Need help with typescript-is?
Click the “chat” button below for chat support from the developer who created it, or find similar developers for support.

About the developer

woutervh-
721 Stars 31 Forks MIT License 376 Commits 30 Opened issues

Services available

!
?

Need anything else?

Contributors list

typescript-is

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: https://github.com/Microsoft/TypeScript/issues/14419)

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.

Example:

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
.

Options

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. | |
transformNonNullExpressions
| Boolean (default:
false
). If
true
, non-null expressions (eg.
foo!.bar
) are checked to not be
null
or
undefined
| |
emitDetailedErrors
| Boolean or
auto
(default:
auto
). The generated validation functions can return detailed error messages, pointing out where and why validation failed. These messages are used by
assertType()
, but are ignored by
is()
. If
false
, validation functions return empty error messages, decreasing code size.
auto
will generate detailed error messages for assertions, but not for type checks.
true
will always generate detailed error messages, matching the behaviour of version 0.18.3 and older. |

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,
                "transformNonNullExpressions": true,
                "emitDetailedErrors": "auto"
            }
        ]
    }
}

⭐ 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(wildString)) { // returns true // wildString can be used as string! } else { // never gets to this branch }

if (is(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(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(object).toFixed(2); // "42.00"

try { const asString = assertType(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';

@ValidateClass() 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

async and
Promise
returning methods

AssertType
can also work correctly with
async
methods, returning promise rejected with
TypeGuardError

To enable this functionality, you need to emit decorators metadata for your TypeScript project.

{
    "compilerOptions": {
      "emitDecoratorMetadata": true
    }
}

Then

AssertType
will work with async methods and
Promise
returning methods automatically. ```typescript import { ValidateClass, AssertType } from 'typescript-is';

@ValidateClass() class A { async method(@AssertType({ async: true }) value: number) { // You can safely use value as a number return value; }

methodPromise(@AssertType({ async: true }) value: number): Promise {
    // You can safely use value as a number
    return Promise.resolve(value);
}

}

new A().method(42).then(value => value === 42 /* true */); new A().method('42' as any).catch(error => { // error will be of TypeGuardError type }) new A().methodPromise('42' as any).catch(error => { // error will be of TypeGuardError type }) ```

If you want to throw synchronously for some reason, you can override the behaviour using with

@AssertType({ async: false })
: ```typescript import { ValidateClass, AssertType } from 'typescript-is';

@ValidateClass() class A { async method(@AssertType({ async: false }) value: number) { // You can safely use value as a number return value; } }

new A().method(42).then(value => value === 42 /* true */); new A().method('42' as any); // will throw error ```

If you cannot or don't want to enable decorators metadata, you still make AssertType reject with promise using

@AssertType({ async: true })
```typescript import { ValidateClass, AssertType } from 'typescript-is';

@ValidateClass() class A { async method(@AssertType({ async: true }) value: number) { // You can safely use value as a number return value; } } ```

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({}); // false, because x is missing equals({ x: 'value' }); // true equals({ 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(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(object: any): object is T { return is(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(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 https://github.com/woutervh-/typescript-is.git
cd typescript-is/
npm install

Building

npm run build

Testing

npm run test

We use cookies. If you continue to browse the site, you agree to the use of cookies. For more information on our use of cookies please see our Privacy Policy.