Source map - Content

A source map is a {{Glossary("JSON")}} file format that maps between minified or transformed code received by the browser and its original unmodified form, allowing the original code to be reconstructed and used when debugging.

Code executed by the browser is often transformed in some way from the original source created by a developer. There are several reasons for this:

To make delivering code from the server more efficient by combining and minifying source files.
To support older browsers by transforming modern features into older equivalents.
To use languages that browsers don't support, like {{Glossary("TypeScript")}} or Sass.

In these situations, debugging the original source is more intuitive than the source in the transformed state that the browser has downloaded. Browsers detect a source map via the {{HTTPHeader("SourceMap")}} HTTP header for a resource, or a sourceMappingURL annotation in the generated code.

Example

For example, consider this SCSS syntax of Sass:

scss

ul {
  list-style: none;
  li {
    display: inline;
  }
}

During the build process, the SCSS is transformed into CSS. A source map file index.css.map is generated and linked to from the CSS in a comment at the end:

css

ul {
  list-style: none;
}
ul li {
  display: inline;
}

/*# sourceMappingURL=index.css.map */

This map file contains not only mappings between the original SCSS and the generated CSS but also the original SCSS source code in encoded form. It's ignored by the browser's CSS parser but used by browser's DevTools:

json

{
  "version": 3,
  "sourceRoot": "",
  "sources": ["index.scss"],
  "names": [],
  "mappings": "AAAA;EACC;;AACA;EACC",
  "file": "index.css"
}

The source map allows the browser's DevTools to link to specific lines in the original SCSS file and display the source code: