为什么console.log停止javascript库中的错误？

Question

I have been toying with creating a small library for a web app I am working on. In creating this library, I can enter a log statement at the top of the script, and everything below works fine. However, if I remove the top console.log statement, I get errors. Code is below. Errors are:

ReferenceError: assignment to undeclared variable TestFirst

Code:

$(document).ready(function() {
    (function() {
        console.log('starting');
        'use strict';
        function define_TestFirst()  {
            function TestFirst () {};
            return TestFirst;
        }

        if (typeof(TestFirst) === 'undefined') {
            console.log('defined');
            TestFirst = define_TestFirst();
            TestFirst.prototype.test = function () {
                console.log('TestFirst object created.');
            }
        } else {
            console.log('TestFirst library already defined!');
        }
    })();
});

Answer 1

The problem is that the "use strict" must be the first statement of your function in order to be used. It's ignored otherwise.

Now, I guess that you see the problem:
The problem is not about removing the top console.log , it's about use strict being no longer ignored.

The problem with your script running in strict mode is that any variable must be declared first :

ReferenceError: assignment to undeclared variable TestFirst

Means that you need to add the var statement at var TestFirst = define_TestFirst();

Answer 2

As the error says, you did not declare TestFirst with var but assign a value to it (or use window.TestFirst = ... ):

TestFirst = define_TestFirst();

You want strict mode, but having the console.log() before the use strict causes the use strict to not enable strict mode.

From MDN:

To invoke strict mode for an entire script, put the exact statement "use strict"; (or 'use strict';) before any other statements.