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JSig Specification

JSig is language to express the types of a JavaScript program.

Jsig types and subtypes

To be able to talk about how JavaScript values and JSig types are related we will introduce some concepts.

A value is of a type

A JavaScript value v is of a JSig type T.

To check whether v is of type T we first infer the Jsig type expression S for v. Then if S is a subtype of T we know that v is of type T

JSig subtypes

A Jsig type expression S can be a subtype of a JSig type expression T

TODO: define subtype semantics

Statements

A JSig file contains a series of statements.

You can also use a statement in a comment in a piece of code.

Assignment statement

someIdentifier : {{TypeExpression}}

An assignment statement is an identifier and a type expression seperated by a : symbol.

An identifier is a reference to some concrete value in JavaScript.

There are multiple ways that identifiers can be resolved to a value in JavaScript, these will be defined later.

The concrete value v that the identifier references to must satisfy the {{TypeExpression}}

Examples:

  • add : (Number, Number) => void
  • myApp : { state: T, render: (T) => DOMElement }
  • HttpServer : { listen: (Number) => void }

Type declaration

type SomeCustomTypeName : {{TypeExpression}}

A type declaration statement is the type keyword, followed by a custom type name followed by the : symbol and followed by a type expression

The custom type name can then be used in the rest of the file to reference to the {{TypeExpression}}

It is also valid for another file to import the SomeCustomTypeName type literal into their file. i.e. all custom types are exported by default.

Example:

  • type Coord : Number
  • type Pos : { x: Number, y: Number }
  • type HttpRequest : { url: String, method: String }

Generic type declaration

TODO

Import statement

import { A, B, ... } from "./other-file.jsig"

An import statement is an import keyword followed by a comma seperated list of type names in curly braces, followed by a from keyword and a file name.

An import statement allows you to import one or more custom types defined in another file.

For the import to be valid the other file should have defined A and B as custom types.

Example:

  • import { AThing } from "./other-file.jsig"
  • import { Callback, EventEmitter } from "types-node.jsig"
  • import { AThing, BThing } from "./other-file.jsig"
  • import { AThing as BThing } from "./other-file.jsig"

Type expression

A type expression is one of the following

  • A builtin type
  • A type literal
  • An object literal
  • A function expression
  • A tuple literal
  • A union expression
  • A compound expression
  • A string literal
  • A null literal
  • A undefined literal
  • A numeric literal
  • A boolean literal

Builtin Types

There are a set of builtin types.

  • String
  • Number
  • Boolean
  • Object
  • void
  • Any
  • Array
  • Error
  • Function

Every builtin type is a valid type expression.

The builtin String type

Any JavaScript String value is of type String.

i.e. A value v is of type String if typeof v === 'string'.

Examples:

  • "foo" is of type String.
  • new String("foo") is not of type String

The builtin Number type

Any JavaScript Number value is of type Number

i.e. A value v is of type Number if typeof v === 'number'

Examples:

  • 42 is of type Number
  • Infinity, -Infinity is of type Number
  • NaN is of type Number
  • new Number(0) is not of type Number

The builtin Boolean type

TODO

The bultin Object type

Any Javascript object is of type Object

i.e. A value v is of type Object if typeof v === 'object' && v !== null

Examples:

  • { foo: 'bar' } is of type Object
  • [1, 2, 3] is of type Object
  • new Object() is of type Object
  • null is not of type Object

The generic variant of Object<Key, Value>

myThing : Object<String, T>

TODO

The builtin void type

If something has no value then it is of type void. In JavaScript the undefined value is of type void

i.e. A value v is of type void if v === void 0

Examples:

  • void 0 is of type void
  • undefined is of type void
  • (function () { /* no return statement */ }()) is of type void
  • null is not of type void

The builtin Any type

Everything is of type Any

i.e. A value v is of type Any if true

Examples

  • undefined is of type Any
  • "foo" is of type Any
  • null is of type Any
  • { foo: 50 } is of type Any

The builtin Array type

Any javascript Array object is of type Array

i.e. A value v is of type Array if Array.isArray(v)

Examples:

  • [1, 2, 3] is of type Array
  • new Array(10) is of type Array
  • { length: 1, 1: 10 } is not of type Array
  • { __proto__: Array.prototype } is not of type Array

The generic variant of Array<Value>

myThing : Array<T>

TODO

The builtin Error type

Any javascript Error object is of type Error

i.e. A value v is of type Error if {}.toString.call(v) === '[object Error]'

Examples:

  • new Error("foo") is of type Error
  • Error("foo") is of type Error
  • { message: "foo" } is not of type Error

The builtin Function type

Any javascript function is of type Function

i.e. A value v is of type Function if typeof v === 'function'

Examples:

  • Object.prototype.toString is of type Function
  • function () {} is of type Function
  • new Function("return 42") is of type Function
  • { call: function () {} } is not of type Function

The generic variant of Function<Value>

myThing : Function<Promise>

TODO

Builtin literals

TODO

string literal

myThing : "foo"

TODO

null literal

myThing : null

TODO

undefined literal

myThing : undefined

TODO

numeric literal

myThing : 13

TODO

boolean literal

myThing : true

TODO

Type literals

A type literal is a type name. All builtin types are type literals.

Any type literal that is not builtin in is a custom type.

Examples:

myThing : MyCustomType

It is expected that a custom type is either defined inside the file using a type declaration statement or is imported into the file using an import statement.

Generic type literals

myThing : MyCustomType<T>

TODO

Object literals

myThing : { foo: Number, bar: String }

TODO

Function expression

myThing : (someStr: String) => Boolean

TODO

Tuple literals

myThing : [Number, Number]

TODO

Union expression

myThing : null | Object

TODO

Compound expression

myThing : { foo: String } & () => Boolean

TODO