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VanillaJS version of ReactJS propTypes
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It's like ReactJS propTypes without React. Actually, it's very heavily inspired by this concept. It's purpose is for normal JavaScript functions rather than just React Components.

Demo Screenshot


$ npm i -S api-check or $bower i -S api-check

api-check utilizes UMD, so you can:

var apiCheck = require('api-check')(/* your custom options, checkers*/);

Also available as an AMD module or as apiCheck on global


Note, there are a bunch of tests. Those should be instructive as well.

var myApiCheck = require('api-check')({
  /* config options */
  output: {
    prefix: 'app/lib Name',
    suffix: 'Good luck!',
    docsBaseUrl: ''
  verbose: false
}, {
  /* custom checkers if you wanna */

// given we have a function like this:
function foo(bar, foobar) {
  // we can define our api as the first argument to myApiCheck.warn
  myApiCheck.warn([myApiCheck.number, myApiCheck.arrayOf(myApiCheck.string)], arguments);
  // do stuff
// the function above can be called like so:
foo(3, ['a','b','c']);

// if it were called like so, a descriptive warning would be logged to the console
foo('whatever', false);

// here's something a little more complex (this is what's in the screenshot and [the demo](,console,output))
var myCheck = require('api-check')({
  output: {
    prefix: 'myApp',
    suffix: 'see docs -->',
    docsBaseUrl: ''
function doSomething(person, options, callback) {
  myCheck.warn([ // you can also do myCheck.throw to throw an exception
      name: myCheck.shape({
        first: myCheck.string,
        last: myCheck.string
      age: myCheck.number,
      isOld: myCheck.bool,
      walk: myCheck.func,
      ipAddress: function(val, name, location) {
        if (!/(\d{1,3}\.){3}\d{1,3}/.test(val)) {
          return myCheck.utils.getError(name, location, 'ipAddress');
      childrenNames: myCheck.arrayOf(myCheck.string).optional
  ], arguments, {
    prefix: 'doSomething',
    suffix: 'Good luck!',
    urlSuffix: 'dosomething-api-check-failure'

  // do stuff

var person = {
  name: {
    first: 'Matt',
    last: 'Meese'
  age: 27,
  isOld: false,
  ipAddress: '',
  walk: function() {}
function callback() {}
var options = 'whatever I want because it is an "any" type';

console.log('Successful call');
doSomething(person, options, callback);

console.log('Successful call (without options)');
doSomething(person, callback); // <-- options is optional

console.log('Failed call (without person)');
doSomething(callback); // <-- this would fail because person is not optional

person.ipAddress = 'Invalid IP Address!!!';

console.log('Failed call (invalid ip address)');
doSomething(person, options, callback); // <-- this would fail because the ipAddress checker would fail

// if you only wish to check the first argument to a function, you don't need to supply an array.

var libCheck = apiCheck(); // you don't HAVE to pass anything if you don't want to.
function bar(a) {
  var errorMessage = libCheck(apiCheck.string, arguments);
  if (!errorMessage) {
    // success
  } else if (typeof errorMessage === 'string') {
    // there was a problem and errorMessage would like to tell you about it
bar('hello!'); // <-- success!

Differences from React's propTypes

Differences in Supported Types noted below with a *

  • All types are required by default, to set something as optional, append .optional
  • checkApi.js does not support element and node types
  • checkApi.js supports a few additional types
  • object fails on null. Use object.nullOk if you don't want that

Similarities to React's propTypes

This project was totally written from scratch, but it (should) support the same api as React's propTypes (with the noted difference above). If you notice something that functions differently, please file an issue.

apiCheck(), apiCheck.warn(), and apiCheck.throw()

These functions do the same thing, with minor differences. In both the warn and throw case, a message is generated based on the arguments that the function was received and the api that was defined to describe what was wrong with the invocation.

