Runtime Type Checks for TypeScript
Clone or download
Latest commit b44caef Oct 28, 2018

Build Status Coverage Status


A package for generating runtime type checks from TypeScript type annotations for JavaScript, using the TypeScript compiler API.

Please note, that this package is still experimental and resulting code is not intended to be used in production. It is a proof of concept for adding runtime type checks by making use of the recently published TypeScript transformation API. Feel free to report bugs and to make suggestions. Pull requests are also very welcome.


You can try ts-runtime on the playground:

Quick Start

$ yarn global add ts-runtime
$ tsr --help

You can also use npm -g install ts-runtime.


Type reflections and assertions for the runtime environment are being made possible by flow-runtime, a runtime type system for JavaScript.


Most of explicit type annotations will be reflected (and checked) at runtime. Checks for implicitly inferred types may be added in a future release. In the following section the most important cases are documented.

Source File

On top of every source file, the runtime type checking library is imported.

import t from 'ts-runtime/lib';


It will be distinguished between reassignable declarations (var and let) and constant declarations (const).

Reassignable Declarations

A variable declaration that may be reassigned at runtime as the following:

let num: number;
num = "Hello World!";

will be reflected and checked as shown below:

let _numType = t.number(), num;
num = _numType.assert("Hello World!")

Constant Declarations

A constant declaration does only need to be checked when declared, and no additional variable holding the variable's type, has to be introduced.

const num: number = "Hello World!";

The const declaration from above, results in:

const num = t.number().assert("Hello World!");


In TypeScript the above assignments would already be flagged by the compiler. By using the type assertion as any, any assignment will be allowed but still caught at runtime with this package.

true as any as number;

The above assertion may not be a real world example, but there are situations where similar things happen. Imagine calling a function, that returns any. While you know that the returned value will be a number, you also want the TypeScript compiler to be aware of it and you assert it as number. Probably this function call is from an external library and a bug is introduced that a boolean value is returned instead, which will remain unnoticed unless checked at runtime:



Function parameters and its return value will be checked as well, and functions will be annotated to extract its type reflection at runtime.

Function Declarations

This function simply creates a number from a string.

function getNumberFromString(str: string): number {
  return Number(str);

The code below is the result, with runtime type checks inserted.

function getNumberFromString(str) {
  let _strType = t.string();
  const _returnType = t.return(t.number());
  t.param("str", _strType).assert(str);
  return _returnType.assert(Number(str));

By default the function is also annotated, to get the type reflection of the object at runtime with type queries:

t.annotate(getNumberFromString, t.function(t.param("str", t.string()), t.return(t.number())));

Annotations can be turned off, by setting the annotate option to false or using the --noAnnotate flag with the CLI.

Function Expressions

Also function expressions will be transformed:

const getNumberFromString = function(str: string): number {
  return Number(str);

This declaration will be converted to:

const getNumberFromString = function (str) {
  let _strType = t.string();
  const _returnType = t.return(t.number());
  t.param("str", _strType).assert(str);
  return _returnType.assert(Number(str));

Again, by default the function is annotated like so:

const getNumberFromString = t.annotate(function (str) {
  // function body
}, t.function(t.param("str", t.string()), t.return(t.number())));

Arrow Functions

Arrow function are also supported, with a similar result to function expressions. If runtime checks have to be inserted into an arrow function without a body, ts-runtime generates it for you:

const getNumberFromString = (str: string): number => Number(str);

This one-liner is great, but in order to assert the values it is transformed to the following:

const getNumberFromString = (str) => {
  let _strType = t.string();
  const _returnType = t.return(t.number());
  t.param("str", _strType).assert(str);
  return _returnType.assert(Number(str));

Of course, also arrow functions are annotated by default:

const getNumberFromString = t.annotate((str) => {
  // arrow function body
}, t.function(t.param("str", t.string()), t.return(t.number())));

Type Queries

In the following example, TypeScript gets the type of a variable and uses it as type for another variable declaration.

let num = 10;
let numType: typeof num = "Hello World!";

In ts-runtime this will be transformed to:

let num = 10;
let _myNumType = t.typeOf(num), myNum = _myNumType.assert("Hello World!");

Please note, that num is not reflected and asserted, because it lacks an explicit type annotation.


The TypeScript compiler option preserveConstEnums will be always set to true by ts-runtime. A warning in the console will let you know.

