add timed functions

src/debug.js

@@ -160,6 +160,37 @@ function createDebug(namespace) {
     return section;
   };
 
+  debug.time = function () {
+    var args = [].slice.call(arguments);
+    if (args.length < 2) {
+      throw new Error('debug.time() takes at least a debug string and a function');
+    }
+
+    var fn = args.pop();
+    if (typeof fn !== 'function') {
+      throw new Error('the last argument to debug.time() must be a function');
+    }
+
+    var isPromise = false;
+    var section = debug.begin.apply(debug, args);
+    try {
+      var result = fn();
+
+      if (typeof Promise === 'function' && result instanceof Promise) { // eslint-disable-line no-undef
+        isPromise = true;
+        result.then(function () {
+          section.end();
+        });
+      }
+
+      return result;
+    } finally {
+      if (!isPromise) {
+        section.end();
+      }
+    }
+  };
+
   // env-specific initialization logic for debug instances
   if ('function' === typeof exports.init) {
     exports.init(debug);

test/debug_spec.js

@@ -82,6 +82,46 @@ describe('debug', function () {
 
         expect(log.log).to.have.been.calledThrice;
       });
+
+      it('times a critical function', function () {
+        log.log = sinon.spy();
+
+        var result = log.time('a critical function', function () {
+          log('hello from inside');
+          return 1234;
+        });
+
+        expect(result).to.equal(1234);
+        expect(log.log).to.have.been.calledThrice;
+      });
+
+      it('should throw if there aren\'t enough arguments', function () {
+        log.log = sinon.stub();
+
+        expect(function () {
+          log.time();
+        }).to.throw('debug.time() takes at least a debug string and a function');
+
+        expect(function () {
+          log.time('hello');
+        }).to.throw('debug.time() takes at least a debug string and a function');
+
+        expect(function () {
+          log.time(function () {});
+        }).to.throw('debug.time() takes at least a debug string and a function');
+      });
+
+      it('should throw if last argument isn\'t a function', function () {
+        log.log = sinon.stub();
+
+        expect(function () {
+          log.time('hello', 1234);
+        }).to.throw('the last argument to debug.time() must be a function');
+
+        expect(function () {
+          log.time('hello', function () {}, 1235);
+        }).to.throw('the last argument to debug.time() must be a function');
+      });
     });
   });
 });