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    0.17.1 • Public • Published


    TypeScript transformer that generates run-time type-checks.

    npm node Travis (.org) npm David David NpmLicense

    💿 Installation

    npm install --save typescript-is
    # Ensure you have the required dependencies at compile time:
    npm install --save-dev typescript
    # If you want to use the decorators, ensure you have reflect-metadata in your dependencies:
    npm install --save reflect-metadata

    💼 Use cases

    If you've worked with TypeScript for a while, you know that sometimes you obtain any or unknown data that is not type-safe. You'd then have to write your own function with type predicates that checks the foreign object, and makes sure it is the type that you need.

    This library automates writing the type predicate function for you.

    At compile time, it inspects the type you want to have checked, and generates a function that can check the type of a wild object at run-time. When the function is invoked, it checks in detail if the given wild object complies with your favorite type.

    In particular, you may obtain wild, untyped object, in the following situations:

    • You're doing a fetch call, which returns some JSON object. You don't know if the JSON object is of the shape you expect.
    • Your users are uploading a file, which is then read by your application and converted to an object. You don't know if this object is really the type you expect.
    • You're reading a JSON string from localStorage that you've stored earlier. Perhaps in the meantime the string has been manipulated and is no longer giving you the object you expect.
    • Any other case where you lose compile time type information...

    In these situations typescript-is can come to your rescue.

    NOTE this package aims to generate type predicates for any serializable JavaScript object. Please check What it won't do for details.

    Similar projects

    🎛️ Configuration

    This package exposes a TypeScript transformer factory at typescript-is/lib/transformer-inline/transformer

    As there currently is no way to configure the TypeScript compiler to use a transformer without using it programatically, the recommended way is to compile with ttypescript. This is basically a wrapper around the TypeScript compiler that injects transformers configured in your tsconfig.json.

    (please vote here to support transformers out-of-the-box:

    Using ttypescript

    First install ttypescript:

    npm install --save-dev ttypescript

    Then make sure your tsconfig.json is configured to use the typescript-is transformer:

        "compilerOptions": {
            "plugins": [
                { "transform": "typescript-is/lib/transform-inline/transformer" }

    Now compile using ttypescript:

    npx ttsc

    Using with ts-node, webpack, Rollup

    Please check the README of ttypescript for information on how to use it in combination with ts-node, webpack, and Rollup.

    Note: This will not work if ts-loader is configured with transpileOnly: true.

    Using with webpack + ts-loader without ttypescript

    If you are using ts-loader in a webpack project, you can use getCustomTransformers as suggested in #54. This means you don't need to use ttypescript or write a custom compilation script.


    const typescriptIsTransformer = require('typescript-is/lib/transform-inline/transformer').default
    module.exports = {
        // I am hiding the rest of the webpack config
        module: {
            rules: [
                    test: /\.ts$/,
                    exclude: /node_modules/,
                    loader: 'ts-loader',
                    options: {
                        getCustomTransformers: program => ({
                            before: [typescriptIsTransformer(program)]

    Note: This will not work if ts-loader is configured with transpileOnly: true.


    There are some options to configure the transformer.

    Property Description
    shortCircuit Boolean (default false). If true, all type guards will return true, i.e. no validation takes place. Can be used for example in production deployments where doing a lot of validation can cost too much CPU.
    ignoreClasses Boolean (default: false). If true, when the transformer encounters a class (except for Date), it will ignore it and simply return true. If false, an error is generated at compile time.
    ignoreMethods Boolean (default: false). If true, when the transformer encounters a method, it will ignore it and simply return true. If false, an error is generated at compile time.
    ignoreFunctions (deprecated, use functionBehavior instead) Boolean (default: false). If true, when the transformer encounters a function, it will ignore it and simply return true. If false, an error is generated at compile time.
    functionBehavior One of error, ignore, or basic (default: error). Determines the behavior of transformer when encountering a function. error will cause a compile-time error, ignore will cause the validation function to always return true, and basic will do a simple function-type-check. Overrides ignoreFunctions.
    disallowSuperfluousObjectProperties Boolean (default: false). If true, objects are checked for having superfluous properties and will cause the validation to fail if they do. If false, no check for superfluous properties is made.

