Calculate the function f(x) for a simple linear sequence (Medium).js

/*
Description:
This is a follow-up from my previous Kata which can be found here: http://www.codewars.com/kata/5476f4ca03810c0fc0000098

This time, for any given linear sequence, calculate the function [f(x)] and return it as a function in Javascript or Lambda/Block in Ruby.

For example:

getFunction([0, 1, 2, 3, 4])(5) => 5
getFunction([0, 3, 6, 9, 12])(10) => 30
getFunction([1, 4, 7, 10, 13])(20) => 61
Assumptions for this kata are:

The sequence argument will always contain 5 values equal to f(0) - f(4).
The function will always be in the format "nx +/- m", 'x +/- m', 'nx', 'x' or 'm'
If a non-linear sequence simply return 'Non-linear sequence' for javascript, ruby, and python. For C#, throw an ArgumentException.
*/
function getFunction(sequence) {
  const n = sequence[1]-sequence[0]
  const check = sequence[1]-sequence[0]===sequence[2]-sequence[1]
  if (!check) return'Non-linear sequence'
  return (x)=>x*n+sequence[0]
}