The JSON files should be formatted according to the .editorconfig file.
All data in the repo must conform to the schema. The formal feature data schema is defined in compat-data.schema.json; see compat-data-schema.md for more info. The browser data schema is defined in browsers.schema.json; see browsers-schema.md for more info.
You can use npm test to validate data against the schema. You might need to install the devDependencies using npm install.
The JSON data is validated against the schema using ajv.
To see how changes will affect the statistics of real (either false or a version number, as defined in #3555), true, and null values, you can run npm run stats [folder]. This generates a Markdown-formatted table of the percentages of real, true, and null values for the eight primary browsers that browser-compat-data is focusing on. The script also takes an optional argument regarding a specific folder (such as api or javascript), which will print statistics result for only that folder.
To find all the entries that are non-real, or of a specified value, you can run npm run traverse <browser> [folder] [value]. The browser may be any single browser defined in the browsers/ folder. The folder may be omitted to search through all data folders, or a comma-separated list of folders to search through. The value may be omitted to search for all non-real values (or more specifically, true and null values), or any value accepted by version_added and version_removed. For example, to search for all Safari entries that are non-real, run npm run traverse safari. To search for all WebView entries that are marked as true in api and javascript, run npm run traverse webview_android api,javascript true.