util: Format negative zero as '-0'

Format negative zero as '-0' instead of as '0', as it does not behave
identically to positive zero. ((-0).toString() still returns '0' as
required by ES5 9.8.1.2).

Fixes joyent/node#6548.
Closes joyent/node#6550.

lib/util.js

@@ -323,8 +323,13 @@ function formatPrimitive(ctx, value) {
                                              .replace(/\\"/g, '"') + '\'';
     return ctx.stylize(simple, 'string');
   }
-  if (isNumber(value))
+  if (isNumber(value)) {
+    // Format -0 as '-0'. Strict equality won't distinguish 0 from -0,
+    // so instead we use the fact that 1 / -0 < 0 whereas 1 / 0 > 0 .
+    if (value === 0 && 1 / value < 0)
+      return ctx.stylize('-0', 'number');
     return ctx.stylize('' + value, 'number');
+  }
   if (isBoolean(value))
     return ctx.stylize('' + value, 'boolean');
   // For some reason typeof null is "object", so special case here.

test/simple/test-util-inspect.js

@@ -34,6 +34,10 @@ Date2.prototype.foo = 'bar';
 var after = util.inspect(d);
 assert.equal(orig, after);
 
+// test positive/negative zero
+assert.equal(util.inspect(0), '0');
+assert.equal(util.inspect(-0), '-0');
+
 // test for sparse array
 var a = ['foo', 'bar', 'baz'];
 assert.equal(util.inspect(a), '[ \'foo\', \'bar\', \'baz\' ]');