fixes a bug: truncate(1.670006528897705e-10, 2) → 1.67, failed to rec…

…ognize the scientific notation...

js/base/functions.js

@@ -169,13 +169,36 @@ const safeValue = (object, key, defaultValue = undefined) => {
     return ((key in object) && object[key]) ? object[key] : defaultValue
 }
 
+
+// See https://stackoverflow.com/questions/1685680/how-to-avoid-scientific-notation-for-large-numbers-in-javascript for discussion
+
+function toFixed (x) { // avoid scientific notation for too large and too small numbers
+
+    if (Math.abs (x) < 1.0) {
+        const e = parseInt (x.toString ().split ('e-')[1])
+        if (e) {
+            x *= Math.pow (10, e-1)
+            x = '0.' + (new Array (e)).join ('0') + x.toString ().substring (2)
+        }
+    } else {
+        const e = parseInt (x.toString ().split ('+')[1])
+        if (e > 20) {
+            e -= 20
+            x /= Math.pow (10, e)
+            x += (new Array (e+1)).join ('0')
+        }
+    }
+    return x
+}
+
 // See https://stackoverflow.com/questions/4912788/truncate-not-round-off-decimal-numbers-in-javascript for discussion
 
 // > So, after all it turned out, rounding bugs will always haunt you, no matter how hard you try to compensate them.
 // > Hence the problem should be attacked by representing numbers exactly in decimal notation.
 
 const truncate_regExpCache = []
     , truncate = (num, precision = 0) => {
+        num = toFixed (num)
         const re = truncate_regExpCache[precision] || (truncate_regExpCache[precision] = new RegExp("([-]*\\d+\\.\\d{" + precision + "})(\\d)"))
         const [,result] = num.toString ().match (re) || [null, num]
         return parseFloat (result)

js/test/test_base.js

@@ -290,6 +290,7 @@ describe ('ccxt base code', () => {
         assert.equal (ccxt.truncate ( 17.569,  2),  17.56)
         assert.equal (ccxt.truncate (49.9999,  4), 49.9999)
         assert.equal (ccxt.truncate (49.99999, 4), 49.9999)
+        assert.equal (ccxt.truncate (1.670006528897705e-10, 4), 0)
     })
 
     it ('parseBalance() works', () => {