type-check

Syntax

White space is ignored. The root node is a Types.

• Identifier = [\$\w]+ - a group of any lower or upper case letters, numbers, underscores, or dollar signs - eg. String
• Type = an Identifier, an Identifier followed by a Structure, just a Structure, or a wildcard * - eg. String, Object{x: Number}, {x: Number}, Array{0: String, 1: Boolean, length: Number}, *
• Types = optionally a comment (an Identifier followed by a ::), optionally the identifier Maybe, one or more Type, separated by | - eg. Number, String | Date, Maybe Number, Maybe Boolean | String
• Structure = Fields, or a Tuple, or an Array - eg. {x: Number}, (String, Number), [Date]
• Fields = a {, followed one or more Field separated by a comma , (trailing comma , is permitted), optionally an ... (always preceded by a comma ,), followed by a } - eg. {x: Number, y: String}, {k: Function, ...}
• Field = an Identifier, followed by a colon :, followed by Types - eg. x: Date | String, y: Boolean
• Tuple = a (, followed by one or more Types separated by a comma , (trailing comma , is permitted), followed by a ) - eg (Date), (Number, Date)
• Array = a [ followed by exactly one Types followed by a ] - eg. [Boolean], [Boolean | Null]

Guide

type-check uses Object.toString to find out the basic type of a value. Specifically,

{}.toString.call(VALUE).slice(8, -1)
{}.toString.call(true).slice(8, -1) // 'Boolean'

A basic type, eg. Number, uses this check. This is much more versatile than using typeof - for example, with document, typeof produces 'object' which isn't that useful, and our technique produces 'HTMLDocument'.

You may check for multiple types by separating types with a |. The checker proceeds from left to right, and passes if the value is any of the types - eg. String | Boolean first checks if the value is a string, and then if it is a boolean. If it is none of those, then it returns false.

Adding a Maybe in front of a list of multiple types is the same as also checking for Null and Undefined - eg. Maybe String is equivalent to Undefined | Null | String.

You may add a comment to remind you of what the type is for by following an identifier with a :: before a type (or multiple types). The comment is simply thrown out.

The wildcard * matches all types.

There are three types of structures for checking the contents of a value: 'fields', 'tuple', and 'array'.

If used by itself, a 'fields' structure will pass with any type of object as long as it is an instance of Object and the properties pass - this allows for duck typing - eg. {x: Boolean}.

To check if the properties pass, and the value is of a certain type, you can specify the type - eg. Error{message: String}.

If you want to make a field optional, you can simply use Maybe - eg. {x: Boolean, y: Maybe String} will still pass if y is undefined (or null).

If you don't care if the value has properties beyond what you have specified, you can use the 'etc' operator ... - eg. {x: Boolean, ...} will match an object with an x property that is a boolean, and with zero or more other properties.

For an array, you must specify one or more types (separated by |) - it will pass for something of any length as long as each element passes the types provided - eg. [Number], [Number | String].

A tuple checks for a fixed number of elements, each of a potentially different type. Each element is separated by a comma - eg. (String, Number).

An array and tuple structure check that the value is of type Array by default, but if another type is specified, they will check for that instead - eg. Int32Array[Number]. You can use the wildcard * to search for any type at all.

Example:

var options = {
  customTypes: {
    Even: {
      typeOf: 'Number',
      validate: function(x) {
        return x % 2 === 0;
      }
    }
  }
};
typeCheck('Even', 2, options); // true
typeCheck('Even', 3, options); // false

customTypes allows you to set up custom types for validation. The value of this is an object. The keys of the object are the types you will be matching. Each value of the object will be an object having a typeOf property - a string, and validate property - a function.

The typeOf property is the type the value should be (optional - if not set only validate will be used), and validate is a function which should return true if the value is of that type. validate receives one parameter, which is the value that we are checking.

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