{"id":13713,"date":"2022-08-25T13:18:18","date_gmt":"2022-08-25T20:18:18","guid":{"rendered":"https:\/\/blog.couchbase.com\/?p=13687"},"modified":"2025-06-13T17:32:00","modified_gmt":"2025-06-14T00:32:00","slug":"from-n1ql-to-javascript-and-back-part-4","status":"publish","type":"post","link":"https:\/\/www.couchbase.com\/blog\/from-n1ql-to-javascript-and-back-part-4\/","title":{"rendered":"From N1QL to JavaScript and back – Part 4: Error Handling"},"content":{"rendered":"

In previous blogs, we have covered executing N1QL from JavaScript functions<\/a>, processing documents through iterators<\/a>, and manipulating data<\/a>.<\/span><\/p>\n

We now move onto handling errors from N1QL statements.<\/span><\/p>\n

Error handling<\/span><\/h2>\n

When an error of any nature is encountered, the jsevaluator<\/em> by default will halt the function execution and return an error:<\/span><\/p>\n

\"\"<\/p>\n

In this particular case, the second INSERT will fail because key <\/span>k10<\/span><\/em> already exists:<\/span><\/p>\n

[\r\n\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\"code\": 10109,\r\n\u00a0\u00a0\u00a0\u00a0\"msg\": \"Error executing function 'doInsert' (n1ql:doInsert)\",\r\n\u00a0\u00a0\u00a0\u00a0\"reason\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"details\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Code\": \"\u00a0 \u00a0 var q2 = N1QL('insert into b1 values(\\\\\\\"k10\\\\\\\", {\\\\\\\"f1\\\\\\\": 10});', {}, true);\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Exception\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"caller\": \"couchbase:2088\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"cause\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"caller\": \"couchbase:1961\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"code\": 17012,\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"key\": \"dml.statement.duplicatekey\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"message\": \"Duplicate Key: k10\"\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0},\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"code\": 12009,\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"icause\": \"Duplicate Key: k10\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"key\": \"datastore.couchbase.DML_error\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"message\": \"DML Error, possible causes include concurrent modification. Failed to perform INSERT on key k10\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"retry\": false\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0},\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Location\": \"functions\/n1ql.js:21\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Stack\": \" \u00a0 at doInsert (functions\/n1ql.js:3:14)\"\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0},\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"type\": \"Exceptions from JS code\"\r\n\u00a0\u00a0\u00a0\u00a0}\r\n\u00a0\u00a0}\r\n]<\/pre>\n
You don’t have to stick to the default behaviour, JavaScript allows to catch errors and handle them:<\/span><\/p>\n
\"\"<\/p>\n
In this case, the function <\/span>doInsert()<\/span><\/em> returns <\/span>failure<\/span> because key <\/span>k10<\/span><\/em> already exists.<\/span><\/p>\n
Return vs throw<\/span><\/h3>\n
The <\/span>catch<\/span><\/em> block can be used to handle the failure in whichever way is useful for your business logic.<\/span><\/p>\n
The available options boil down to 1) take some evasive action and continue execution, 2) return early with success, and 3) return early with failure.<\/span><\/p>\n
The difference between success and failure is simply using <\/span>return<\/span> to return a value as in the example above vs <\/span>throw<\/span><\/em> to return an error.<\/span><\/p>\n
\"\"<\/p>\n
The difference between the two is important, because <\/span>if you choose to return a result, the caller request will continue execution. <\/b>While if you throw an error, the whole request will fail with the error, so do be mindful about which you want to use, because it will make the difference between your request working correctly and misbehaving.<\/span><\/p>\n
Returned expression<\/span><\/h3>\n
