Front End Developer

Inline CSS or Javascript has bad impact on site performance.

Your HTML code will weigh more as you use inline scripts, whereas external scripts reduces HTML file size which helps fast rendering of webpage.

HTML code will never be cached so inline scripts. Contrary to that, external dependencies, such as CSS and JavaScript files, will be cached by the visitor's web browser. So it reduces https requests each time user click through web pages.

It is hard to maintain Inline CSS and Javascript code. Where having code in just one centralized location is a lot more preferable than changing exactly the same kind of code snippets spread all over the files in the web site.

JavaScript is read line by line. Sometimes, this can result in what seems like a subsequent line of code being executed prior to an earlier line of code. A callback function is used to prevent this from happening, because it is not called until the previous line of code has fully executed.

null is an object with no value. undefined is a type.

typeof null; // "object"

typeof undefined; // "undefined"

JavaScript variables have functional scope.

IIFE stands for immediately-invoked function expression; it executes immediately after created by adding a () after the function.

Lazy loading is a design pattern commonly used in computer programming to defer initialization of an object until the point at which it is needed.

Lazy loading is loading code only once user needs it. For Example, there is a button on the page, which shows different layout once user pressed it. So there is no need to load code for that layout on initial page load.

this and that are important to variable scope in JavaScript. Here are a few stackoverflow posts on this, that and self.