mini-assert

// var assert = require('mini-assert'); // NPM, `npm install mini-assert`
// import assert from "mini-assert"; // JSPM, `jspm install npm:mini-assert`

var bool = Math.random()>0.5;

// use this function to bring to attention
// an unexpected code execution, i.e. a bug
assert(bool, "should not be <=0.5");

// use this function to bring to attention
// a wrong usage of an API
assert.expect(bool, "should be >0.5");

// use this function to bring to attention
// a warning, i.e. something non-critical
assert.warning(bool, "be careful; <= 0.5");

assert.onerror = function(){
    // track errors
};

the only differences between assert, assert.expect, and assert.warning are; - the error message is prepended with a different name: ASSERTION-FAIL: /*...*/, WRONG-USAGE: /*...*/, WARNING: /*...*/ - assert, assert.expect throw an error whereas assert.warning only does a console.log
If in the browser and if location.hostname!=='localhost' then no error is ever thrown and only console.log is used