Omit needless names

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FrontEndMasters Hardcore Functional
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Functional Programming JavaScript Fundamentals
Through separations and recognitions

Separate inputs from environment 

Shows a date manipulation example. The function does not take an argument, but uses ​new Date()​ internally. Then the improved version takes year and month as arguments and returns a calculated value.

The above jsbin example shows, that a function that clearly separates its inputs are much easier to test (like the one above). You don't have to mock out secret inputs (like new Date inside)