1 files changed, 28 insertions, 0 deletions
diff --git a/js/misc/params.js b/js/misc/params.js
new file mode 100644
index 0000000..817d66c
--- /dev/null
+++ b/js/misc/params.js
@@ -0,0 +1,28 @@
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+/* exported parse */
+
+// parse:
+// @params: caller-provided parameter object, or %null
+// @defaults-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 the values from @defaults.
+//
+// Return value: a new object, containing the merged parameters from
+// @params and @defaults
+function parse(params = {}, defaults, allowExtras) {
+    if (!allowExtras) {
+        for (let prop in params) {
+            if (!(prop in defaults))
+                throw new Error(`Unrecognized parameter "${prop}"`);
+        }
+    }
+
+    let defaultsCopy = Object.assign({}, defaults);
+    return Object.assign(defaultsCopy, params);
+}