notes/node_modules/postcss/docs/source-maps.md

PostCSS and Source Maps

PostCSS has great source maps support. It can read and interpret maps from previous transformation steps, autodetect the format that you expect, and output both external and inline maps.

To ensure that you generate an accurate source map, you must indicate the input and output CSS file paths — using the options from and to, respectively.

To generate a new source map with the default options, simply set map: true. This will generate an inline source map that contains the source content. If you don’t want the map inlined, you can set map.inline: false.

processor
    .process(css, {
        from: 'app.sass.css',
        to:   'app.css',
        map: { inline: false },
    })
    .then(function (result) {
        result.map //=> '{ "version":3,
                   //      "file":"app.css",
                   //      "sources":["app.sass"],
                   //       "mappings":"AAAA,KAAI" }'
    });

If PostCSS finds source maps from a previous transformation, it will automatically update that source map with the same options.

Options

If you want more control over source map generation, you can define the map option as an object with the following parameters:

• inline boolean: indicates that the source map should be embedded in the output CSS as a Base64-encoded comment. By default, it is true. But if all previous maps are external, not inline, PostCSS will not embed the map even if you do not set this option.

  If you have an inline source map, the result.map property will be empty, as the source map will be contained within the text of result.css.

• prev string, object, boolean or function: source map content from a previous processing step (for example, Sass compilation). PostCSS will try to read the previous source map automatically (based on comments within the source CSS), but you can use this option to identify it manually. If desired, you can omit the previous map with prev: false.

• sourcesContent boolean: indicates that PostCSS should set the origin content (for example, Sass source) of the source map. By default, it is true. But if all previous maps do not contain sources content, PostCSS will also leave it out even if you do not set this option.

• annotation boolean or string: indicates that PostCSS should add annotation comments to the CSS. By default, PostCSS will always add a comment with a path to the source map. PostCSS will not add annotations to CSS files that do not contain any comments.

  By default, PostCSS presumes that you want to save the source map as opts.to + '.map' and will use this path in the annotation comment. A different path can be set by providing a string value for annotation.

  If you have set inline: true, annotation cannot be disabled.

• from string: by default, PostCSS will set the sources property of the map to the value of the from option. If you want to override this behaviour, you can use map.from to explicitly set the source map's sources property.