Enum Declarations

enum Action {

The enum from above will be transformed to the following by TypeScript:

var Action;
(function (Action) {
  Action[Action["None"] = 0] = "None";
  Action[Action["Save"] = 1] = "Save";
  Action[Action["Update"] = 2] = "Update";
})(Action || (Action = {}));

and annotated by ts-runtime with the reflection below:

t.annotate(Action, t.enum(t.enumMember(0), t.enumMember(1), t.enumMember(2)));

Enum References

When using the enum as a type reference, only the numbers 0, 1, and 2 can be assigned to action:

let action: Action;
let _actionType = t.enumRef(Action), action;

The same is true when using a specific enum members as reference. In this example only the number 1 can be assigned to saveAction:

let saveAction: Action.Save;
let _saveActionType = t.enumMember(Action.Save), saveAction;

Type Aliases

Type aliases are removed entirely by the TypeScript compiler.

type MyType = {
  property: string;
  optional?: number;
  method: (param: boolean) => void;

The type alias declaration from above will be replaced with the following reflection:

const MyType = t.type("MyType", t.object("property", t.string()),"optional", t.number(), true),"method", t.function(t.param("param", t.boolean()), t.return(t.void())))

Self references are supported.


Also interfaces would be compiled away.

interface BaseInterface {
  [index: string]: any;

interface MyInterface extends BaseInterface {
  prop: string;

With ts-runtime they will be replaced with a reflection:

const BaseInterface = t.type("BaseInterface", t.object(
  t.indexer("index", t.string(), t.any()))

const MyInterface = t.type("MyInterface", t.intersect(t.ref(BaseInterface), t.object("prop", t.string())


Classes are transformed with support for properties, static properties, static and non-static methods, deriving from other classes (extends), implementing interfaces (implements), as well as method overloading.

class MyClass {
  method(param?: number): void {


At this point, only a very minimal class transformation, with a single method, is shown:

class MyClass {
  method(param) {
    let _paramType = t.number();
    const _returnType = t.return(t.void());
    t.param("param", _paramType, true).assert(param);

By default also classes are annotated:

@t.annotate(t.class("MyClass","method", t.function(t.param("param", t.number(), true), t.return(t.void())))
class MyClass {
  // class body


Method overloading is supported for type aliases, interfaces and classes, and generates union types based on the overloads.

class MyInterface {
  method(param: number): string;
  method(param: boolean): string;
  method(param: any): any {
    // implementation

While all overloads are considered (excluding merged declarations) when generating a reflection, the implementation itself is ignored:

@t.annotate(t.class("MyInterface","method", t.function(
    t.param("param", t.union(t.number(), t.boolean())), t.return(t.string()))
class MyInterface {
  method(param) {
    // implementation


Generics are supported for functions, classes, interfaces and type aliases.

function asArray<T>(val: T): T[] {
  return [val];

The above snippet shows a simple function that makes use of generics to specify the return type, which results in the following transformation:

function asArray(val) {
  const T = t.typeParameter("T");
  let _valType = t.flowInto(T);
  const _returnType = t.return(t.array(T));
  t.param("val", _valType).assert(val);
  return _returnType.assert([val]);

Externals and Ambient Declarations

We were seeing a couple of different transformations based on local variables. What about external packages, declaration files and ambient declarations? They are collected and emitted to a single file.


Imagine the following type reference:

import * as ts from 'typescript';
let flowNode: ts.FlowNode;

It points to the interface FlowNode inside typescript.d.ts:

interface FlowNode {
  flags: FlowFlags;
  id?: number;

The reference is removed and replaced by a string. This string holds the fully qualified name of the reference, and the hashed file name as a suffix, to prevent naming clashes:

let _flowNodeType = t.ref("ts.FlowNode.82613696"), flowNode;

The actual reflections go into a single file (tsr-declaration.js by default):

t.declare(t.type("ts.FlowFlags.82613696", t.enum(/* enum members */)));
t.declare(t.type("ts.FlowNode.82613696", t.object(/* properties */)));

By default declarations from built in libs (such as DOM, or ES6) are not reflected, but inferred at runtime.


Also local declarations will be included in tsr-declarations.js:

declare module MyModule {
  class MyClass {


The code from above will be reflected as follows:

t.declare(t.class("MyModule.MyClass.3446180452", t.object()));

The generated file will be located in the common directory of all entry files or in the root of outDir or outFile. For some controls regarding this file, have a look at the options.


