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YSlow Command Line

YSlow analyzes web pages and why they're slow based on Yahoo!'s rules for high performance web sites

YSlow for Command Line runs on Node.JS and requires a HAR file as input source in order to analyze page performance. It is currently available as a NPM package for installation.


$ npm install yslow -g


$ yslow --help
  Usage: yslow [options] [file ...]


    -h, --help               output usage information
    -V, --version            output the version number
    -i, --info <info>        specify the information to display/log (basic|grade|stats|comps|all) [basic]
    -f, --format <format>    specify the output results format (json|xml|plain) [json]
    -r, --ruleset <ruleset>  specify the YSlow performance ruleset to be used (ydefault|yslow1|yblog) [ydefault]
    -b, --beacon <url>       specify an URL to log the results
    -d, --dict               include dictionary of results fields
    -v, --verbose            output beacon response information


    yslow file.har
    yslow -i grade -f xml -b file1.har file2.har
    yslow --info all --format plain /tmp/*.har
    yslow -i basic --rulseset yslow1 -d < file.har
    curl | yslow -i grade -b -v

  More Info:

As a module

$ node
> require('fs').readFile('', function (err, data) {
    var har = JSON.parse(data),
        YSLOW = require('yslow').YSLOW,
        doc = require('jsdom').jsdom(),
        res =, har, 'ydefault'),
        content = YSLOW.util.getResults(res.context, 'basic');

{ w: 98725, o: 89, u: '', r: 9, i: 'ydefault', lt: 981 }