Add params.js, for function param object parsing.

js/misc/Makefile.am

@@ -2,4 +2,5 @@ jsmiscdir = $(pkgdatadir)/js/misc
 
 dist_jsmisc_DATA =		\
 	docInfo.js		\
-	format.js
+	format.js		\
+	params.js

js/misc/params.js

@@ -0,0 +1,33 @@
+/* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+
+// parse:
+// @params: caller-provided parameter object, or %null
+// @default: function-provided defaults object
+// @allowExtras: whether or not to allow properties not in @default
+//
+// Examines @params and fills in default values from @defaults for
+// any properties in @defaults that don't appear in @params. If
+// @allowExtras is not %true, it will throw an error if @params
+// contains any properties that aren't in @defaults.
+//
+// If @params is %null, this returns @defaults.
+//
+// Return value: the updated params
+function parse(params, defaults, allowExtras) {
+    if (!params)
+        return defaults;
+
+    if (!allowExtras) {
+	for (let prop in params) {
+	    if (!(prop in defaults))
+		throw new Error('Unrecognized parameter "' + prop + '"');
+	}
+    }
+
+    for (let prop in defaults) {
+	if (!(prop in params))
+	    params[prop] = defaults[prop];
+    }
+
+    return params;
+}