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Capacitor Data Types

Data moving between the web runtime and native environments in Capacitor have to be serialized and deserialized so that they can be stored natively in each language. The supported data types are those that can be represented in JSON such as numbers, strings, booleans, arrays, and objects (or dictionaries or key-value stores).


While Swift is the preferred language on iOS, it interoperates with Objective-C (upon which the system frameworks are built) and so the platform supports the intersection of three languages. Most data types will be translated as expected but there are some cases that may require special attention.

Null Values

Objective-C does not support storing null values in collections such as arrays, dictionaries, or sets. Instead it uses a special placeholder object, NSNull, to represent a null value. In contrast, Swift uses Optionals to describe a value that might be null. Swift can manipulate NSNull values but Objective-C cannot handle Optionals (although, in some contexts, the runtime will automatically map optionals into the underlying value or NSNull). These NSNull objects can appear regardless of which language you are using.

As an example, consider the following object being passed to a Capacitor plugin call:

{ 'foo': null, 'bar': [1, 2, null, 4]}


CAPPluginCall stores this data as its options property but has a variety of convenience accessors that operate on it. The accessors will cast the value to the expected type(s) so NSNull values will get filtered out.

if let value = call.getString("foo") {
    // GOOD: `value` is nil, so this block won't run

However, accessing the storage property directly can return an NSNull object.

if call.options["foo"] != nil {
    // BAD: the key returned a truthy `NSNull` object, so this block will run

It is not recommended to rely on the presence of a key to convey meaning. Always type-check the corresponding value to evaluate it.


Since accessing an array typically requires typing the whole collection, it is important to consider if it contains a single type or might be heterogeneous.

if let values = call.getArray("bar") {
    // NEUTRAL: the array is all valid objects, so this block will run, but each value will need to be typed individually
if let values = call.getArray("bar", Int?) {
    // BAD: the array is a mix of `Int` and `NSNull` and can't be cast to `Int?`, so this block won't run

To help with this behavior, Capacitor includes a convenience extension that can map an array with NSNull values into an array of optionals. It works on the JSValue protocol, which represents all of the valid types that can be bridged between environments, but can be cast to a specific subtype.

if let values = call.getArray("bar").capacitor.replacingNullValues() as? [Int?] {
    // GOOD: `values` is now cast to `Int?` with `nil` at index 2


In most situations, dates should work as expected. Any Date object sent from JavaScript or Date or NSDate object returned from a plugin will be serialized into an ISO 8601 string.

However, part of this behavior can be changed if needed. Data moving from the web runtime to native iOS code uses a different mechanism than data going in the other direction. WKWebView automatically transforms JavaScript Date objects into native Date objects. For consistency with other platforms and to match developer expectations, Capacitor will serialize these objects before passing them to the plugin starting in 3.0. If you want to opt-out of this behavior, set the shouldStringifyDatesInCalls property on your plugin.

override func load() {
    shouldStringifyDatesInCalls = false

The CAPPluginCall convenience accessor getDate will handle both data types and return a Date object.

Data moving from native code to the web view will be serialized as JSON. Since JSON does not officially define dates, including a Date object in a plugin’s results would throw an exception prior to 3.0. But Capacitor will now automatically serialize any Date objects into strings as per convention. If your plugin needs to handle dates differently, serialize them into some other supported JSON type first.

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