I have a similar but opposite requirement. I want to allow null but not allow undefined.

facebook/react#3163 (comment)

I was thinking of adding isDefined to complement isRequired

airbnb-prop-types has explicitNull for this use case.

@ljharb Perhaps I'm missing something, but explicitNull requires null. In my case I disallow null and allow undefined only when not required. Currently prop-types allows null or undefined when not required. What we're trying to achieve is choosing one or the other, not both.

@lukescott ah, true. explicitNull does indeed require null - if you'd file an issue on airbnb-prop-types, i can add the ability to disallow it :-)

(Forbidding undefined is the same as "requiring null" or "requiring some other value", or the composition of those two, so it really doesn't need its own API imo)

@ljharb This was the best stand-alone work-around I could come up with:

// Usage: notNull(PropType.string)

if (process.env.NODE_ENV !== "production") {
	var secret = require("prop-types/lib/ReactPropTypesSecret")
}

function notNull(validator) {
	if (process.env.NODE_ENV !== "production") {
		if (!validator.isRequired) {
			return validator
		}
		return (props, propName, componentName, location, propFullName) => {
			const propValue = props[propName]
			if (propValue === null) {
				return new Error(
					"The " + location +
					" `" + propFullName +
					"` in `" + componentName +
					"` must not be `null`."
				)
			}
			return validator(
				props, propName, componentName, location, propFullName, secret
			)
		}
	}
}

module.exports = function noNulls(types) {
	if (process.env.NODE_ENV !== "production") {
		const newTypes = {}
		Object.keys(types).forEach(key => {
			newTypes[key] = notNull(types[key])
		})
		return newTypes
	} else {
		return types
	}
}

The goal of the PR was to get something built-in though. Especially since my work-around relies on ReactPropTypesSecret.

That's never required - change it to return (props, propName, componentName, location, propFullName, ...rest) => { and props, propName, componentName, location, propFullName, ...rest.