HAR file format: Log and pages

HAR 1.2, edited by Jan Odvarko, was taken up as a W3C Web Performance Working Group draft (August 2012). The W3C never published it as a standard and the draft now says “DO NOT USE … abandoned”; it remains the published text of HAR 1.2, and this page follows it as served at w3c.github.io/web-performance/specs/HAR/Overview.html. This page covers the file as a whole and its top-level objects; requests and responses are on the entries page.

General rules

A HAR file is a JSON document (RFC 4627). It must be UTF-8; no other encoding is allowed. Writers may put a byte order mark at the start and readers must ignore one.
The document is an object with a single member, log.
Times of day are ISO 8601 strings with a time zone, such as 2009-07-24T19:20:30.45+01:00. Durations are numbers of milliseconds. Where a duration or size does not apply or is not known, the draft uses −1.
Every object in the format may have an optional comment string (new in 1.2 for most objects).

Versions and custom fields

log.version is major.minor. A change that older readers can still handle raises the minor number; a change that breaks them raises the major number. The fields added in 1.2 over 1.1 are all optional: the comments on pages, page timings, entries, requests, responses, cookies, post data, parameters, content, cache and timings, and serverIPAddress, connection, cookies[].secure, content.encoding and timings.ssl.

A program may add its own fields; their names must start with an underscore, and no standard field ever does. Readers ignore custom fields they did not write, and ignore unknown standard fields in a file whose minor version is newer than they know.

log

Field	Type	Required	Meaning
`version`	string	yes	Version number of the format, such as `1.2`.
`creator`	object	yes	The program that wrote the file; see creator and browser.
`browser`	object	no	The browser the traffic came from.
`pages`	array	no	The pages loaded, in page objects. Left out when the program does not group requests by page.
`entries`	array	yes	One entry per HTTP request. Writers should sort them by `startedDateTime`, oldest first; readers should not rely on the order.
`comment`	string	no	A comment from the user or the program.

creator and browser

Both objects have the same fields.

Field	Type	Required	Meaning
`name`	string	yes	The program’s or browser’s name.
`version`	string	yes	Its version.
`comment`	string	no	A comment.

pages

Each item of log.pages:

Field	Type	Required	Meaning
`startedDateTime`	string	yes	When the page started loading, ISO 8601.
`id`	string	yes	A name for the page, unique in the file. Entries refer to it in `pageref`.
`title`	string	yes	The page’s title.
`pageTimings`	object	yes	Page load times; see below.
`comment`	string	no	A comment (new in 1.2).

pageTimings

Field	Type	Required	Meaning
`onContentLoad`	number	no	Milliseconds from the page’s start until its content was loaded (the `DOMContentLoaded` event or `readyState` “interactive”, depending on the browser); −1 when it does not apply.
`onLoad`	number	no	Milliseconds from the page’s start until the `load` event; −1 when it does not apply.
`comment`	string	no	A comment (new in 1.2).

The draft does not say in words that the first four fields of a page are required; it simply does not mark them optional, which is how it shows required fields throughout.

Example

{
  "log": {
    "version": "1.2",
    "creator": { "name": "WebInspector", "version": "537.36" },
    "pages": [
      {
        "startedDateTime": "2023-11-14T22:13:20.123Z",
        "id": "page_1",
        "title": "https://example.com/",
        "pageTimings": { "onContentLoad": 412.5, "onLoad": 980.2 }
      }
    ],
    "entries": [ … ]
  }
}

The top of a HAR file, with one page.

.HAR File Format – Log and pages