Deferred: Warn on exceptions that are likely programming errors

Fixes gh-2736
Closes gh-2737

src/deferred.js

@@ -157,12 +157,17 @@ jQuery.extend( {
 											mightThrow();
 										} catch ( e ) {
 
+											if ( jQuery.Deferred.exceptionHook ) {
+												jQuery.Deferred.exceptionHook( e,
+													process.stackTrace );
+											}
+
 											// Support: Promises/A+ section 2.3.3.3.4.1
 											// https://promisesaplus.com/#point-61
 											// Ignore post-resolution exceptions
 											if ( depth + 1 >= maxDepth ) {
 
-												// Only substitue handlers pass on context
+												// Only substitute handlers pass on context
 												// and multiple values (non-spec behavior)
 												if ( handler !== Thrower ) {
 													that = undefined;
@@ -182,6 +187,12 @@ jQuery.extend( {
 							if ( depth ) {
 								process();
 							} else {
+
+								// Call an optional hook to record the stack, in case of exception
+								// since it's otherwise lost when execution goes async
+								if ( jQuery.Deferred.getStackHook ) {
+									process.stackTrace = jQuery.Deferred.getStackHook();
+								}
 								window.setTimeout( process );
 							}
 						};

src/deferred/exceptionHook.js

@@ -0,0 +1,19 @@
+define( [
+	"../core",
+	"../deferred"
+], function( jQuery ) {
+
+// These usually indicate a programmer mistake during development,
+// warn about them ASAP rather than swallowing them by default.
+var rerrorNames = /^(Eval|Internal|Range|Reference|Syntax|Type|URI)Error$/;
+
+jQuery.Deferred.exceptionHook = function( error, stack ) {
+
+	// Support: IE9
+	// Console exists when dev tools are open, which can happen at any time
+	if ( window.console && window.console.warn && error && rerrorNames.test( error.name ) ) {
+		window.console.warn( "jQuery.Deferred exception: " + error.message, stack );
+	}
+};
+
+} );

src/jquery.js

@@ -4,6 +4,7 @@ define( [
 	"./traversing",
 	"./callbacks",
 	"./deferred",
+	"./deferred/exceptionHook",
 	"./core/ready",
 	"./data",
 	"./queue",

test/unit/deferred.js

@@ -525,6 +525,65 @@ QUnit.test( "jQuery.Deferred.then - spec compatibility", function( assert ) {
 	} catch ( _ ) {}
 } );
 
+QUnit[ window.console ? "test" : "skip" ]( "jQuery.Deferred.exceptionHook", function( assert ) {
+
+	assert.expect( 1 );
+
+	var done = assert.async(),
+		defer = jQuery.Deferred(),
+		oldWarn = window.console.warn;
+
+	window.console.warn = function( msg ) {
+		assert.ok( /barf/.test( msg ), "Message: " + msg );
+	};
+	jQuery.when(
+		defer.then( function() {
+			// Should get an error
+			jQuery.barf();
+		} ).then( null, jQuery.noop ),
+		defer.then( function() {
+			// Should NOT get an error
+			throw new Error( "Make me a sandwich" );
+		} ).then( null, jQuery.noop )
+	).then( function( ) {
+		window.console.warn = oldWarn;
+		done();
+	} );
+
+	defer.resolve();
+} );
+
+QUnit[ window.console ? "test" : "skip" ]( "jQuery.Deferred.exceptionHook with stack hooks", function( assert ) {
+
+	assert.expect( 2 );
+
+	var done = assert.async(),
+		defer = jQuery.Deferred(),
+		oldWarn = window.console.warn;
+
+	jQuery.Deferred.getStackHook = function() {
+		// Default exceptionHook assumes the stack is in a form console.warn can log,
+		// but a custom getStackHook+exceptionHook pair could save a raw form and
+		// format it to a string only when an exception actually occurs.
+		// For the unit test we just ensure the plumbing works.
+		return "NO STACK FOR YOU";
+	};
+
+	window.console.warn = function( msg, stack ) {
+		assert.ok( /cough_up_hairball/.test( msg ), "Function mentioned: " + msg );
+		assert.ok( /NO STACK FOR YOU/.test( stack ), "Stack trace included: " + stack );
+	};
+	defer.then( function() {
+		jQuery.cough_up_hairball();
+	} ).then( null, function( ) {
+		window.console.warn = oldWarn;
+		delete jQuery.Deferred.getStackHook;
+		done();
+	} );
+
+	defer.resolve();
+} );
+
 QUnit.test( "jQuery.Deferred - 1.x/2.x compatibility", function( assert ) {
 
 	assert.expect( 8 );