JavaScript function to convert underscore-case to camel-case (and vice-versa)
Tests

Your test code isn't the clearest. Assuming the rest of the code works as it should, then you should look into making your tests clearer and more reliable.

As it stands, you don't give any expected value for the test results. Having that would enable you to throw an error if the result is wrong as well as if the method throws. Consider something like this:

var tests = [
    {
        'toChange': 'ThisIsPascalCase',
        'toCase': 'toUnderscoreCase',
        'expected': 'this_is_pascal_case'
    }
];

tests.forEach(function(test) {
    var result = changeCase(test.toChange, test.toCase);
    if(!result) {
        console.error("Something has gone wrong!");
        // log extra error details if you want them
    }
    else if(result !== test.expected) {
        console.error("Result doesn't match expected result!");
        // log extra error and environment/stack details
    }
    else {
        console.log("Test passed successfully.");
    }
});


That makes your test code clearer, and provides you with accuracy checking.