| 'use strict'; | |
| var log = Math.log; | |
| module.exports = function (x) { | |
| if (isNaN(x)) return NaN; | |
| x = Number(x); | |
| if (x < -1) return NaN; | |
| if (x > 1) return NaN; | |
| if (x === -1) return -Infinity; | |
| if (x === 1) return Infinity; | |
| if (x === 0) return x; | |
| return 0.5 * log((1 + x) / (1 - x)); | |
| }; |