In all cases, an object is returned with the following properties:

argTypes (arrayOf[Object])

This is an array of objects representing the types of the arguments passed.

apiTypes (arrayOf[Object])

This is an object representing the types of the api. It's a whole language of its own that you'll hopefully get after looking at it for a while.

failed (boolean)

Will be false when the check passes, and true when it fails

passed (boolean)

Will be true when the check passes, and false when it fails

message (string)

If the check failed, this will be a useful message for display to the user. If it passed, this will be an empty string

Also note that if you only have one argument, then the first argument to the apiCheck function can simply be the checker function. For example:

apiCheck(apiCheck.bool, arguments);

The second argument can either be an arguments-like object or an array.

Supported types


apiCheck.array([]); // <-- pass
apiCheck.array(23); // <-- fail


apiCheck.bool(false); // <-- pass
apiCheck.bool('me bool too?'); // <-- fail


apiCheck.func(function() {}); // <-- pass
apiCheck.func(new RegExp()); // <-- fail

func.withProperties *

Not available in React's propTypes

var checker = apiCheck.func.withProperties({
  type: apiCheck.oneOfType([apiCheck.object, apiCheck.string]),
  help: apiCheck.string.optional
function winning(){}
winning.type = 'awesomeness';
checker(winning); // <--pass

function losing(){}
checker(losing); // <-- fail


apiCheck.number(423.32); // <-- pass
apiCheck.number({}); // <-- fail

object *

null fails, use object.nullOk to allow null to pass

apiCheck.object({}); // <-- pass
apiCheck.object([]); // <-- fail
apiCheck.object(null); // <-- fail

object.nullOk *

Not available in React's propTypes

apiCheck.object.nullOk({}); // <-- pass
apiCheck.object.nullOk([]); // <--- false
apiCheck.object.nullOk(null); // <-- pass

emptyObject *

Not available in React's propTypes

apiCheck.emptyObject({}); // <-- pass
apiCheck.emptyObject([]); // <-- fail
apiCheck.emptyObject(null); // <-- fail
apiCheck.emptyObject({"foo": "bar"}) // <-- fail


apiCheck.string('I am a string!'); // <-- pass
apiCheck.string([]); // <-- fail


apiCheck.range(0, 10)(4); // <-- pass
apiCheck.range(-100, 100)(500); // <-- fail


apiCheck.greaterThan(100)(200); // <-- pass
apiCheck.greaterThan(-10)(-20); // <-- fail
apiCheck.greaterThan(50)('Frogs!'); // <-- fail


apiCheck.lessThan(100)(50); // <-- pass
apiCheck.lessThan(-10)(0); // <-- fail
apiCheck.lessThan(50)('Frogs!'); // <-- fail


apiCheck.instanceOf(RegExp)(new RegExp); // <-- pass
apiCheck.instanceOf(Date)('wanna go on a date?'); // <-- fail


apiCheck.oneOf(['Treek', ' Wicket Wystri Warrick'])('Treek'); // <-- pass
apiCheck.oneOf(['Chewbacca', 'Snoova'])('Snoova'); // <-- fail


apiCheck.oneOfType([apiCheck.string, apiCheck.object])({}); // <-- pass
apiCheck.oneOfType([apiCheck.array, apiCheck.bool])('Kess'); // <-- fail


apiCheck.arrayOf(apiCheck.string)(['Huraga', 'Japar', 'Kahless']); // <-- pass
)([[[1,2,3], [4,5,6], [7,8,9]], [[1,2,3], [4,5,6], [7,8,9]]]); // <-- pass (for realz)
apiCheck.arrayOf(apiCheck.bool)(['a', 'b', 'c']); // <-- fail

typeOrArrayOf *

Not available in React's propTypes

Convenience checker that combines oneOfType with arrayOf and whatever you specify. So you could take this:

  apiCheck.string, apiCheck.arrayOf(apiCheck.string)



which is a common enough use case to justify the checker.

