improve typing

[jimmywarting]

No description provided.

[codecov]

Codecov Report

Merging #105 (c73241f) into master (52f296c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #105   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          401       411   +10     
  Branches        59        60    +1     
=========================================
+ Hits           401       411   +10     

Impacted Files	Coverage Δ
file.js	100.00% <100.00%> (ø)
from.js	100.00% <100.00%> (ø)
index.js	100.00% <100.00%> (ø)
streams.cjs	100.00% <100.00%> (ø)

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