I'm lost in the node documentation :)

Adelin · Accepted Answer · 2018-07-04T06:30:47.770

You cannot, without tampering with the 3^rd party lib assert

Behind the scenes, uses a fail function, that is private in the context of assert and you cannot tell assert to use a custom fail function.

this is the function used behind the scenes:

function fail(actual, expected, message, operator, stackStartFunction) {
  throw new assert.AssertionError({
    message: message,
    actual: actual,
    expected: expected,
    operator: operator,
    stackStartFunction: stackStartFunction
  });
}

As such, you have three options:

(recommended) Fork the lib on github. Implement either some watchers like onFail or allow it to be extensible and create a pull request.
(not recommended) Overwrite the fail function in the node_modules\assert\assert.js file yourself so that, besides triggering the usual stuff, it does what you want as well.

While quick, this will cause a broken dependency forever.
Look for other assertions libraries (if there are any that fit your needs)

score 0 · Answer 2 · answered Oct 05 '18 at 20:14

0

My answer

const assert = require('assert');
describe('Set Custom Error Message for Assert (Node.js)', () => {
    it('Message Assert', () => {
       assert.fail(21, 42, 'This is a message custom', '##');
    });
});

Reference

answered Oct 05 '18 at 20:14

Erick Ribeiro

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Set Custom Error Message for Assert (Node.js)

2 Answers2