just as; right after; as soon as; at that moment
Used after a verb in the past tense or a noun to indicate that something happens immediately after the preceding action or state. Often implies an unexpected or sudden result.
Just as I left the house, it started raining.
He burst out laughing the moment he heard that story.
The moment I opened the door, a cat jumped out.
I stood up. At that very moment, the lights went out.
突然 describes something happening abruptly with no set-up, while 途端に always follows a specific preceding action or event that triggers the result.
From 途端 (とたん, 'the moment when something happens') + particle に. The noun 途端 itself is composed of 途 (way, path) and 端 (edge, end), literally 'the edge of the path', metaphorically the exact moment.