TypeScript is JavaScript with syntax for types.

JavaScript (also known as ECMAScript) started its life as a simple scripting language for browsers. At the time it was invented, it was expected to be used for short snippets of code embedded in a web page — writing more than a few dozen lines of code would have been somewhat unusual. Due to this, early web browsers executed such code pretty slowly. Over time, though, JS became more and more popular, and web developers started using it to create interactive experiences.

Web browser developers responded to this increased JS usage by optimizing their execution engines (dynamic compilation) and extending what could be done with it (adding APIs), which in turn made web developers use it even more. On modern websites, your browser is frequently running applications that span hundreds of thousands of lines of code. This is the long and gradual growth of “the web”, starting as a simple network of static pages, and evolving into a platform for rich applications of all kinds.

More than this, JS has become popular enough to be used outside the context of browsers, such as implementing JS servers using node.js. The “run anywhere” nature of JS makes it an attractive choice for cross-platform development. There are many developers these days that use only JavaScript to program their entire stack!

To summarize, we have a language that was designed for quick uses, and then grew to a full-fledged tool to write applications with millions of lines. Every language has its own quirks — oddities and surprises, and JavaScript’s humble beginning makes it have many of these. Some examples: