From Quora: https://www.quora.com/What-is-the-most-elegant-piece-of-JavaScript-code-you-can-write
I actually have two candidates for the title of “most powerful line of code ever written in JavaScript”. The first is this line:
1 | element.innerHTML = "..."; |
The innerHTML property was introduced by Microsoft in the Internet Explorer 4.0 browser (IE4), which was released in October 1997. Prior to this, the only way to modify the HTML content of a page from JavaScript was using the document.write method. Unfortunately, calling document.write on a loaded document automatically calls document.open, which clears the entire document. In other words, it was only with the introduction of innerHTML that JavaScript gained the ability to modify a specific portion of a page, by inserting a bit of arbitrary HTML inside an existing element. And thus, modern Web applications were born.
My other candidate is this line:
1 | var xhr = new XMLHttpRequest(); |
Prior to this, content could only be downloaded into the browser by requesting a whole new page. Developers resorted to all sorts of tricks, primarily using hidden frames, to circumvent this limitation, but such code was very unwieldy, and had problematic side-effects. It was Microsoft again that changed the game by introducing the XMLHttpRequest object in Internet Explorer 5.0 (IE5) in March 1999. And just like that, AJAX was created, and along with it Single-Page Applications, and the modern Web we know today.