    If you are using ttypescript, you can include the options in your tsconfig.json:

        "compilerOptions": {
            "plugins": [
                    "transform": "typescript-is/lib/transform-inline/transformer",
                    "shortCircuit": true,
                    "ignoreClasses": true,
                    "ignoreMethods": true,
                    "functionBehavior": "ignore",
                    "disallowSuperfluousObjectProperties": true

    How to use

    Before using, please make sure you've completed configuring the transformer.

    In your TypeScript code, you can now import and use the type-check function is (or createIs), or the type assertion function assertType (or createAssertType).

    Validation (is and createIs)

    For example, you can check if something is a string or number and use it as such, without the compiler complaining:

    import { is } from 'typescript-is';
    const wildString: any = 'a string, but nobody knows at compile time, because it is cast to `any`';
    if (is<string>(wildString)) { // returns true
        // wildString can be used as string!
    } else {
        // never gets to this branch
    if (is<number>(wildString)) { // returns false
        // never gets to this branch
    } else {
        // Now you know that wildString is not a number!

    You can also check your own interfaces:

    import { is } from 'typescript-is';
    interface MyInterface {
        someObject: string;
        without: string;
    const foreignObject: any = { someObject: 'obtained from the wild', without: 'type safety' };
    if (is<MyInterface>(foreignObject)) { // returns true
        const someObject = foreignObject.someObject; // type: string
        const without = foreignObject.without; // type: string

    Assertions (assertType and createAssertType)

    Or use the assertType function to directly use the object:

    import { assertType } from 'typescript-is';
    const object: any = 42;
    assertType<number>(object).toFixed(2); // "42.00"
    try {
        const asString = assertType<string>(object); // throws error: object is not a string
        asString.toUpperCasse(); // never gets here
    } catch (error) {
        // ...

    Decorators (ValidateClass and AssertType)

    You can also use the decorators to automate validation in class methods. To enable this functionality, you should make sure that experimental decorators are enabled for your TypeScript project.

        "compilerOptions": {
            "experimentalDecorators": true

    You should also make sure the peer dependency reflect-metadata is installed.

    npm install --save reflect-metadata

    You can then use the decorators:

    import { ValidateClass, AssertType } from 'typescript-is';
    class A {
        method(@AssertType() value: number) {
            // You can safely use value as a number
            return value;
    new A().method(42) === 42; // true
    new A().method('42' as any); // will throw error

    Strict equality (equals, createEquals, assertEquals, createAssertEquals)

    This family of functions check not only whether the passed object is assignable to the specified type, but also checks that the passed object does not contain any more than is necessary. In other words: the type is also "assignable" to the object. This functionality is equivalent to specifying disallowSuperfluousObjectProperties in the options, the difference is that this will apply only to the specific function call. For example:

    import { equals } from 'typescript-is';
    interface X {
        x: string;
    equals<X>({}); // false, because `x` is missing
    equals<X>({ x: 'value' }); // true
    equals<X>({ x: 'value', y: 'another value' }); // false, because `y` is superfluous

    To see the declarations of the functions and more examples, please check out index.d.ts.

    For many more examples, please check out the files in the test/ folder. There you can find all the different types that are tested for.

    What it won't do

    • This library aims to be able to check any serializable data.
    • This library will not check functions. Function signatures are impossible to check at run-time.
    • This library will not check classes (except the global Date). Instead, you are encouraged to use the native instanceof operator. For example:
    import { is } from 'typescript-is';
    class MyClass {
        // ...
    const instance: any = new MyClass();
    is<MyClass>(instance); // error -> classes are not supported.
    // Instead, use instanceof:
    if (instance instanceof MyClass) {
        // ...
    • This library will not magically check unbound type parameters. Instead, make sure all type parameters are bound to a well-defined type when invoking the is function. For example:
    import { is } from 'typescript-is';
    function magicalTypeChecker<T>(object: any): object is T {
        return is<T>(object); // error -> type `T` is not bound.

    If you stumble upon anything else that is not yet supported, please open an issue or submit a PR. 😉

    🗺️ Road map

    Features that are planned:

    • Promise support. Something like assertOrReject<Type>(object) will either resolve(object) or reject(error).
    • Optimize the generated conditions. Things like false || "key" === "key" can be simplified. Might be more interesting to publish a different library that can transform a TypeScript AST, and then use it here, or use an existing one. Might be out of scope, as there are plenty of minifiers/uglifiers/manglers out there already.

    🔨 Building and testing

    git clone
    cd typescript-is/
    npm install
    # Building
    npm run build
    # Testing
    npm run test


    npm i @detachhead/typescript-is

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