🦉 Props Validation 🦉

As an application becomes complex, it may be quite unsafe to define props in an informal way. This leads to two issues:

• hard to tell how a component should be used, by looking at its code.
• unsafe, it is easy to send wrong props into a component, either by refactoring a component, or one of its parents.

A props type system solves both issues, by describing the types and shapes of the props. Here is how it works in Owl:

• props key is a static key (so, different from this.props in a component instance)
• it is optional: it is ok for a component to not define a props key.
• props are validated whenever a component is created/updated
• props are only validated in dev mode (see config page)
• if a key does not match the description, an error is thrown
• it validates keys defined in (static) props. Additional keys given by the parent will cause an error.

For example:

class ComponentA extends owl.Component {
    static props = ['id', 'url'];

    ...
}

class ComponentB extends owl.Component {
  static props = {
    count: {type: Number},
    messages: {
      type: Array,
      element: {type: Object, shape: {id: Boolean, text: String }
    },
   date: Date,
   combinedVal: [Number, Boolean]
  };

  ...
}

• it is an object or a list of strings
• a list of strings is a simplified props definition, which only lists the name of the props. Also, if the name ends with ?, it is considered optional.
• all props are by default required, unless they are defined with optional: true (in that case, validation is only done if there is a value)
• valid types are: Number, String, Boolean, Object, Array, Date, Function, and all constructor functions (so, if you have a Person class, it can be used as a type)
• arrays are homogeneous (all elements have the same type/shape)

For each key, a prop definition is either a boolean, a constructor, a list of constructors, or an object:

• a boolean: indicate that the props exists, and is mandatory.
• a constructor: this should describe the type, for example: id: Number describe the props id as a number
• a list of constructors. In that case, this means that we allow more than one type. For example, id: [Number, String] means that id can be either a string or a number.
• an object. This makes it possible to have more expressive definition. The following sub keys are then allowed (but not mandatory):
  • type: the main type of the prop being validated
  • element: if the type was Array, then the element key describes the type of each element in the array. If it is not set, then we only validate the array, not its elements,
  • shape: if the type was Object, then the shape key describes the interface of the object. If it is not set, then we only validate the object, not its elements,
  • validate: this is a function which should return a boolean to determine if the value is valid or not. Useful for custom validation logic.

Examples:

  // only the existence of those 3 keys is documented
  static props = ['message', 'id', 'date'];

  // size is optional
  static props = ['message', 'size?'];

  static props = {
    messageIds: {type: Array, element: Number},  // list of number
    otherArr: {type: Array},   // just array. no validation is made on sub elements
    otherArr2: Array,   // same as otherArr
    someObj: {type: Object},  // just an object, no internal validation
    someObj2: {
      type: Object,
      shape: {
        id: Number,
        name: {type: String, optional: true},
        url: String
      ]},    // object, with keys id (number), name (string, optional) and url (string)
    someFlag: Boolean,     // a boolean, mandatory (even if `false`)
    someVal: [Boolean, Date],   // either a boolean or a date
    otherValue: true,     // indicates that it is a prop
    kindofsmallnumber: {
      type: Number,
      validate: n => (0 <= n && n <= 10)
    },
    size: {
      validate:  e => ["small", "medium", "large"].includes(e)
    },
  };