D = (obj, prop, value) => {
	if (obj[prop] === undefined) {
		obj[prop] = value;
	}
}
Sets a constant default value when undefined
@template T
@template {keyof T} P
@param {T} obj an object
@param {P} prop a property of this object
@param {T[P]} value a default value of the property
@returns {void}