The concept of Typeahead can best be illustrated by way of example. Consider a simple INPUT that has a SLEEP to simulate some processing in an AFTER FIELD. If the user types 123 [TAB] 456, what should happen to the 456 that the user types whilst the SLEEP / processing is occurring? After 123 has appeared in field1, a) should 456 appear in field1 as the user types, b) should 456 appear in field2 when the SLEEP finishes and the focus moves to field2, or c) should 456 not appear at all because the program was busy when it was typed.
What if 123 was not a valid value for field1 and a NEXT FIELD field1 occurred as part of the AFTER FIELD field1 logic? What if the value of field1 meant that field2 was no longer active and field3 was the next field? What if an OK/Cancel dialog appeared and needed to be acknowledged?
In all cases you want a consistent user experience no matter how long the processing takes and how quickly the user types.
Read more at
https://4js.com/ask-reuben/ig-154/
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