--- /dev/null
+'use strict';
+
+var log = Math.log, sqrt = Math.sqrt;
+
+module.exports = function (x) {
+ if (isNaN(x)) return NaN;
+ x = Number(x);
+ if (x === 0) return x;
+ if (!isFinite(x)) return x;
+ if (x < 0) {
+ x = -x;
+ return -log(x + sqrt(x * x + 1));
+ }
+ return log(x + sqrt(x * x + 1));
+};