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1.2.4 • Public • Published



Licensed under the MIT license


resourcetiming-compression.js compresses data from ResourceTiming. A companion script, resourcetiming-decompression.js, converts the compressed data back to the original form.

ResourceTiming is a W3C web-perf API that exposes all of the page's resources' network timing information to the developer and is available in most modern browsers. The interface performance.getEntriesByType('resource') returns a list of resources with information about each resource's URL, why it was downloaded, and a dozen timestamps. Collecting this information is easy, but beaconing all of this data back to a data warehouse can be a challenge because of the amount of data available for each resource. On a typical page, which might have over 100 resources, you could easily see 50 KB of ResourceTiming data per page-load.

resourcetiming-compression.js applies several data-compression techniques to reduce the size of your serialized ResourceTiming data to about 15% of it's original size in many cases. See this blog post for a description of these techniques.

resourcetiming-decompression.js is a companion script that will take the compressed ResourceTiming data and builds it back to its original ResourceTiming form (eg. performance.getEntriesByType('resource')) for analysis.

NOTE: resourcetiming-compression.js is the same code that drives the restiming.js plugin for Boomerang, but also includes the resourcetiming-decompression.js component.


Releases are available for download from GitHub.

Development: resourcetiming-compression.js - 30kb

Production: resourcetiming-compression.min.js - 2.4kb (minified / gzipped)

Development: resourcetiming-decompression.js - 8.8kb

Production: resourcetiming-decompression.min.js - 1kb (minified / gzipped)

resourcetiming-compression.js is also available as the npm resourcetiming-compression module. You can install using Node Package Manager (npm):

npm install resourcetiming-compression

resourcetiming-compression.js is also available via bower. You can install using:

bower install resourcetiming-compression


Please see the W3C ResourceTiming API Reference for details on how to use the ResourceTiming API.


To include resourcetiming-compression.js, simply include it via a script tag:

<script type="text/javascript" src="resourcetiming-compression.min.js"></script>

Once included in the page, a top-level ResourceTimingCompression object is available on window. If AMD or CommonJS environments are detected, it will simply expose itself via those methods.

From the NPM module:

var ResourceTimingCompression = require("resourcetiming-compression").ResourceTimingCompression;

To get all of the compressed resources, you can simply call:

var {restiming, servertiming} = ResourceTimingCompression.getResourceTiming();


To include resourcetiming-decompression.js, simply include it via a script tag:

<script type="text/javascript" src="resourcetiming-decompression.min.js"></script>

Once included in the page, a top-level ResourceTimingDecompression object is available on window. If AMD or CommonJS environments are detected, it will simply expose itself via those methods.

From the NPM module:

var ResourceTimingDecompression = require("resourcetiming-compression").ResourceTimingDecompression;

To decompress your resources, you can simply call:

var original = ResourceTimingDecompression.decompressResources(restiming, servertiming);

Resource Dimensions

If available, when compressing the resources via compressResourceTiming(), any resource that has a visible component on the page (such as an IMG element) will have its height width top and left values captured and included in the compressed data as well.

This information is encoded as a "special value" in the resource's timings array.

For each resource, multiple hits to the same URL are separated by a pipe (|) character:

  // this resource was loaded twice with timings 70,1z,1c and 90,1,2
  "": "370,1z,1c|390,1,2"

(See the blog post for a description of how to interpret the values)

If the resource has visible elements on the page, they will be appended to this list of timings with a special prefix of *0 (Base36 encoded):

  // this resource was loaded twice with timings 70,1z,1c and 90,1,2 and had
  // dimensions of height = 1, width = 5, top = 10 and left = 11
  "": "370,1z,1c|390,1,2|*01,5,a,b"

Resource Sizes

If available via ResourceTiming2, when compressing the resources via compressResourceTiming(), the resource's transferSize, encodedBodySize and decodedBodySize will be captured and included in the compressed data as well.

This information is encoded as a "special value" in the resource's timings array. They will be appended to the list of timings with a special prefix of *1:

The data will be stored in the order of: [e, t, d].

  • e: encodedBodySize is the Base36 decoded value (encodedBodySize = parseInt(e, 36))
  • t:
    • If a Base36 encoded number, t is the transferSize increase in size over encodedBodySize (transferSize = parseInt(e, 36) + parseInt(t, 36))
    • If the value of "_", transferSize is 0
    • If missing, transferSize was either 0 or undefined
  • d:
    • If a Base36 encoded number, d is the decodedBodySize increase in size over encodedBodySize (decodedBodySize = parseInt(e, 36) + parseInt(d, 36))
    • If 0, encodedBodySize is 0
    • If missing, encodedBodySize was either 0 or undefined

Taking the following example:

  // this resource was loaded twice with timings 70,1z,1c and 90,1,2 and had
  // transferSize
  "": "370,1z,1c|390,1,2|*1a,b,c"

Results in:

  • encodedBodySize = parseInt("a", 36) = 10
  • transferSize = parseInt("a", 36) + parseInt("b", 36) = 21
  • decodedBodySize = parseInt("a", 36) + parseInt("c", 36) = 22

Resource Contributions

We call contribution of a resource to a page the proportion of the total load time that can be blamed on that resource. We want contribution scores to encourage parallelization and not only short resources.

It enables us to study resource impact in a more meaningful way over simply looking at raw load times.

Here is an example with only 2 resources to get some intuition into how it works:

    0                                        100ms
A   |-----------------------------------------|

                      50ms                   100ms
B                      |----------------------|

The contribution of resource A is 50ms for the 0-50ms time range and 50ms/2 for the 50ms to 100ms time range because it overlaps with B. We get:

contribution_A = (50 + 50 / 2) / 100 = .75

contribution_B = (50 / 2) / 100 = .25

This is computed based on all the resources in a single beacon. It is not done by default because it is computationally expensive.

