Running b-=a, b*=a, b/=a or b%=a on global variables like: var a=1, b=2 the return type is wrong, we get double instead of int.
I tried with b = b - a and it worked, I got an int.

Repro:
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var inspect = Java.type("jdk.nashorn.test.tools.StaticTypeInspector").inspect
var a=1,b=2;
print(inspect(b-=a, " assignment substraction "))