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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"async-wrap: no way to catch errors without changing the throw origin","articleBody":"Consider some long stack trace module. It can use `async_wrap` to manage the the callSite objects correctly and use the v8 `Error` hooks to modify the `.stack` property. However because v8 sets the `.stack` property in a lazy way, it is necessary to do a `try {} finally {}` around the callback.\n\nSee https://github.com/AndreasMadsen/trace/blob/master/trace.js#L53 for an example with the `tracing` module.\n\nAn okay solution solution is to use the `uncaughtException` event like this:\n\n``` js\nvar asyncWrap = process.binding('async_wrap');\n\n// Enable asyncWrap and call init hook function on async initialization\nvar asyncHooksObject = {};\nvar kCallInitHook = 0;\nasyncWrap.setupHooks(\n  asyncHooksObject,\n  asyncFunctionInitialized,\n  asyncCallbackBefore,\n  asyncCallbackAfter);\nasyncHooksObject[kCallInitHook] = 1;\n\nfunction asyncFunctionInitialized() {}\n\nfunction asyncCallbackBefore() {\n  process.once('uncaughtException', asyncCallbackError);\n}\n\nfunction asyncCallbackError(error) {\n  // Set stack by v8 magic\n  error.stack;\n\n  // changes throw origin, should not be necessary\n  throw error;\n}\n\nfunction asyncCallbackAfter() {\n  process.removeListener('uncaughtException', asyncCallbackError);\n}\n\nsetTimeout(function () {\n  badluck();\n}, 10);\n```\n\nHowever this changes the throw origin:\n\n```\n/Users/Andreas/Sites/node_modules/trace/test.js:26\n  throw error;\n        ^\nReferenceError: badluck is not defined\n    at null._onTimeout (/Users/Andreas/Sites/node_modules/trace/test.js:34:3)\n    at Timer.listOnTimeout (timers.js:90:15)\n```\n\nIf `uncaughtException` isn't used then this is the error:\n\n```\n/Users/Andreas/Sites/node_modules/trace/test.js:34\n  badluck();\n  ^\nReferenceError: badluck is not defined\n    at null._onTimeout (/Users/Andreas/Sites/node_modules/trace/test.js:34:3)\n    at Timer.listOnTimeout (timers.js:90:15)\n```\n\nThis is much more informative. It would be really nice if `async_wrap` or some other mechanism allowed something similar to the old `tracing.addAsyncListener({ error: handler })` behaviour. Such that the throw origin can be preserved.\n\n_issue tracking: https://github.com/AndreasMadsen/trace/issues/12_\n_issue tracking: https://github.com/iojs/tracing-wg/issues/7_\n","author":{"url":"https://github.com/AndreasMadsen","@type":"Person","name":"AndreasMadsen"},"datePublished":"2015-01-30T17:47:08.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":18},"url":"https://github.com/669/node/issues/669"}