mastermatt · August 29, 2015 13:56 · Feb 7, 2014 · Feb 7, 2014
diff --git a/precisionRound.js b/precisionRound.js
@@ -1,4 +1,3 @@
-
 /**
  * Returns the value of a float rounded using the number of decimal points provided as precision.
  *
@@ -39,7 +38,7 @@ Math.precisionRound = function( number, precision ) {
         return NaN;
     }
 
-    // In case the number is already in scientific when casted to string, eSplit off the exponent
+    // In case the number is already in scientific when casted to string, split off the exponent
     eSplit = ("" + castNumber).split( "e" );
 
     // Increase the exponent by the given precision and create a scientific notion string

diff --git a/precisionRound.js b/precisionRound.js
@@ -0,0 +1,56 @@
+
+/**
+ * Returns the value of a float rounded using the number of decimal points provided as precision.
+ *
+ * Fixes binary rounding issues eg. Math.round(1.005 * 100) / 100 === 1 that present
+ * problems for accounting and finance-related software.
+ *
+ * The normal technique for rounding with decimals is to multiply the number by the log10 of the
+ * precision, round then divide by the same log eg. for 2 decimal points `round(num * 100) / 100`.
+ * The binary rounding issue, however, causes this:  1.005 * 100 === 100.49999999999999.
+ *
+ * This method plays on the use of converting scientific notation to a number,
+ * eg 1.005e2 === 100.5 so casting: `round(num + "e2") + "e-2"` to a number will result
+ * in the desired result.
+ *
+ * @param   {number}  number        The value to round
+ * @param   {int}     precision     The optional number of decimal digits to round to
+ *
+ * @returns {number} The adjusted value
+ *
+ * Based on MDN function
+ * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/round
+ */
+Math.precisionRound = function( number, precision ) {
+    "use strict";
+
+    var nativeRound = Math.round,
+        castNumber = +number,
+        castPrecision = +precision,
+        scaledNumber, eSplit, eString;
+
+    // If the exp is undefined or zero, just use native rounding
+    if( typeof precision === "undefined" || 0 === castPrecision ) {
+        return nativeRound( number );
+    }
+
+    // If the value is not a number or the exp is not an integer...
+    if( isNaN( castNumber ) || !(typeof castPrecision === "number" && 0 === castPrecision % 1) ) {
+        return NaN;
+    }
+
+    // In case the number is already in scientific when casted to string, eSplit off the exponent
+    eSplit = ("" + castNumber).split( "e" );
+
+    // Increase the exponent by the given precision and create a scientific notion string
+    eString = (eSplit[0] + "e" + (eSplit[1] ? (+eSplit[1] + castPrecision) : castPrecision));
+
+    // Cast to number and round
+    scaledNumber = nativeRound( +eString );
+
+    // Do the same as before, backwards
+    eSplit = ("" + scaledNumber).split( "e" );
+    eString = (eSplit[0] + "e" + (eSplit[1] ? (+eSplit[1] - castPrecision) : -castPrecision));
+
+    return +eString;
+};