The read-only rangeUnderflow property of a ValidityState object indicates if the value of an <input>, after having been edited by the user, does not conform to the constraints set by the element's min attribute.

If the field is numeric in nature, including the date, month, week, time, datetime-local, number and range types and a min value is set, if the value doesn't conform to the constraints set by the min value, the rangeUnderflow property will be true.

Given the following:

<input type="number" min="20" max="40" step="2" />

if value < 20, rangeUnderflow will be true. When true, the element matches the :invalid and :out-of-range CSS pseudo-classes.

• ValidityState.rangeOverflow
• Constraint validation
• Forms: Data form validation
• step attribute
• max attribute