30-seconds-of-code: purity of dropElements

[larrybotha]

Up until dropElements all functions are pure.

dropElements, however, is destructive because of Array.shift.

It can be made pure using Array.prototype.slice:

const dropElements = (arr, func) => {
  while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1);
  return arr;
};
// dropElements([1, 2, 3, 4], n => n >= 3) -> [3,4]

Not sure if this is even a concern, but I'm happy to make a PR :)

[Chalarangelo]

Unless explicitly stated in the description, functions should not mutate their arguments. Feel free to submit a PR fixing this!

[larrybotha]

Also, for anyone who comes across this issue and is interested in the subtlety, slice will always return an array, while shift could return undefined:

// Array.prototype.slice :: array -> array
[].slice()
// => []

// Array.prototype.shift :: array -> array || undefined
[].shift()
// => undefined 

Functions that always return the same type eliminate the need to check for failure cases:

const myArr = [];

myArr.slice(1)
    .map(x => x + 1)
    .filter(x => x > 1)
// []

myArr.shift()
    // throws an error here: Cannot read property 'map' of undefined
    .map(x => x + 1)
    .filter(x => x > 1)

[lock[bot]]

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