formatPrefix takes a value, not a prefix.

README.md

@@ -122,9 +122,9 @@ The available *type* values are:
 
 The type `n` is also supported as shorthand for `,g`.
 
-<a name="locale_formatPrefix" href="#locale_formatPrefix">#</a> <i>locale</i>.<b>formatPrefix</b>(<i>specifier</i>, <i>prefix</i>)
+<a name="locale_formatPrefix" href="#locale_formatPrefix">#</a> <i>locale</i>.<b>formatPrefix</b>(<i>specifier</i>, <i>value</i>)
 
-Equivalent to [*locale*.format](#locale_format), except the returned function will convert values to the units of the specified SI *prefix* before formatting in fixed point notation. The following prefixes are supported:
+Equivalent to [*locale*.format](#locale_format), except the returned function will convert values to the units of the appropriate [SI prefix](https://en.wikipedia.org/wiki/Metric_prefix#List_of_SI_prefixes) for the specified numeric *value* before formatting in fixed point notation. The following prefixes are supported:
 
 * `y` - yocto, 10⁻²⁴
 * `z` - zepto, 10⁻²¹
@@ -144,10 +144,10 @@ Equivalent to [*locale*.format](#locale_format), except the returned function wi
 * `Z` - zetta, 10²¹
 * `Y` - yotta, 10²⁴
 
-Unlike [*locale*.format](#locale_format) with the `s` format type, this method allows you to specify the SI *prefix* explicitly, rather than computing it dynamically based on the formatted number. In addition, the *precision* for the given *specifier* represents the number of digits past the decimal point (as with `f` fixed point notation), not the number of significant digits. For example:
+Unlike [*locale*.format](#locale_format) with the `s` format type, this method returns a formatter with a consistent SI prefix, rather than computing the prefix dynamically for each number. In addition, the *precision* for the given *specifier* represents the number of digits past the decimal point (as with `f` fixed point notation), not the number of significant digits. For example:
 
 ```js
-var f = formatPrefix(",.0", "µ");
+var f = formatPrefix(",.0", 1e-6);
 f(.00042); // "420µ"
 f(.0042); // "4,200µ"
 ```

src/localeFormat.js

@@ -100,10 +100,11 @@ export default function(locale) {
     };
   }
 
-  function formatPrefix(specifier, prefix) {
+  function formatPrefix(specifier, value) {
     var f = format((specifier = formatSpecifier(specifier), specifier.type = "f", specifier)),
-        i = prefixes.indexOf(prefix),
-        k = Math.pow(10, (i < 0 ? (prefix = "", 0) : 8 - i) * 3);
+        i = Math.max(-8, Math.min(8, Math.floor((Math.log(value) / (Math.LN10 * 3) + 1e-12)))),
+        k = Math.pow(10, -i * 3),
+        prefix = prefixes[8 + i];
     return function(value) {
       return f(k * value) + prefix;
     };

test/formatPrefix-test.js

@@ -1,20 +1,25 @@
 var tape = require("tape"),
     format = require("../");
 
-tape("formatPrefix(\"s\", prefix)(number) formats with the specified SI prefix", function(test) {
-  test.equal(format.formatPrefix(",.0s", "µ")(.00042), "420µ");
-  test.equal(format.formatPrefix(",.0s", "µ")(.0042), "4,200µ");
-  test.equal(format.formatPrefix(",.3s", "m")(.00042), "0.420m");
+tape("formatPrefix(\"s\", value)(number) formats with the SI prefix appropriate to the specified value", function(test) {
+  test.equal(format.formatPrefix(",.0s", 1e-6)(.00042), "420µ");
+  test.equal(format.formatPrefix(",.0s", 1e-6)(.0042), "4,200µ");
+  test.equal(format.formatPrefix(",.3s", 1e-3)(.00042), "0.420m");
   test.end();
 });
 
-tape("formatPrefix(\"s\", prefix)(number) treats unknown SI prefix as the empty string", function(test) {
-  test.equal(format.formatPrefix(",.0s", "F")(42), "42");
+tape("formatPrefix(\"s\", value)(number) uses yocto for very small reference values", function(test) {
+  test.equal(format.formatPrefix(",.0s", 1e-27)(1e-24), "1y");
   test.end();
 });
 
-tape("formatPrefix(\"$,s\", prefix)(number) formats with the specified SI prefix", function(test) {
-  var f = format.formatPrefix(" $12,.1s", "M");
+tape("formatPrefix(\"s\", value)(number) uses yotta for very small reference values", function(test) {
+  test.equal(format.formatPrefix(",.0s", 1e27)(1e24), "1Y");
+  test.end();
+});
+
+tape("formatPrefix(\"$,s\", value)(number) formats with the specified SI prefix", function(test) {
+  var f = format.formatPrefix(" $12,.1s", 1e6);
   test.equal(f(-42e6),  "      -$42.0M");
   test.equal(f(+4.2e6), "        $4.2M");
   test.end();