feat: better error message for Map

Closes chaijs#1228

lib/chai/utils/inspect.js

@@ -131,6 +131,7 @@ function formatValue(ctx, value, recurseTimes) {
   var base = ''
     , array = false
     , typedArray = false
+    , map = false
     , braces = ['{', '}'];
 
   if (isTypedArray(value)) {
@@ -144,6 +145,12 @@ function formatValue(ctx, value, recurseTimes) {
     braces = ['[', ']'];
   }
 
+  // Make Map say that they are Map
+  if (isMap(value)) {
+    map = true;
+    keys = Array.from(value.keys());
+  }
+
   // Make functions say that they are functions
   if (typeof value === 'function') {
     name = getName(value);
@@ -185,6 +192,10 @@ function formatValue(ctx, value, recurseTimes) {
     output = formatArray(ctx, value, recurseTimes, visibleKeys, keys);
   } else if (typedArray) {
     return formatTypedArray(value);
+  } else if (map) {
+    output = keys.map(function(key) {
+      return formatValue(ctx, key, recurseTimes) + ' => ' + formatValue(ctx, value.get(key), recurseTimes);
+    });
   } else {
     output = keys.map(function(key) {
       return formatProperty(ctx, value, recurseTimes, visibleKeys, key, array);
@@ -359,6 +370,10 @@ function isArray(ar) {
          (typeof ar === 'object' && objectToString(ar) === '[object Array]');
 }
 
+function isMap(m) {
+  return typeof m === 'object' && objectToString(m) === '[object Map]';
+}
+
 function isRegExp(re) {
   return typeof re === 'object' && objectToString(re) === '[object RegExp]';
 }

test/assert.js

@@ -366,6 +366,13 @@ describe('assert', function () {
     err(function () {
       assert.deepEqual(obj1, obj2);
     }, "expected { tea: \'chai\' } to deeply equal { tea: \'black\' }");
+
+    var map1 = new Map([['a', 'a']])
+      , map2 = new Map([['a', 'b']]);
+
+    err(function () {
+      assert.deepEqual(map1, map2);
+    }, "expected { 'a' => 'a' } to deeply equal { 'a' => 'b' }")
   });
 
   it('deepEqual (ordering)', function() {