JSON: why are forward slashes escaped?

The JSON spec says you CAN escape forward slash, but you don't have to.

I asked the same question some time ago and had to answer it myself. Here's what I came up with:

It seems, my first thought [that it comes from its JavaScript roots] was correct.

'\/' === '/' in JavaScript, and JSON is valid JavaScript. However, why are the other ignored escapes (like \z) not allowed in JSON?

The key for this was reading http://www.cs.tut.fi/~jkorpela/www/revsol.html, followed by http://www.w3.org/TR/html4/appendix/notes.html#h-B.3.2. The feature of the slash escape allows JSON to be embedded in HTML (as SGML) and XML.

JSON doesn't require you to do that, it allows you to do that. It also allows you to use "\u0061" for "A", but it's not required, like Harold L points out:

The JSON spec says you CAN escape forward slash, but you don't have to.

Harold L answered Oct 16 '09 at 21:59

Allowing \/ helps when embedding JSON in a <script> tag, which doesn't allow </ inside strings, like Seb points out:

This is because HTML does not allow a string inside a <script> tag to contain </, so in case that substring's there, you should escape every forward slash.

Seb answered Oct 16 '09 at 22:00 (#1580667)

Some of Microsoft's ASP.NET Ajax/JSON API's use this loophole to add extra information, e.g., a datetime will be sent as "\/Date(milliseconds)\/". (Yuck)