Custom React PropType validators that we use at Airbnb. Use of airbnb-js-shims or the equivalent is recommended.Since PropTypes are typically not included in production builds of React, this library’s functionality serves no useful purpose. As such, when the NODE_ENV environment variable is "production", instead of exporting the implementations of all these prop types, we export mock functions - in other words, something that ensures that no exceptions are thrown, but does no validation. When environment variables are inlined (via a browserify transform or webpack plugin), then tools like webpack or uglify are able to determine that only the mocks will be imported - and can avoid including the entire implementations in the final bundle that is sent to the browser. This allows for a much smaller ultimate file size, and faster in-browser performance, without sacrificing the benefits of PropTypes themselves.