Did you
Didja
/ˈdɪdʒə/AssimilationReduction
In real speech
What happens to the sounds
d + y blend into /dʒ/ (“j”), and you weakens to /ə/: did you → “DID-ja”.
When to use it
Didja see that? Even polite speakers say it this way when talking fast — it's natural pronunciation, not slang.
Casual → formal
Slang
didja
Neutral
did you
Sound notes
Assimilation: neighbouring sounds change each other, e.g. t + y → “ch”, d + y → “j”.