This article will help you with debugging modern Web aps. ‘Modern’ here specifically means apps where source code undergoes a major transformation before execution, e.g. you are using Babel to enable features from ES2015-ES2018, TypeScript, and/or frameworks like Mobx that rely on transformations like decorators and transpile down to ES5. Because of the above, code that is actually executed by JS runtime is very different from what you see in your IDE.

You may be thinking, ‘but we’ve got source maps !’ While source maps are good, transforming ES2015-ES2018 features into ES5 changes code too dramatically, a single source line often becomes 3 or more during execution, this becomes _this, variable names change due to transpilation of let and const into var, etc. All of these changes really mess with debugging features like step over or examining variable values by their name. Debugging it can be a pain, but it does not have to.

JavaScript is among the most open-source friendly languages. Thus, when faced with a question “why doesn’t it call this function?” or “What changes this property?”, most of the time we have the code of 3rd-party libraries ready and available for us to dive into for debug. Still, sometimes you only have the minified version of a library, or a normal version so big and complicated - you give up after pressing “step into method” over 30 times. You need to track the exact place where one of your objects properties is used.

Solution? Use Chrome and the most awesome feature of ES6 – Proxies! Your users are locked into IE 8 and no ES6? What’s wrong with your organization? Ok, but you as a developer, surely can use Chrome, just for this debug session? Proxies are supported by all modern browser releases for at least a year , and current versions of Node.