The second thing to bear in mind is that, whether you return or throw an error, you may very well want to give meaningful information.<\/span><\/p>\n
Consider the following code:<\/span><\/p>\n
\"\"<\/p>\n
This will fail with something like:<\/span><\/p>\n
[\r\n\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\"code\": 10109,\r\n\u00a0\u00a0\u00a0\u00a0\"msg\": \"Error executing function 'doInsert' (n1ql:doInsert)\",\r\n\u00a0\u00a0\u00a0\u00a0\"reason\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"details\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Code\": \" \u00a0 \u00a0 \u00a0 throw\u00a0 err;\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Exception\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"caller\": \"couchbase:2088\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"cause\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"caller\": \"couchbase:1961\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"code\": 17012,\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"key\": \"dml.statement.duplicatekey\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"message\": \"Duplicate Key: k10\"\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0},\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"code\": 12009,\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"icause\": \"Duplicate Key: k10\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"key\": \"datastore.couchbase.DML_error\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"message\": \"DML Error, possible causes include concurrent modification. Failed to perform INSERT on key k10\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"retry\": false\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0},\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Location\": \"functions\/n1ql.js:6\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Stack\": \" \u00a0 at doInsert (functions\/n1ql.js:3:17)\"\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0},\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"type\": \"Exceptions from JS code\"\r\n\u00a0\u00a0\u00a0\u00a0}\r\n\u00a0\u00a0}\r\n]<\/pre>\n
Much like the previous example, what we gather from this is that\u00a0<\/span>err<\/span><\/em> is an object, and is returned in the <\/span>reason<\/span><\/em> field.<\/span><\/p>\n
However, if you amend the <\/span>throw<\/span><\/em> statement like so:<\/span><\/p>\n
\"\"<\/p>\n
The <\/span>doInsert()<\/span><\/em> function will now throw a string and not an object:<\/span><\/p>\n
[\r\n\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\"code\": 10109,\r\n\u00a0\u00a0\u00a0\u00a0\"msg\": \"Error executing function 'doInsert' (n1ql:doInsert)\",\r\n\u00a0\u00a0\u00a0\u00a0\"reason\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"details\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Code\": \" \u00a0 \u00a0 \u00a0 throw \\\"failure\\\" + err;\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Exception\": \"failure {\\\"caller\\\":\\\"couchbase:2088\\\",\\\"cause\\\":{\\\"caller\\\":\\\"couchbase:1961\\\",\\\"code\\\":17012,\\\"key\\\":\\\"dml.statement.duplicatekey\\\",\\\"message\\\":\\\"Duplicate Key: k10\\\"},\\\"code\\\":12009,\\\"icause\\\":\\\"Duplicate Key: k10\\\",\\\"key\\\":\\\"datastore.couchbase.DML_error\\\",\\\"message\\\":\\\"DML Error, possible causes include concurrent modification. Failed to perform INSERT on key k10\\\",\\\"retry\\\":false}\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"Location\": \"functions\/n1ql.js:6\"\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0},\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"type\": \"Exceptions from JS code\"\r\n\u00a0\u00a0\u00a0\u00a0}\r\n\u00a0\u00a0}\r\n]<\/pre>\n
The <\/span>err<\/span><\/em> object is now marshalled into a string, and all of a sudden, your error is not as legible.<\/span><\/p>\n
This applies to return<\/em> too: in particular, by default <\/span>err<\/span><\/em> contains strings containing marshalled JSON objects.<\/span><\/p>\n
For instance:<\/span><\/p>\n
\"\"<\/p>\n
Returns:<\/span><\/p>\n
