Parentheses for negative values. Related d3#5.

README.md

@@ -98,6 +98,7 @@ The *sign* can be:
 
 * `-` - nothing for positive and a minus sign for negative. (Default behavior.)
 * `+` - a plus sign for positive and a minus sign for negative.
+* `(` - nothing for positive and parentheses for negative.
 * ` ` (space) - a space for positive and a minus sign for negative.
 
 The *symbol* can be:

src/formatSpecifier.js

@@ -1,7 +1,7 @@
 import formatTypes from "./formatTypes";
 
 // [[fill]align][sign][symbol][0][width][,][.precision][type]
-var re = /^(?:(.)?([<>=^]))?([+\- ])?([$#])?(0)?(\d+)?(,)?(\.\d+)?([a-z%])?$/i;
+var re = /^(?:(.)?([<>=^]))?([+\-\( ])?([$#])?(0)?(\d+)?(,)?(\.\d+)?([a-z%])?$/i;
 
 export default function(specifier) {
   return new FormatSpecifier(specifier);

src/localeFormat.js

@@ -57,13 +57,14 @@ export default function(locale) {
 
       // Convert negative to positive, and compute the prefix.
       // Note that -0 is not less than 0, but 1 / -0 is!
-      var valuePrefix = (value < 0 || 1 / value < 0 ? (value *= -1, "-") : sign === "-" ? "" : sign) + prefix;
+      var valueNegative = (value < 0 || 1 / value < 0) && (value *= -1, true),
+          valuePrefix = (valueNegative ? (sign === "(" ? sign : "-") : sign === "-" || sign === "(" ? "" : sign) + prefix;
 
       // Perform the initial formatting.
       value = formatType(value, precision);
 
       // Compute the suffix.
-      var valueSuffix = suffix + (type === "s" ? prefixes[8 + prefixExponent / 3] : "");
+      var valueSuffix = suffix + (type === "s" ? prefixes[8 + prefixExponent / 3] : "") + (valueNegative && sign === "(" ? ")" : "");
 
       // Break the formatted value into the integer “value” part that can be
       // grouped, and fractional or exponential “suffix” part that is not.

test/format-type-none-test.js

@@ -46,3 +46,15 @@ tape("format(\"$\") can output a currency", function(test) {
   test.equal(f(-4.2), "-$4.2");
   test.end();
 });
+
+tape("format(\"($\") can output a currency with parentheses for negative values", function(test) {
+  var f = format.format("($");
+  test.equal(f(0), "$0");
+  test.equal(f(.042), "$0.042");
+  test.equal(f(.42), "$0.42");
+  test.equal(f(4.2), "$4.2");
+  test.equal(f(-.042), "($0.042)");
+  test.equal(f(-.42), "($0.42)");
+  test.equal(f(-4.2), "($4.2)");
+  test.end();
+});