Final answer:
In most programming languages, an input property is set to undefined when no value has been provided. Developers can specify a default value, but without one, undefined is the standard.
Step-by-step explanation:
When an input property has not been provided a value, the value is typically set to undefined. This is the default behavior in most programming languages, such as JavaScript. In some circumstances, a developer may set an explicit default value to be used when no value is provided; this ensures that the property has a predictable value. If a default value is not explicitly set by the developer, then the property will have the value undefined.