  • Only as number syntax for type assertions (no angle-bracket syntax: <number>).
  • No reflection of mapped types, indexed access types and type operators yet.
  • readonly is currently only checked for classes.
  • Class visibility modifiers are not asserted.
  • Class type parameters are only checked when extending, at this time.
  • Types with self references and generics are not asserted correctly.
  • No class expressions (const A = class { }), only class declarations (class A { }) can be used.
  • ExpressionWithTypeArguments can only contain PropertyAccessExpressions as expression with an Identifier as name, recursively.
  • No JSX support.



Type: boolean
Default: false

Specifies if classes and function should be annotated.


Type: ts.CompilerOptions

  moduleResolution: ts.ModuleResolutionKind.NodeJs,
  module: ts.ModuleKind.ES2015,
  target: ts.ScriptTarget.ES2015,
  lib: ["lib.es2015.d.ts"],
  strictNullChecks: true,
  experimentalDecorators: true,
  sourceMap: false,
  removeComments: true,
  preserveConstEnums: true,

The option preserveConstEnum will always be set to true by ts-runtime.


Type: string
Default: "tsr-declarations"

The file name where all external and ambient declarations will be written to. Excludes a path or an extension.


Type: boolean
Default: false

Specifies if the generated file should be imported on top of every entry file.


Type: boolean
Default: false

Specifies if the library import on top of every file should be omitted.


Type: boolean
Default: false

Try to continue if TypeScript compiler diagnostics occurred.


Type: boolean
Default: false

Do not delete temporary files on finish.


Type: string
Default: ".tsr"

Name of the directory, where temporary files should be written to.


Type: string
Default: ""

Prefix for the default library import.


Type: string
Default: "t"

Identifier of the default library import, prefixed by its namespace. Looks like the following by default

import t from "ts-runtime/lib";


Type: boolean
Default: false

By default, built in libraries, such as DOM or ES6, are not reflected, but inferred at runtime.


Type: string
Default: "_"

If new variables are introduced while transforming, they will be prefixed with this specified string.


Type: boolean
Default: false

Adds import "module-alias/register"; on top of every file.


Type: number
Default: 3

Limit the output of the stack trace. This only takes effect when using the CLI.


Type: boolean
Default: true

Log messages to the console. This option is not available via the CLI.


It is easy to make use of ts-runtime via Node.js. entryFiles is a string[] and an Options object may optionally be passed as a second parameter.

import { transform } from 'ts-runtime';

It is also possible to listen for various events:

import { transform, bus } from 'ts-runtime';

bus.on(, () => {
  // callback if processing is about to start

transform(entryFiles, { log: false });


  Usage: tsr <file...> [options]

  ---------  ts-runtime  ---------
  Turns TypeScript type assertions
  into runtime type checks for you


    -h, --help                               output usage information
    -v, --version                            output the version number
    -a, --noAnnotate                         do not annotate classes and functions
    -c, --tsConfig <path>                    use the compiler options of the given tsconfig.json
    -C, --compilerOptions <compilerOptions>  set TypeScript compiler options. defaults to "{}"
    -d, --declarationFileName <fileName>     set file name for global declarations. defaults to "tsr-declarations"
    -e, --excludeDeclarationFile             do not automatically import ambient declarations in the entry file. default to false
    -E, --excludeLib                         do not automatically import the runtime library. defaults to false
    -f, --force                              try to finish on TypeScript compiler error. defaults to false
    -F, --fast                               no fancy status for the command line, but faster processing. defaults to false
    -l, --libIdentifier <name>               lib import name. defaults to "t"
    -L, --libDeclarations                    reflect declarations from global libs (e.g. DOM). defaults to false
    -m, --moduleAlias                        import package module-alias on top of every file.
    -n, --libNamespace <namespace>           prefix for lib and code additions. defaults to ""
    -p, --declarationPrefix <namespace>      prefix for added variables. defaults to "_"
    -s, --stackTraceOutput <limit>           output a specified number of lines of the stack trace. defaults to 3


    $ tsr entry.ts --force
    $ tsr src/entry1 bin/entry2 lib/entry3
    $ tsr entry.ts -c tsconfig.json


$ git checkout
$ cd ts-runtime
$ yarn build