apiCheck.typeOrArrayOf(apiCheck.string)('string'); // <-- pass
apiCheck.typeOrArrayOf(apiCheck.string)(['array', 'of strings']); // <-- pass
apiCheck.typeOrArrayOf(apiCheck.bool)(['array', false]); // <-- fail
apiCheck.typeOrArrayOf(apiCheck.object)(32); // <-- fail


apiCheck.objectOf(apiCheck.arrayOf(apiCheck.bool))({a: [true, false], b: [false, true]}); // <-- pass
apiCheck.objectOf(apiCheck.number)({a: 'not a number?', b: 'yeah, me neither (◞‸◟;)'}); // <-- fail

shape *

Note: React propTypes does support shape, however it does not support the strict option

If you add .strict to the shape, then it will enforce that the given object does not have any extra properties outside those specified in the shape. See below for an example...

  name: checkers.shape({
    first: checkers.string,
    last: checkers.string
  age: checkers.number,
  isOld: checkers.bool,
  walk: checkers.func,
  childrenNames: checkers.arrayOf(checkers.string)
  name: {
    first: 'Matt',
    last: 'Meese'
  age: 27,
  isOld: false,
  walk: function() {},
  childrenNames: []
}); // <-- pass
  mint: checkers.bool,
  chocolate: checkers.bool
})({mint: true}); // <-- fail

Example of strict

var strictShape = apiCheck.shape({
  cookies: apiCheck.bool,
  milk: apiCheck.bool,
  popcorn: apiCheck.bool.optional
}).strict; // <-- that!

  cookies: true,
  milk: true,
  popcorn: true,
  candy: true
}); // <-- fail because the extra `candy` property

  cookies: true,
  milk: true
}); // <-- pass because it has no extra properties and `popcorn` is optional

Note, you can also append .optional to the .strict (as in: apiCheck.shape({}).strict.optional)

shape.onlyIf *

Not available in React's propTypes

This can only be used in combination with shape

  cookies: apiCheck.shape.onlyIf(['mint', 'chips'], apiCheck.bool)
})({cookies: true, mint: true, chips: true}); // <-- pass

  cookies: apiCheck.shape.onlyIf(['mint', 'chips'], apiCheck.bool)
})({chips: true}); // <-- pass (cookies not specified)

  cookies: apiCheck.shape.onlyIf('mint', apiCheck.bool)
})({cookies: true}); // <-- fail

shape.ifNot *

Not available in React's propTypes

This can only be used in combination with shape

  cookies: apiCheck.shape.ifNot('mint', apiCheck.bool)
})({cookies: true}); // <-- pass

  cookies: apiCheck.shape.ifNot(['mint', 'chips'], apiCheck.bool)
})({cookies: true, chips: true}); // <-- fail

requiredIfNot *

Not available in React's propTypes

This can only be used in combination with shape

checker = checkers.shape({
  foobar: checkers.shape.requiredIfNot(['foobaz', 'baz'], checkers.bool),
  foobaz: checkers.object.optional,
  baz: checkers.string.optional,
  foo: checkers.string.optional
  foo: [1, 2],
  foobar: true
}); // <-- passes

checker({foo: 'bar'}); // <-- fails

Not available in React's propTypes

This can only be used in combination with shape.requiredIfNot

checker = checkers.shape({
  foobar: checkers.shape.requiredIfNot.all(['foobaz', 'baz'], checkers.bool),
  foobaz: checkers.object.optional,
  baz: checkers.string.optional,
  foo: checkers.string.optional
  foo: [1, 2]
}); // <-- fails

  foo: [1, 2],
  foobar: true
}); // <-- passes

  foo: [1, 2],
  baz: 'foo'
}); // <-- passes

args *

Not available in React's propTypes

This will check if the given item is an arguments-like object (non-array object that has a length property)

function foo(bar) {
  apiCheck.args(arguments); // <-- pass
apiCheck.args([]); // <-- fail
apiCheck.args({}); // <-- fail
apiCheck.args({length: 3}); // <-- pass
apiCheck.args({length: 'not-number'}); // <-- fail


