CODEWITHSUNDEEP
javascriptdynamicmocha.js
Martins Untals
Martins Untals

Reputation: 2288

How to provide context of failed assertion in mocha

If I use mocha.js with dynamically generated tests, then how do I print out context of what were variable values when test failed?

If we take that example from mocha.js documentation, 

var assert = require('assert');

function add() {
  return Array.prototype.slice.call(arguments).reduce(function(prev, curr) {
    return prev + curr;
  }, 0);
}

describe('add()', function() {
  var tests = [
    {args: [1, 2],       expected: 3},
    {args: [1, 2, 3],    expected: 6},
    {args: [1, 2, 3, 4], expected: 10}
  ];

  tests.forEach(function(test) {
    it('correctly adds ' + test.args.length + ' args', function() {
      var res = add.apply(null, test.args);
      assert.equal(res, test.expected);
    });
  });
});

then how do I print out {args: [1, 2, 3], expected: 6}, if second test fails?

Upvotes: 0

Views: 297

Answers (2)

Martin Schneider
Martin Schneider

Reputation: 3268

you can add an additional parameter message to the assert.XYZ methods which will be output when the assert fails. You can add your desired output there

tests.forEach(function(test) {
    it('correctly adds ' + test.args.length + ' args', function() {
        var res = add.apply(null, test.args);
        assert.equal(res, test.expected, "Failed with data " + JSON.stringify(test));
    });
});

Upvotes: 1

Krzysztof Safjanowski
Krzysztof Safjanowski

Reputation: 7438

You can pass additional comments as third argument for assert method

  tests.forEach(function(test) {
    it('correctly adds ' + test.args.length + ' args', function() {
      var res = add.apply(null, test.args);
      assert.equal(res, test.expected, JSON.stringify(test)); // third argument passed
    });
  });

will produce output like:

1) add() correctly adds 3 args:

  AssertionError: {"args":[1,2,3],"expected":8}
  + expected - actual

  -6
  +8

Upvotes: 1

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