fix(flow): add flowtype tests

[evenchange4]

resolved #66

[codecov]

Codecov Report

Merging #67 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #67   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files           5        5           
  Lines          44       44           
  Branches        6        6           
=======================================
  Hits           42       42           
  Misses          2        2

Impacted Files	Coverage Δ
src/index.js	66.66% <ø> (ø)	⬆️

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