---
title: Check
description: Documentation on how to use check, Meteor's type checking library.
---

The `check` package includes pattern checking functions useful for checking the types and structure
of variables and an [extensible library of patterns](#matchpatterns) to specify which types you are
expecting.

To add `check` (or `Match`) to your application, run this command in your terminal:

```bash
meteor add check
```

{% apibox "check" %}

Meteor methods and publish functions can take arbitrary [EJSON](#ejson) types as arguments, but most
functions expect their arguments to be of a particular type. `check` is a lightweight function for
checking that arguments and other values are of the expected type. For example:

```js
Meteor.publish('chatsInRoom', function (roomId) {
  // Make sure `roomId` is a string, not an arbitrary Mongo selector object.
  check(roomId, String);
  return Chats.find({ room: roomId });
});

Meteor.methods({
  addChat(roomId, message) {
    check(roomId, String);
    check(message, {
      text: String,
      timestamp: Date,
      // Optional, but if present must be an array of strings.
      tags: Match.Maybe([String])
    });

    // Do something with the message...
  }
});
```

If the match fails, `check` throws a `Match.Error` describing how it failed. If
this error gets sent over the wire to the client, it will appear only as
`Meteor.Error(400, 'Match Failed')`. The failure details will be written to the
server logs but not revealed to the client.

By default, `check` will throw immediately at the first error encountered. Pass in `{ throwAllErrors: true }` to throw an array of all errors. For example:
```js
check(message, {/* ... */}, {throwAllErrors: true})
```

{% apibox "Match.test" %}

`Match.test` can be used to identify if a variable has a certain structure.

```js
// Will return true for `{ foo: 1, bar: 'hello' }` or similar.
Match.test(value, { foo: Match.Integer, bar: String });

// Will return true if `value` is a string.
Match.test(value, String);

// Will return true if `value` is a string or an array of numbers.
Match.test(value, Match.OneOf(String, [Number]));
```

This can be useful if you have a function that accepts several different kinds
of objects, and you want to determine which was passed in.

<h2 id="matchpatterns">Match Patterns</h2>

The following patterns can be used as pattern arguments to
[`check`](#check) and `Match.test`:

<dl>
{% dtdd name:"<code>Match.Any</code>" %}
Matches any value.
{% enddtdd %}

{% dtdd name:"<code>String</code>, <code>Number</code>, <code>Boolean</code>, <code>undefined</code>, <code>null</code>" %}
Matches a primitive of the given type.
{% enddtdd %}

{% dtdd name:"<code>Match.Integer</code>" %}
Matches a signed 32-bit integer. Doesn't match `Infinity`, `-Infinity`, or `NaN`.
{% enddtdd %}

{% dtdd name:"<code>[<em>pattern</em>]</code>" %}
A one-element array matches an array of elements, each of which match
*pattern*. For example, `[Number]` matches a (possibly empty) array of numbers;
`[Match.Any]` matches any array.
{% enddtdd %}

<dt><span class="name"><code>{ <em>key1</em>: <em>pattern1</em>, <em>key2</em>: <em>pattern2</em>, ... }</code></span></dt>
<dd>
Matches an Object with the given keys, with values matching the given patterns.
If any *pattern* is a `Match.Maybe` or `Match.Optional`, that key does not need to exist
in the object. The value may not contain any keys not listed in the pattern.
The value must be a plain Object with no special prototype.
</dd>

<dt><span class="name"><code>Match.ObjectIncluding({ <em>key1</em>: <em>pattern1</em>, <em>key2</em>: <em>pattern2</em>, ... })</code></span></dt>
<dd>
Matches an Object with the given keys; the value may also have other keys
with arbitrary values.
</dd>

{% dtdd name:"<code>Object</code>" %}
Matches any plain Object with any keys; equivalent to
`Match.ObjectIncluding({})`.
{% enddtdd %}



{% dtdd name:"<code>Match.Maybe(<em>pattern</em>)</code>" %}

Matches either `undefined`, `null`, or _pattern_. If used in an object, matches only if the key is
not set as opposed to the value being set to `undefined` or `null`. This set of conditions was
chosen because `undefined` arguments to Meteor Methods are converted to `null` when sent over the
wire.

{% codeblock lang:js %}
// In an object
const pattern = { name: Match.Maybe(String) };

check({ name: 'something' }, pattern); // OK
check({}, pattern); // OK
check({ name: undefined }, pattern); // Throws an exception
check({ name: null }, pattern); // Throws an exception

// Outside an object
check(null, Match.Maybe(String)); // OK
check(undefined, Match.Maybe(String)); // OK
{% endcodeblock %}
{% enddtdd %}

{% dtdd name:"<code>Match.Optional(<em>pattern</em>)</code>" %}

Behaves like `Match.Maybe` except it doesn't accept `null`. If used in an object, the behavior is
identical to `Match.Maybe`.

{% enddtdd %}

{% dtdd name:"<code>Match.OneOf(<em>pattern1</em>, <em>pattern2</em>, ...)</code>" %}
Matches any value that matches at least one of the provided patterns.
{% enddtdd %}

{% dtdd name:"Any constructor function (eg, <code>Date</code>)" %}
Matches any element that is an instance of that type.
{% enddtdd %}

{% dtdd name:"<code>Match.Where(<em>condition</em>)</code>" %}
Calls the function *condition* with the value as the argument. If *condition*
returns true, this matches. If *condition* throws a `Match.Error` or returns
false, this fails. If *condition* throws any other error, that error is thrown
from the call to `check` or `Match.test`. Examples:

{% codeblock lang:js %}
check(buffer, Match.Where(EJSON.isBinary));

const NonEmptyString = Match.Where((x) => {
  check(x, String);
  return x.length > 0;
});

check(arg, NonEmptyString);
{% endcodeblock %}
{% enddtdd %}
</dl>