as soon as; the minute (that); no sooner than
Used after the dictionary form of a verb to indicate that one action happens immediately after another. Often found in written or formal contexts.
彼は家に着くや否や、電話をかけた。
As soon as he arrived home, he made a phone call.
ベルが鳴るや否や、生徒たちは教室を飛び出し
Rare and somewhat archaic use meaning 'whether or not'. Modern Japanese typically uses かどうか or 否か instead.
行くや否や、まだ決めていない。
I haven't decided yet whether or not to go.
The minute the bell rang, the students rushed out of the classroom.
や否や is a more literary, formal way to say 'as soon as', whereas 途端に is the common, everyday choice and often adds a nuance of surprise.
や否や is a more formal, literary way of saying 'as soon as', while 途端 is common in everyday speech and writing.
A classical Japanese construction combining the particle や (expressing doubt or question) and 否 (いな, 'no'), literally meaning 'whether or not'. Over time, it developed the sense of immediate succession after a verb.