JSON is a lightweight data interchange format based on a subset of JavaScript. You can read all about JSON at json.org. json.el is a JSON parser and generator for Emacs Lisp, which can produce an Emacs Lisp data structure from a JSON object and vice-versa. It’s been Included with Emacs since February 2008.

Using it is pretty straightforward; here are some examples.

* (require 'json)

JSON's primitive values are strings, numbers, and the keywords true, false, and null.

* (json-read-from-string "true")
* (json-encode t)
* (json-read-from-string "4.5")
* (json-read-from-string "\\"foo\\"")

JSON's compound values are arrays and dictionaries.

* (json-read-from-string "[true, 4.5]")
[t 4.5]
* (json-read-from-string "{\\"foo\\": true}")
((foo . t))

Notice that we read the JSON array as a lisp vector and the JSON dictionary as an alist. We could just have read the array as a list, and the dictionary as a plist or hashtable. json.el allows for all of these representations. (Also, note that the alist keys are symbols; we could read these as keywords or strings.)

  • dictionary-as-plist:
    * (let ((json-object-type 'plist))
        (json-read-from-string "{\\"foo\\": true}"))
    (:foo t)
  • key-as-string:
    * (let ((json-key-type 'string))
        (json-read-from-string "{\\"foo\\": true}"))
    (("foo" . t))
  • dictionary-as-hashtable:
    * (let ((json-object-type 'hash-table))
        (json-read-from-string "{\\"foo\\": true}"))
    #<hash-table 'equal nil 1/65 0x314f800>
    * (gethash "foo" *)

json.el generally does the right thing when encoding idiomatic lisp data structures:

* (json-encode '(1 2 3))
"[1, 2, 3]"
* (json-encode '(:foo 1 :bar 2 :baz 3))
"{\\"foo\\":1, \\"bar\\":2, \\"baz\\":3}"


  1. Brian Palmer

    Just curious, did you develop this for any reason in particular? I've been looking at json lately for a project I'm going to try to ajaxify...

    Brian Palmer, 1 April 2006