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function expm1 (x) {
    // Returns exp(number) - 1, computed in a way that accurate even when the value of number is close to zero  
    // 
    // version: 1008.1718
    // discuss at: http://phpjs.org/functions/expm1    // +   original by: Brett Zamir (http://brett-zamir.me)
    // %          note 1: Precision 'n' can be adjusted as desired
    // *     example 1: expm1(1e-15);
    // *     returns 1: 1.0000000000000007e-15
    var ret=0, n = 50; // degree of precision    var factorial = function factorial (n) {
       if ((n === 0) || (n === 1)) {
           return 1;
       }
       else {          var result = (n * factorial(n-1) );
          return result;
       }
    };
    for (var i=1; i < n; i++) {        ret += Math.pow(x, i)/factorial(i);
    }
    return ret;
}

1

expm1(1e-15);

1

1.0000000000000007e-15