Restrict a function to one argument

Array.prototype.map calls its callback with the element, the index and the whole array. Most of the time the extra two are harmless, but hand it a function that accepts a second parameter — parseInt(string, radix) is the famous one — and you get nonsense: parseInt("7", 1) is NaN and parseInt("11", 2) is 3.

Implement unary(fn). It returns a new function that calls fn with exactly one argument, whatever else it was given: the wrapper's first argument. If the wrapper is called with no arguments it still calls fn with exactly one argument, whose value is undefined, so arguments.length inside fn is always 1. Return whatever fn returns.

What it has to do

• Pass only the first argument through to fn, dropping any others.
• Call fn with exactly one argument every time, even when the wrapper receives none.
• Return fn's return value unchanged.

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