ConstrainULong type is used to specify a constraint for a property whose value is an integral number. It extends the ULongRange dictionary (which provides the ability to specify a permitted range of property values) to also support an exact value and/or an ideal value the property should take on. In addition, you can specify the value as a simple long integer value, in which case the user agent does its best to match the value once all other more stringent constraints are met.


If the value of a ConstrainULong is an object rather than a number, it may have the properties below in addition to the properties it inherits from ULongRange .

An integer specifying precise, required, value the property must have to be considered acceptable.
An integer specifying a value the property would ideally have, but which can be considered optional if necessary to find a match.


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The definition of 'ConstrainULong' in that specification.
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Technically, ConstrainULong is actually based on an intermediary dictionary named ConstrainULongRange , which adds exact and ideal to ULongRange , with ConstrainULong being a type that can be either a long integer or a ULongRange . However, for the sake of documentation clarity, the intermediate type (present only because of quirks in WebIDL syntax) is ignored here.


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