typescanner

typescanner

typescanner is a simple library for implementing Type Guard in TypeScript.

It can be easily implemented from basic primitive type guard to type guard for types defined in Type Aliases.

Install

npm i -D typescanner

Example

// define the union type
const Lang = {
  ja: "ja",
  en: "en",
} as const;
type Lang = typeof Lang[keyof typeof Lang]; // "ja" | "en"

const langList = Object.values(Lang);

type Post = {
  id: number;
  author: string | null;
  body: string;
  lang: Lang;
  isPublic: boolean;
  createdAt: Date;
  tags?: string[] | null;
};

// create a scanner
const isPost = scanner<Post>({
  id: number,
  author: union(string, Null),
  body: string,
  lang: list(langList),
  isPublic: boolean,
  createdAt: date,
  tags: optional(array(string), Null),
});

const data = {
  id: 1,
  author: "taro",
  body: "Hello!",
  lang: "ja",
  isPublic: true,
  createdAt: new Date(),
  tags: ["tag1", "tag2"],
} as unknown;

// scan
const post = scan(data, isPost);

post.body; // OK

Usage

fields

fields is used when creating a scanner. fields returns a Condition or an array of conditions as defined below.

type Condition<T> = (value: unknown) => value is T;

string; // string
number; // number
boolean; // boolean
symbol; // symbol
bigint; // bigint
Undefined; // undefined
Null; // null
data; // Date

union(string, null); // string | null
// If you want to use multiple types other than null and undefined,
// you need to write the types explicitly.
union<string | number>(string, number); // string | number

// Passes the Condition for the values in the array.
array(string); // string[]
array<string | number>(string, number); // (string | number)[]

optional(string); // string | undefined
optional<string | number>(string, number); // string | number | undefined

// Check if the value passed when determining the type is contained 
// in the array passed by list().
list(["a", "b", "c"]); // "a" | "b" | "c"

//　Pass a constructor as argument
instanceOf(Error)

scanner

scanner is a function to implement type guard for objects. It returns a "type guard function" of the type defined by Type Aliase by setting a field to the value of each property.

  type Foo = {
    a: string;
    b: number;
    c: boolean;
    d: Date;
    e: string[];
    f?: string;
    g: "a" | "b" | "c";
    h: string | null;
    i: string | number;
    j: number;
  };
  
  // You can extend fields by defining your own type guard functions.
  const even = (value: unknown): value is number =>
    isNumber(value) && value % 2 === 0;
  
  const isFoo = scanner<Foo>({
    a: string,
    b: number,
    c: boolean,
    d: date,
    e: array(string),
    f: optional(string),
    g: list(["a", "b", "c"]),
    h: union(string, Null),
    i: union<string | number>(string, number),
    j: even, // Custom field
  });
  
  const foo = {
    a: "a",
    b: 2,
    c: true,
    d: new Date(),
    e: ["a", "b"],
    f: "f",
    g: "a",
    j: 2,
  } as unknown;
  
  if (isFoo(foo)) {
    foo.a // OK
  }

scan

scan is used with the first argument being the value you want to validate and the second argument being the Condition. If the verification is successful, the "narrowed value" will be returned. If it fails, it throws an exception.

  // success
  const data = scan(foo, isFoo);
  data.a // OK
  
  // Error!
  const data = scan(bar, isFoo); // Error: type assertion is failed.

other

Basic type guard functions with function names beginning with "is".

// primitive

isString("a")

isNumber(1)

isBoolean(true)

isUndefined(undefined)

isNull(null)

isDate(new Data())

isSymbol(Symbol("a"))

isBigint(BigInt(1))

// isArray

isArray(["a", "b"], isString) // string[]

isArray<string | number>(["a", 1], isString, isNumber) // (string | number)[]

isArray(["a", null, undefined], isString, isNull, isUndefined) // (string | null | undefined)[]

// isOptional

isOptional("a", isString) // true

isOptional(undefined, isString) // true

// isList

const Lang = {
  ja: "ja",
  en: "en",
} as const;

type Lang = typeof Lang[keyof typeof Lang]; // "ja" | "en"
const langList = Object.values(Lang);

isList("ja", langList) // true

// isInstanceOf

try {
  ...
} catch (error) {
  if (isInstanceOf(error, Error)) {
    error.message // OK
  }
}

Contribution

wellcome

License

MIT