[\r\n\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\"message\": \"{\\\"caller\\\":\\\"couchbase:2088\\\",\\\"cause\\\":{\\\"caller\\\":\\\"couchbase:1961\\\",\\\"code\\\":17012,\\\"key\\\":\\\"dml.statement.duplicatekey\\\",\\\"message\\\":\\\"Duplicate Key: k10\\\"},\\\"code\\\":12009,\\\"icause\\\":\\\"Duplicate Key: k10\\\",\\\"key\\\":\\\"datastore.couchbase.DML_error\\\",\\\"message\\\":\\\"DML Error, possible causes include concurrent modification. Failed to perform INSERT on key k10\\\",\\\"retry\\\":false}\",\r\n\u00a0\u00a0\u00a0\u00a0\"stack\": \"Error\\n\u00a0 \u00a0 at doInsert (functions\/n1ql.js:3:17)\"\r\n\u00a0\u00a0}\r\n]<\/pre>\n
If you want to obtain a nice returned value, you have to parse the error.message<\/em> object before returning it:<\/span><\/p>\n
\"\"<\/p>\n
Which yields:<\/span><\/p>\n
[\r\n\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\"caller\": \"couchbase:2088\",\r\n\u00a0\u00a0\u00a0\u00a0\"cause\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"caller\": \"couchbase:1961\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"code\": 17012,\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"key\": \"dml.statement.duplicatekey\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"message\": \"Duplicate Key: k10\"\r\n\u00a0\u00a0\u00a0\u00a0},\r\n\u00a0\u00a0\u00a0\u00a0\"code\": 12009,\r\n\u00a0\u00a0\u00a0\u00a0\"icause\": \"Duplicate Key: k10\",\r\n\u00a0\u00a0\u00a0\u00a0\"key\": \"datastore.couchbase.DML_error\",\r\n\u00a0\u00a0\u00a0\u00a0\"message\": \"DML Error, possible causes include concurrent modification. Failed to perform INSERT on key k10\",\r\n\u00a0\u00a0\u00a0\u00a0\"retry\": false\r\n\u00a0\u00a0}\r\n]<\/pre>\n
And if you want to include the stack too, you have to construct a new object like so:<\/span><\/p>\n
\"\"<\/p>\n
Which yields:<\/span><\/p>\n
[\r\n\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\"caller\": \"couchbase:2088\",\r\n\u00a0\u00a0\u00a0\u00a0\"cause\": {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"caller\": \"couchbase:1961\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"code\": 17012,\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"key\": \"dml.statement.duplicatekey\",\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"message\": \"Duplicate Key: k10\"\r\n\u00a0\u00a0\u00a0\u00a0},\r\n\u00a0\u00a0\u00a0\u00a0\"code\": 12009,\r\n\u00a0\u00a0\u00a0\u00a0\"icause\": \"Duplicate Key: k10\",\r\n\u00a0\u00a0\u00a0\u00a0\"key\": \"datastore.couchbase.DML_error\",\r\n\u00a0\u00a0\u00a0\u00a0\"message\": \"DML Error, possible causes include concurrent modification. Failed to perform INSERT on key k10\",\r\n\u00a0\u00a0\u00a0\u00a0\"retry\": false,\r\n\u00a0\u00a0\u00a0\u00a0\"stack\": \"Error\\n\u00a0 \u00a0 at doInsert (functions\/n1ql.js:3:17)\"\r\n\u00a0\u00a0}\r\n]<\/pre>\n
The moral of the story is: be mindful of how you return your error, and be mindful how you format it.<\/span><\/p>\n
Conclusion<\/span><\/h2>\n
Over the course of several blogs, we have covered executing N1QL, processing documents and manipulating data, and handling errors.<\/span><\/p>\n
Next in the series, we’ll move to prepared statements.<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"
In previous blogs, we have covered executing N1QL from JavaScript functions, processing documents through iterators, and manipulating data. We now move onto handling errors from N1QL statements. Error handling When an error of any nature is encountered, the jsevaluator by […]<\/p>\n","protected":false},"author":1782,"featured_media":13716,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":""},"categories":[1814,1816,2273,1812,2201],"tags":[1316,2133,8911],"ppma_author":[8924],"class_list":["post-13713","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-application-design","category-couchbase-server","category-eventing","category-n1ql-query","category-tools-sdks","tag-error-handling","tag-functions","tag-udf"],"acf":[],"yoast_head":"\nFrom N1QL to JavaScript and back - Part 4: Error Handling - The Couchbase Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.couchbase.com\/blog\/from-n1ql-to-javascript-and-back-part-4\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"From N1QL to JavaScript and back - Part 4: Error Handling\" \/>\n<meta property=\"og:description\" content=\"In previous blogs, we have covered executing N1QL from JavaScript functions, processing documents through iterators, and manipulating data. We now move onto handling errors from N1QL statements. 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