The ConstrainDOMString dictionary is used to specify a constraint for a property whose value is a string. It allows you to specify one or more exact string values from which one must be the parameter's value, or a set of ideal values which should be used if possible. You can also specify a single string (or an array of strings) which the user agent will do its best to match once all more stringent constraints have been applied.


The value of a ConstrainDOMString can be any of the following:

  • A single DOMString
  • An array of DOMString objects
  • An object with one or both of the following properties:
Either a single DOMString which must be the value of the property, or an array of DOMString objects one of which must be the property's value. If the property can't be set to one of the listed values, matching will fail.
Either a single DOMString or an arrray of DOMStrings specifying ideal values for the property. If possible, one of the listed values will be used, but if it's not possible, the user agent will use the closest possible match.


Specification Status Comment
Media Capture and Streams
The definition of 'ConstrainDOMString' in that specification.
Candidate Recommendation Initial definition

Technically, ConstrainDOMString is actually based on an intermediary dictionary named ConstrainDOMStringParameters, which adds exact and ideal to DOMString. However, for the sake of documentation clarity, the intermediate type (present only because of quirks in WebIDL syntax) is ignored here.

Browser compatibility

ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
ConstrainDOMStringChrome Full support YesEdge Full support ≤79Firefox Full support 50IE No support NoOpera Full support YesSafari ? WebView Android Full support YesChrome Android Full support YesFirefox Android Full support 50Opera Android Full support YesSafari iOS ? Samsung Internet Android Full support Yes


Full support
Full support
No support
No support
Compatibility unknown
Compatibility unknown

See also