node: allow configurable util.inspect() options

Via env variables by default. So to get more object depth,
you pass the `DEBUG_DEPTH=10` env var.

For the `showHidden` option, you set `DEBUG_SHOW_HIDDEN=on`.

See the Node.js docs for the complete list of `util.inspect()` options:
https://nodejs.org/api/util.html#util_util_inspect_object_options

debug.js

@@ -125,6 +125,11 @@ function createDebug(namespace) {
   debug.useColors = exports.useColors();
   debug.color = selectColor(namespae);
 
+  // env-specific initialization logic for debug instances
+  if ('function' === typeof exports.init) {
+    exports.init(debug);
+  }
+
   return debug;
 }
 

node.js

@@ -12,6 +12,7 @@ var util = require('util');
  */
 
 exports = module.exports = require('./debug');
+exports.init = init;
 exports.log = log;
 exports.formatArgs = formatArgs;
 exports.save = save;
@@ -24,6 +25,32 @@ exports.useColors = useColors;
 
 exports.colors = [6, 2, 3, 4, 5, 1];
 
+/**
+ * Build up the default `inspectOpts` object from the environment variables.
+ *
+ *   $ DEBUG_COLORS=no DEBUG_DEPTH=10 DEBUG_SHOW_HIDDEN=enabled node script.js
+ */
+
+exports.inspectOpts = Object.keys(process.env).filter(function (key) {
+  return /^debug_/i.test(key);
+}).reduce(function (obj, key) {
+  // camel-case
+  var prop = key
+    .substring(6)
+    .toLowerCase()
+    .replace(/_([a-z])/, function (_, k) { return k.toUpperCase() });
+
+  // coerce string value into JS value
+  var val = process.env[key];
+  if (/^(yes|on|true|enabled)$/i.test(val)) val = true;
+  else if (/^(no|off|false|disabled)$/i.test(val)) val = false;
+  else if (val === 'null') val = null;
+  else val = Number(val);
+
+  obj[prop] = val;
+  return obj;
+}, {});
+
 /**
  * The file descriptor to write the `debug()` calls to.
  * Set the `DEBUG_FD` env variable to override with another value. i.e.:
@@ -41,39 +68,28 @@ var stream = 1 === fd ? process.stdout :
  */
 
 function useColors() {
-  var debugColors = (process.env.DEBUG_COLORS || '').trim().toLowerCase();
-  if (0 === debugColors.length) {
-    return tty.isatty(fd);
-  } else {
-    return '0' !== debugColors
-        && 'no' !== debugColors
-        && 'false' !== debugColors
-        && 'disabled' !== debugColors;
-  }
+  return 'colors' in exports.inspectOpts
+    ? Boolean(exports.inspectOpts.colors)
+    : tty.isatty(fd);
 }
 
 /**
- * Map %o to `util.inspect()`, since Node doesn't do that out of the box.
+ * Map %o to `util.inspect()`, all on a single line.
  */
 
-var inspect = (4 === util.inspect.length ?
-  // node <= 0.8.x
-  function (v, colors) {
-    return util.inspect(v, void 0, void 0, colors);
-  } :
-  // node > 0.8.x
-  function (v, colors) {
-    return util.inspect(v, { colors: colors });
-  }
-);
-
 exports.formatters.o = function(v) {
-  return inspect(v, this.useColors)
+  this.inspectOpts.colors = this.useColors;
+  return util.inspect(v, this.inspectOpts)
     .replace(/\s*\n\s*/g, ' ');
 };
 
+/**
+ * Map %o to `util.inspect()`, allowing multiple lines if needed.
+ */
+
 exports.formatters.O = function(v) {
-  return inspect(v, this.useColors);
+  this.inspectOpts.colors = this.useColors;
+  return util.inspect(v, this.inspectOpts);
 };
 
 /**
@@ -99,11 +115,11 @@ function formatArgs(args) {
 }
 
 /**
- * Invokes `console.error()` with the specified arguments.
+ * Invokes `util.format()` with the specified arguments and writes to `stream`.
  */
 
 function log() {
-  return stream.write(util.format.apply(this, arguments) + '\n');
+  return stream.write(util.format.apply(util, arguments) + '\n');
 }
 
 /**
@@ -202,6 +218,17 @@ function createWritableStdioStream (fd) {
   return stream;
 }
 
+/**
+ * Init logic for `debug` instances.
+ *
+ * Create a new `inspectOpts` object in case `useColors` is set
+ * differently for a particular `debug` instance.
+ */
+
+function init (debug) {
+  debug.inspectOpts = util._extend({}, exports.inspectOpts);
+}
+
 /**
  * Enable namespaces listed in `process.env.DEBUG` initially.
  */