The phrase "just did so" is correct and can be used in written English.
It is typically used to emphasize that someone recently completed an action or task. Example: I forgot to turn off the stove, but my roommate just did so before we left the apartment.
Exact(11)
They just did so for less money and no credit.
"Guys like this just did so much for American music, and America doesn't even know about them," Mr. Powers said.
But they have never won 116 games in a season, as the Seattle Mariners just did, so they understand their position.
Things did get better for workers in that bottom 50%; they just did so moe slowly.And as we noted, income inequality has at least one positive effect:Whether these shifts were good or bad depends on your political persuasion.
What's the matter, dogsie, afraid the Davepound knows how to scrap?! Do I have to mention that I wrote two articles for CNN.com, one about the iPad and one about Ubisoft's DRM? Well I just did, so there.
It just did so through the lunacy of Samberg's Peralta.
Similar(48)
They may just do so.
She just does so much".
Have I just done so?
He just does so many things well.
It may just do so.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com