Use of かと思ったら / かと思うと (in the context of as soon as, just when)


かと思ったら / かと思うと mean "just when; no sooner than" but what is their use
(oral/written, formal/normal) ? Do native use that construction naturally or
is it replaced by another one ?

And I am also curious about their origin: how did this construction acquire
its current meaning ?

> 彼女は忙しい人で、来た **かと思ったら** 、もう帰ってしまった。 She’s a busy person. Just when I thought
> she had arrived, she had already left.
>
> 急に空が暗くなった **かと思うと** 、激しく雨が降り始めた。 As soon as the sky turned black, it started
> raining heavily.

Note: this is not a duplicate with the question [Meaning of
のかと思ったら](https://japanese.stackexchange.com/questions/74881/meaning-
of-%e3%81%ae%e3%81%8b%e3%81%a8%e6%80%9d%e3%81%a3%e3%81%9f%e3%82%89)




I think it's a general-use type of idiom, not specifically for formal or
written use only.

The meaning of the idiom is almost the literal meaning of the words. "As soon
as I thought about it, ..." I'm guessing that the "no sooner than" translation
is more along the lines of "no sooner than had she arrived, ..." rather than
"arrive no sooner than 3pm."




I think it's a general-use type of idiom, not specifically for formal or
written use only.

The meaning of the idiom is almost the literal meaning of the words. "As soon
as I thought about it, ..." I'm guessing that the "no sooner than" translation
is more along the lines of "no sooner than had she arrived, ..." rather than
"arrive no sooner than 3pm."

