Fix SI-prefix rounding carry below 1e-24

src/formatPrefixAuto.js

@@ -7,10 +7,21 @@ export default function(x, p) {
   if (!d) return prefixExponent = undefined, x.toPrecision(p);
   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;
+      i = exponent - (prefixExponent = Math.max(-8, Math.min(8, Math.floor(exponent / 3))) * 3) + 1;
+
+  // Below the smallest prefix (yocto), the value is shown as “0.…”, which
+  // leaves fewer significant digits; re-round to that reduced precision. This
+  // rounding can carry into a higher power of ten (e.g. 9.99 → 10), so derive
+  // the digit position from the re-rounded exponent rather than the original.
+  if (i <= 0) {
+    d = formatDecimalParts(x, Math.max(0, p + i - 1));
+    coefficient = d[0];
+    i = d[1] - prefixExponent + 1;
+  }
+
+  var 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") + formatDecimalParts(x, Math.max(0, p + i - 1))[0]; // less than 1y!
+      : "0." + new Array(1 - i).join("0") + coefficient; // less than 1y!
 }

test/format-type-s-test.js

@@ -69,6 +69,14 @@ test("format(\"s\") formats numbers smaller than 1e-24 with yocto", () => {
   assert.strictEqual(f(-1.29e-21), "−1.2900000z");
 });
 
+test("format(\"s\") carries into a higher power when rounding below 1e-24", () => {
+  assert.strictEqual(format(".3s")(9.99e-25), "1.0y"); // 0.999y → 1.0y, not 0.10y
+  assert.strictEqual(format(".2s")(9.9e-25), "1y"); // 0.99y → 1y, not 0.1y
+  assert.strictEqual(format(".3s")(9.99e-26), "0.1y"); // 0.0999y → 0.1y, not 0.01y
+  assert.strictEqual(format(".1s")(9.5e-25), "1y");
+  assert.strictEqual(format(".5s")(9.9999e-25), "1.000y");
+});
+
 test("format(\"s\") formats numbers larger than 1e24 with yotta", () => {
   const f = format(".8s");
   assert.strictEqual(f(1.23e+21), "1.2300000Z");