Is it a Harshad Number?

Jelly, 4 bytes

bSḍḷ

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How it works

bSḍḷ  Main link. Arguments: n (integer), k (base)

b     Convert n to base k.
 S    Take the sum.
   ḷ  Left; yield n.
  ḍ   Test for divisibility.

Python 2, 46 bytes

lambda n,b:n%sum(n/b**i%b for i in range(n))<1

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Python 3, 73 bytes

def f(n,b):
 if b<2:return 1
 s=0;c=n
 while n:s+=n%b;n//=b
 return c%s<1

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1 is truthy, you know.