intertype

InterType

A JavaScript type checker with helpers to implement own types and do object shape validation.

Declaring New Types

intertype.declare() allows to add new type specifications to intertype.specs. It may be called with one to three arguments. The three argument types are:

• type is the name of the new type. It is often customary to call intertype.declare 'mytype', { ... }, but it is also possible to name the type within the spec and forego the first argument, as in intertype.declare { type: 'mytype', ... }.

• spec is an object that describes the type. It is essentially what will end up in intertype.specs, but it will get copied and possibly rewritten in the process, depending on its content and the other arguments. The spec object may have a property type that names the type to be added, and a property tests which, where present, must be an object with one or more (duh) tests. It is customary but not obligatory to name a single test 'main'. In any event, the ordering in which tests are executed is the ordering of the properties of spec.tests (which corresponds to the ordering in which those tests got attached to spec.tests). The spec may also have further attributes, for which see below.

• test is an optional boolean function that accepts one or more arguments (a value x to be tested and any number of additional parameters P where applicable; together these are symbolized as xP) and returns whether its arguments satisfy a certain condition. The test argument, where present, will be registered as the 'main' (and only) test for the new type, spec.tests.main. The rule of thumb is that when one wants to declare a type that can be characterized by a single, concise test, then giving a single anonymous one-liner (typically an arrow function) is OK; conversely, when a complex type (think: structured objects) needs a number of tests, then it will be better to write a suite of named tests (most of them typically one-liners) and pass them in as properties of spec.tests.

The call signatures are:

• intertype.declare spec—In this form, spec must have a type property that names the new type, as well as a tests property.

• intertype.declare type, spec—This form works like the above, except that, if spec.type is set, it must equal the type argument. It is primarily implemented for syntactical reasons (see examples).

• intertype.declare type, test—This form is handy for declaring types without any further details: you just name it, define a test, done. For example, to declare a type for positive numbers: @declare 'positive', ( x ) => ( @isa.number x ) and ( x > 0 ). Also see the next.

• intertype.declare type, spec, test—This form is handy for declaring types with a minimal set of details and a short test. For example, to define a type for NodeJS buffers: @declare 'buffer', { size: 'length', }, ( x ) => Buffer.isBuffer x (here, the size spec defines how InterType's size_of() method should deal with buffers).