--- /dev/null
+'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));
+};