Record stack for async throws assertions

If the assertion fails, the AssertionError no longer has access to the
stack of when the assertion was called. Record it before entering the
promise chain.

Author: novemberborn

lib/assert.js

@@ -29,6 +29,12 @@ class AssertionError extends Error {
 }
 exports.AssertionError = AssertionError;
 
+function getStack() {
+	const obj = {};
+	Error.captureStackTrace(obj, getStack);
+	return obj.stack;
+}
+
 function wrapAssertions(callbacks) {
 	const pass = callbacks.pass;
 	const pending = callbacks.pending;
@@ -138,7 +144,7 @@ function wrapAssertions(callbacks) {
 				coreAssertThrowsErrorArg = err;
 			}
 
-			const test = fn => {
+			const test = (fn, stack) => {
 				let actual;
 				let threw = false;
 				try {
@@ -160,13 +166,16 @@ function wrapAssertions(callbacks) {
 					throw new AssertionError({
 						assertion: 'throws',
 						message,
+						stack,
 						values
 					});
 				}
 			};
 
 			if (promise) {
-				const intermediate = promise.then(makeNoop, makeRethrow).then(test);
+				// Record stack before it gets lost in the promise chain.
+				const stack = getStack();
+				const intermediate = promise.then(makeNoop, makeRethrow).then(fn => test(fn, stack));
 				pending(this, intermediate);
 				// Don't reject the returned promise, even if the assertion fails.
 				return intermediate.catch(noop);
@@ -196,20 +205,23 @@ function wrapAssertions(callbacks) {
 				return;
 			}
 
-			const test = fn => {
+			const test = (fn, stack) => {
 				try {
 					coreAssert.doesNotThrow(fn);
 				} catch (err) {
 					throw new AssertionError({
 						assertion: 'notThrows',
 						message,
+						stack,
 						values: [formatAssertError.formatWithLabel('Threw:', err.actual)]
 					});
 				}
 			};
 
 			if (promise) {
-				const intermediate = promise.then(noop, reason => test(makeRethrow(reason)));
+				// Record stack before it gets lost in the promise chain.
+				const stack = getStack();
+				const intermediate = promise.then(noop, reason => test(makeRethrow(reason), stack));
 				pending(this, intermediate);
 				// Don't reject the returned promise, even if the assertion fails.
 				return intermediate.catch(noop);