Added an option to throw an error on no matching substitutions.

index.js

@@ -4,9 +4,14 @@ module.exports = template
 
 function template(string) {
     var args
+    var opts
 
     if (arguments.length === 2 && typeof arguments[1] === "object") {
         args = arguments[1]
+    } else if (arguments.length === 3 && typeof arguments[1] === "object" &&
+               typeof arguments[2] === "object") {
+        args = arguments[1]
+        opts = arguments[2]
     } else {
         args = new Array(arguments.length - 1)
         for (var i = 1; i < arguments.length; ++i) {
@@ -25,9 +30,13 @@ function template(string) {
             string[index + match.length] === "}") {
             return i
         } else {
-            result = args.hasOwnProperty(i) ? args[i] : null
-            if (result === null || result === undefined) {
-                return ""
+            if (opts && opts.rejectNoMatch && !args.hasOwnProperty(i)) {
+              throw new Error('No matching substitution available for the pattern');
+            } else {
+                result = args.hasOwnProperty(i) ? args[i] : null
+                if (result === null || result === undefined) {
+                  return ""
+                }
             }
 
             return result

test/string-template.js

@@ -225,3 +225,21 @@ test("Template string with underscores", function (assert) {
     assert.equal(result, "Hello James Bond, how are you?")
     assert.end()
 })
+
+test("Reject no matching substitution", function (assert) {
+    assert.throws(function() {
+      format("I am {name}. I am {age} years old",
+             {name: "Anna"}, {rejectNoMatch: true})
+    })
+    assert.end()
+})
+
+test("Allow no matching substitution", function (assert) {
+    var result
+    assert.doesNotThrow(function() {
+      result = format("I am {name}. I am {age} years old",
+             {name: "Anna"}, {rejectNoMatch: false})
+    })
+    assert.equal(result, "I am Anna. I am  years old");
+    assert.end()
+})