Fix rounding error with values less than 1y.

src/formatPrefixAuto.js

@@ -0,0 +1,16 @@
+import formatDecimal from "./formatDecimal";
+
+export var prefixExponent;
+
+export default function(x, p) {
+  var d = formatDecimal(x, p);
+  if (!d) return x + "";
+  var coefficient = d[0],
+      exponent = d[1],
+      i = exponent - (prefixExponent = Math.max(-8, Math.min(8, Math.floor(exponent / 3))) * 3) + 1,
+      n = coefficient.length;
+  return i === n ? coefficient
+      : i > n ? coefficient + new Array(i - n + 1).join("0")
+      : i > 0 ? coefficient.slice(0, i) + "." + coefficient.slice(i)
+      : "0." + new Array(1 - i).join("0") + formatDecimal(x, p + i - 1)[0]; // less than 1y!
+};

src/formatRounded.js

@@ -4,12 +4,8 @@ export default function(x, p) {
   var d = formatDecimal(x, p);
   if (!d) return x + "";
   var coefficient = d[0],
-      exponent = d[1],
-      i = exponent + 1;
-  return i <= 0
-      ? "0." + new Array(1 - i).join("0") + coefficient
-      : coefficient.slice(0, i)
-      + (coefficient.length > i
-      ? "." + coefficient.slice(i)
-      : new Array(i - coefficient.length + 1).join("0"));
+      exponent = d[1];
+  return exponent < 0 ? "0." + new Array(-exponent).join("0") + coefficient
+      : coefficient.length > exponent + 1 ? coefficient.slice(0, exponent + 1) + "." + coefficient.slice(exponent + 1)
+      : coefficient + new Array(exponent - coefficient.length + 2).join("0");
 };

src/formatTypes.js

@@ -1,7 +1,6 @@
-import formatAutoPrefix from "./formatAutoPrefix";
 import formatDefault from "./formatDefault";
+import formatPrefixAuto from "./formatPrefixAuto";
 import formatRounded from "./formatRounded";
-import formatRoundedPercentage from "./formatRoundedPercentage";
 
 export default {
   "": formatDefault,
@@ -13,9 +12,9 @@ export default {
   "f": function(x, p) { return x.toFixed(p); },
   "g": function(x, p) { return x.toPrecision(p); },
   "o": function(x) { return x.toString(8); },
-  "p": formatRoundedPercentage,
+  "p": function(x, p) { return formatRounded(x * 100, p); },
   "r": formatRounded,
-  "s": formatAutoPrefix,
+  "s": formatPrefixAuto,
   "X": function(x) { return x.toString(16).toUpperCase(); },
   "x": function(x) { return x.toString(16); }
 };

src/localeFormat.js

@@ -2,7 +2,7 @@ import exponent from "./exponent";
 import formatGroup from "./formatGroup";
 import formatSpecifier from "./formatSpecifier";
 import formatTypes from "./formatTypes";
-import {prefixExponent} from "./formatAutoPrefix";
+import {prefixExponent} from "./formatPrefixAuto";
 
 var prefixes = ["y","z","a","f","p","n","µ","m","","k","M","G","T","P","E","Z","Y"];
 

test/format-type-s-test.js

@@ -42,26 +42,26 @@ tape("format(\"[.precision]s\") outputs SI-prefix notation with precision signif
 
 tape("format(\"s\") formats numbers smaller than 1e-24 with yocto", function(test) {
   var f = format.format(".8s");
-  test.equal(f(1.23e-30), "0.0000012y");
-  test.equal(f(1.23e-29), "0.0000123y");
-  test.equal(f(1.23e-28), "0.0001230y");
-  test.equal(f(1.23e-27), "0.0012300y");
-  test.equal(f(1.23e-26), "0.0123000y");
-  test.equal(f(1.23e-25), "0.1230000y");
-  test.equal(f(1.23e-24), "1.2300000y");
-  test.equal(f(1.23e-23), "12.300000y");
-  test.equal(f(1.23e-22), "123.00000y");
-  test.equal(f(1.23e-21), "1.2300000z");
-  test.equal(f(-1.23e-30), "-0.0000012y");
-  test.equal(f(-1.23e-29), "-0.0000123y");
-  test.equal(f(-1.23e-28), "-0.0001230y");
-  test.equal(f(-1.23e-27), "-0.0012300y");
-  test.equal(f(-1.23e-26), "-0.0123000y");
-  test.equal(f(-1.23e-25), "-0.1230000y");
-  test.equal(f(-1.23e-24), "-1.2300000y");
-  test.equal(f(-1.23e-23), "-12.300000y");
-  test.equal(f(-1.23e-22), "-123.00000y");
-  test.equal(f(-1.23e-21), "-1.2300000z");
+  test.equal(f(1.29e-30), "0.0000013y"); // Note: rounded!
+  test.equal(f(1.29e-29), "0.0000129y");
+  test.equal(f(1.29e-28), "0.0001290y");
+  test.equal(f(1.29e-27), "0.0012900y");
+  test.equal(f(1.29e-26), "0.0129000y");
+  test.equal(f(1.29e-25), "0.1290000y");
+  test.equal(f(1.29e-24), "1.2900000y");
+  test.equal(f(1.29e-23), "12.900000y");
+  test.equal(f(1.29e-22), "129.00000y");
+  test.equal(f(1.29e-21), "1.2900000z");
+  test.equal(f(-1.29e-30), "-0.0000013y"); // Note: rounded!
+  test.equal(f(-1.29e-29), "-0.0000129y");
+  test.equal(f(-1.29e-28), "-0.0001290y");
+  test.equal(f(-1.29e-27), "-0.0012900y");
+  test.equal(f(-1.29e-26), "-0.0129000y");
+  test.equal(f(-1.29e-25), "-0.1290000y");
+  test.equal(f(-1.29e-24), "-1.2900000y");
+  test.equal(f(-1.29e-23), "-12.900000y");
+  test.equal(f(-1.29e-22), "-129.00000y");
+  test.equal(f(-1.29e-21), "-1.2900000z");
   test.end();
 });