JS Assert for Input Checking

test_assert.js

var assert = function(condition, msg) {
    if (!condition) {
        // throw message || "Assertion failed";
        console.log('FAIL ----- ' + msg);
    }
    else {
        console.log('PASS ----- ' + msg);
    }
};

var testNum = function(input) {
    if (!isNaN(input) && input !== null) {
        return Number(input) * 2;
    }
    else {
        return null;
    }
};

var testStr = function(input) {
    if (typeof(input) === 'string') {
        return input.slice(0);
    }
    if (typeof(input) === 'number' && !isNaN(input)) {
        return String(input).slice(0);   
    }
    else {
        return null;
    }
};

var assertAll = function(testType, testFunc, optional) {
    console.log('Running asserts...');
    var vals = [5, 'hi', '5', null, undefined, -5, NaN, [], ['hi'], {}];
    for (var i=0; i<vals.length; i++) {
        var result = testFunc(vals[i]);
        assert(testType === typeof(result) || result === optional, 'Input: ' +  String(vals[i]) + '. Result: ' + result);
    }
};

assertAll('number', testNum, null);
assertAll('string', testStr, null);

http://jsfiddle.net/xeus/WvDwY/