Make suggestions when using an unknown assertion

lib/unexpected.js

@@ -66,7 +66,13 @@ var root = this;
             var handler = assertionRule.handler;
             handler.apply(assertion, args);
         } else {
-            throw new Error('Unknown assertion "' + testDescriptionString + '"');
+            var message =
+                'Unknown assertion "' + testDescriptionString + '", ' +
+                'did you mean:\n';
+
+            var similarAssertions = expect.findAssertionSimilarTo(testDescriptionString);
+            message += similarAssertions.join('\n');
+            throw new Error(message);
         }
     }
 
@@ -170,6 +176,61 @@ var root = this;
         });
     };
 
+    /**
+     * Levenshtein distance algorithm from wikipedia
+     * http://en.wikibooks.org/wiki/Algorithm_Implementation/Strings/Levenshtein_distance#JavaScript
+     */
+    function levenshteinDistance(a, b){
+        if (a.length === 0) return b.length;
+        if (b.length === 0) return a.length;
+
+        var matrix = [];
+
+        // increment along the first column of each row
+        var i;
+        for(i = 0; i <= b.length; i++){
+            matrix[i] = [i];
+        }
+
+        // increment each column in the first row
+        var j;
+        for(j = 0; j <= a.length; j++){
+            matrix[0][j] = j;
+        }
+
+        // Fill in the rest of the matrix
+        for(i = 1; i <= b.length; i++){
+            for(j = 1; j <= a.length; j++){
+                if(b.charAt(i-1) == a.charAt(j-1)){
+                    matrix[i][j] = matrix[i-1][j-1];
+                } else {
+                    matrix[i][j] = Math.min(matrix[i-1][j-1] + 1, // substitution
+                                            Math.min(matrix[i][j-1] + 1, // insertion
+                                                     matrix[i-1][j] + 1)); // deletion
+                }
+            }
+        }
+
+        return matrix[b.length][a.length];
+    }
+
+    expect.findAssertionSimilarTo = function (text) {
+        var editDistrances = [];
+        forEach(getKeys(assertions), function (assertion) {
+            var distance = levenshteinDistance(text, assertion);
+            editDistrances.push({
+                assertion: assertion,
+                distance: distance
+            });
+        });
+        editDistrances.sort(function (x, y) {
+            return x.distance - y.distance;
+        });
+        return map(editDistrances.slice(0, 5), function (editDistrance) {
+            return editDistrance.assertion;
+        });
+    };
+
     expect.addAssertion('to [not] be', function (value) {
         this.assert(this.obj === value);
     });
@@ -356,7 +417,7 @@ var root = this;
     expect.eql = eql;
 
     ///////////////////////// Helper functions ///////////////////////////////
-    
+
     function bind(fn, scope) {
         return function () {
             return fn.apply(scope, arguments);
@@ -1148,7 +1209,7 @@ var root = this;
         return JSON;
     })();
     //////////////////////////////////////////////////////////////////////////
-    
+
     // Support three module loading scenarios
     if (typeof require === 'function' && typeof exports === 'object' && typeof module === 'object') {
         // CommonJS/Node.js