ES6 In Depth: Arrow functions

Arrows

<script language="javascript">
<!--
  document.bgColor = "brown";  // red
// -->
</script>

Old browsers would see two unsupported tags and a comment; only new browsers would see JS code.

To support this odd hack, the JavaScript engine in your browser treats the characters <!-- as the start of a one-line comment. No joke. This has really been part of the language all along, and it works to this day, not just at the top of an inline <script> but everywhere in JS code. It even works in Node.

As it happens, this style of comment is standardized for the first time in ES6.

The arrow sequence --> also denotes a one-line comment. Weirdly, while in HTML characters before the --> are part of the comment, in JS the rest of the line after the --> is a comment.

It gets stranger. This arrow indicates a comment only when it appears at the start of a line. That’s because in other contexts, --> is an operator in JS, the goes to operator!

function countdown(n) {
  while (n --> 0)  // "n goes to zero"
    alert(n);
  blastoff();
}

A new arrow in your quiver

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Using arrows to pierce the dark heart of computer science

原文：ES6 In Depth: Arrow functions