[Feature]: Create Schema for JSON and YAML Config Files

🚀 Feature Proposal

Write a draft v4 JSON schema defining Jest config files for inclusion in JSON SchemaStore.

Motivation

Jest config files are powerful, complex, and offer many options. A number of them (e.g., coverageReporters, coverageThreshold, moduleNameMapper, reporters, transform) have complex types, and a number (e.g., *PathIgnorePatterns) have long names.

Example

Offering a schema makes it easy for projects that desire to validate their config files to do so with minimal effort and receive precise, helpful, real-time in-editor warnings and autocomplete suggestions regarding typos in key names. A schema would also make it easy to validate Jest config files in CI. JSON Schema Store recommends use of draft v4 JSON schemas for maximum compatibility.

Pitch

"Quis custodiet ipsos custodes?" ("Who will guard the guards themselves?") ~ Satires (Satire IV lines 347-348), Decimus Junius Juvenalis (a.k.a., Juvenal)

Or as retold via Gru from Despicable Me:

I don't know whether or not a schema would catch any issues that Jest itself wouldn't catch when run, but it's generally preferable to catch issues as they are introduced, and auto-completion of keys is an appreciated productivity boost. A JSON schema is extremely precise documentation, leaving no ambiguity as to what constitutes a valid JSON/YAML config file. Using a JSON schema may simplify Jest's validation logic, facilitate automatically testing whether the JSON/YAML and JavaScript/TypeScript config file options are in sync with one another, and allow documentation to be auto-generated if these things aren't happening already. In the meantime, users who want stronger type checking and auto-completion can presently use a TypeScript file. However, dynamic code execution introduces another possible point of failure, and TypeScript config files require some extra setup (e.g., introducing a dev dependency on ts-node, which can be especially burdensome in high-security settings). Many projects also prefer to use static JSON or YAML config files for simplicity.