apiCheck.any({}); // <-- pass
apiCheck.any([]); // <-- pass
apiCheck.any(true); // <-- pass
apiCheck.any(false); // <-- pass
apiCheck.any(/* seriously, anything, except undefined */); // <-- fail
apiCheck.any.optional(/* unless you specify optional :-) */); // <-- pass
apiCheck.any(3); // <-- pass
apiCheck.any(3.1); // <-- pass
apiCheck.any(3.14); // <-- pass
apiCheck.any(3.141); // <-- pass
apiCheck.any(3.1415); // <-- pass
apiCheck.any(3.14159); // <-- pass
apiCheck.any(3.141592); // <-- pass
apiCheck.any(3.1415926); // <-- pass
apiCheck.any(3.14159265); // <-- pass
apiCheck.any(3.141592653); // <-- pass
apiCheck.any(3.1415926535); // <-- pass
apiCheck.any(3.14159265359); // <-- pass
apiCheck.any(jfio,.jgo); // <-- Syntax error.... ಠ_ಠ


apiCheck.null(null); // <-- pass
apiCheck.null(undefined); // <-- fail
apiCheck.null('hello'); // <-- fail

Custom Types

You can specify your own type. You do so like so:

var myCheck = require('api-check')({
  output: {prefix: 'myCheck'}

function ipAddressChecker(val, name, location) {
  if (!/(\d{1,3}\.){3}\d{1,3}/.test(val)) {
    return apiCheck.utils.getError(name, location, ipAddressChecker.type);
ipAddressChecker.type = 'ipAddressString';

function foo(string, ipAddress) {
  ], arguments);

Then, if you invoked that function like this:

foo('hello', 'not-an-ip-address');

It would result in a warning like this:

myCheck apiCheck failed! `Argument 1` passed, `value` at `Argument 2` must be `ipAddressString`

You passed:

With the types:

The API calls for:

There's actually quite a bit of cool stuff you can do with custom types checkers. If you want to know what they are, look at the tests or file an issue for me to go document them. :-)


When writing custom types, you may find the utils helpful. Please file an issue to ask me to improve documentation for what's available. For now, check out api-check-utils.test.js


Note, obviously, these things are specific to apiCheck and not part of React propTypes

When you create your instance of apiCheck, you can configure it with different options as part of the first argument.


You can specify some extra options for the output of the message.

var myApiCheck = require('api-check')({
  output: {
    prefix: 'Global prefix',
    suffix: 'global suffix',
    docsBaseUrl: ''
  verbose: false, // <-- defaults to false
  disabled: false // <-- defaults to false, set this to true in production

You can also specify an output object to each apiCheck(), apiCheck.throw(), and apiCheck.warn() request:

myApiCheck(apiCheck.bool, arguments, {
  prefix: 'instance prefix:',
  suffix: 'instance suffix',
  urlSuffix: 'example-error-additional-info'

A failure with the above configuration would yield something like this:

Global prefix instance prefix {{error message}} instance suffix global suffix

As an alternative to urlSuffix, you can also specify a url:

myApiCheck(apiCheck.bool, arguments, {
  url: ''


This is the method that apiCheck uses to get the message it throws or console.warns. If you don't like it, feel free to make a better one by simply: apiCheck.getErrorMessage = function(api, args, output) {/* return message */}


This is the method that apiCheck uses to throw or warn the message. If you prefer to do your own thing, that's cool. Simply apiCheck.handleErrorMessage = function(message, shouldThrow) { /* throw or warn */ }

Disable apiCheck

It's a good idea to disable the apiCheck in production. You can disable your own instance of apiCheck as part of the options, but it's probably just better to disable apiCheck globally. I recommend you do this before you (or any of you dependencies) create an instance of apiCheck. Here's how you would do that:

var apiCheck = require('api-check');
apiCheck.globalConfig.disabled = true;


This library was written by Kent C. Dodds. Again, big credits go to the team working on React for thinking up the api. This library was written from scratch, but I'd be lying if I didn't say that I referenced their functions a time or two.

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