Here is a code example to add contributions to your own resources:

var original = ResourceTimingDecompression.decompressResources(rtData);
// "original" is now an array of resource timings.
// Resources in "original" now have a field "contribution".

Server Timing

Server Timing entries are included on Resource- and NavigationTiming entries as serverTiming. They must have a name, might have a non-empty description, and will likely have a non-zero duration. This compression is build on the presumption that resources will have server timing entries with unique durations pointing mostly to the same name and descriptions. There are two parts to this compression:

  1. A "lookup" data structure containing all of the unique name and description pairs (an array of arrays, sorted with most-common first)
  2. For each resource timing entry, a list of duration and key pairs, where duration is the duration of the server timing entry and the key maps to the name and description in (1)

Take the following example:

performance.getEntriesByName(<path/to/resource1>)[0].serverTiming === [{
  name: 'm1',
  duration: 1,
  description: 'desc1'
}, {
  name: 'm2',
  duration: 2,
  description: 'desc3'
performance.getEntriesByName(<path/to/resource2>)[0].serverTiming === [{
  name: 'm1',
  duration: 3,
  description: 'desc1'
}, {
  name: 'm1',
  duration: 4,
  description: 'desc2'
  • getResourceTiming() will return a servertiming "lookup" will all of the unique pairs of name and description, equal to:

    [[m1, desc1, desc2], [m2, desc3]]

  • We supplement the compressed resource timing data with comma-separated list of the form: <duration>:<entryIndex>.<descriptionIndex>.

    • For "resource1", we add: 1:0.0,2:1.0
    • For "resource2", we add: 3:0.0,4:0.1
  • To save bytes, we will omit the zeroes, and irrelevant separators. So, from our example:

    • For "resource1", we add: 1,2:1
    • For "resource2", we add: 3,4:.1
  • And lastly, were there only one description for a given name and it was empty-string, then we simplify that array entry:

    [["description1", ""], ...] would become ["description1", ...]


resourcetiming-compression.js tests

Tests are provided in the test/ directory, and can be run via mocha:

mocha test/*

Or via gulp:

gulp test

Version History

  • v1.2.4 - 2019-07-16
    • Optional skipDimensions for getResourceTiming() and compressResourceTiming()
  • v1.2.3 - 2019-05-09
    • Decode initiatorType above 9 properly
  • v1.2.2 - 2019-05-09
    • Add additional initiatorType values
  • v1.2.1 - 2018-11-26:
    • Add root-level compression and decompression exports
  • v1.2.0 - 2018-11-26:
    • ResourceTimingCompression.getVisibleEntries: Fixed onload bug, updated to work with <PICTURE> elements
    • ResourceTimingCompression: Dimensions work with HOSTNAMES_REVERSED
    • Tests: Added getResourceTiming() tests
  • v1.1.0 - 2018-09-11:
    • Decompresses properly when multiple special entries exist
    • Compress and decompress LINK rel and SCRIPT attributes
    • Decompress namespaced data
    • Adds new INITIATOR_TYPES
  • v1.0.2 - 2018-07-06:
    • Fixed decompression when multiple special entries exist
  • v1.0.1 - 2018-04-13:
    • Make hostname reversal configurable (#20)
    • Add naturalHeight and naturalWidth to dimensionData (#20)
  • v1.0.0 - 2017-08-29:
    • Breaking Changes:
      • Reverses hostnames in Trie entries ([#10]) for better compression
      • getResourceTiming() now returns an object of { restiming, servertiming } instead of just restiming (#17)
    • New Features:
      • Adds ServerTiming data if available (#17)
      • Adds new initiator types (#15): beacon, fetch
      • Resource contribution scores (#16)
      • Adds async, body and defer flags for SCRIPT types (#16)
      • Adds naturalHeight and naturalWidth for dimensions (#16)
    • Bug Fixes:
      • Fixes max IFRAME recursion depth of 10 (#9)
      • Fixes compression algorithm for gzipped zero-byte payloads (#14)
  • v0.3.4 - 2016-10-20:
    • Better src attribute capture and HREF handling (3796c2ae)
    • Look at rect.height|width for dimensions (3796c2ae)
  • v0.3.3 - 2016-10-20:
    • Handle SVG:image elements (0177ee6e)
  • v0.3.2 - 2016-10-20:
    • Decodes resource dimensions (#6)
  • v0.3.1 - 2016-07-15:
    • Fixed capturing of resource sizes (bytes) (#4)
  • v0.3.0 - 2016-07-11:
    • Captures dimensions (px) of resources (d54d5be4)
    • Captures resource sizes (bytes) from ResourceTiming2 (d54d5be4)
    • Breaks certain URLs up slightly so they don't trigger XSS filters (d54d5be4)
    • Limits URLs to 500 characters, and adds the ability to trim other URLs (d54d5be4)
    • Don't go more than 10 IFRAMEs deep (to avoid recursion bugs) (d54d5be4)
    • Fixes browser bugs with incorrect timings (d54d5be4)
  • v0.2.2 - 2016-06-01:
    • Add 'html' initiatorType for root page (91a91404)
  • v0.2.1 - 2016-04-04:
    • Protect against X-O frame access that crashes some browsers (f48c1915)
  • v0.2.0 - 2015-11-23:
    • Adds a CLI (#2)
    • Export both ResourceTimingCompression and ResourceTimingDecompression from main module (567682b7)
  • v0.1.2 - 2015-02-25:
    • Fixed initiatorType parsing (#1)
  • v0.1.1 - 2015-02-13:
    • Fixed how redirectStart and fetchStart are calculated (567682b7)
  • v0.1.0 - 2014-10-17:
    • Initial version




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