pgj commented on code in PR #5221: URL: https://github.com/apache/couchdb/pull/5221#discussion_r1746623961
########## src/docs/src/best-practices/jsdevel.rst: ########## @@ -46,3 +46,254 @@ tips and tricks that will ease the difficulty. - Be sure to guard all document accesses to avoid exceptions when fields or subfields are missing: ``if (doc && doc.myarray && doc.myarray.length)...`` + +=========================== +JavaScript engine versions +=========================== + +Until version 3.4 Apache CouchDB used only Spidermonkey as the it's +underlying JavaScript engine. With version 3.4, it's possible to configure +CouchDB to use QuickJS. + +Recent versions of CouchDB may use the node-local ``_versions`` API endpoint to +get the current engine type and version: + +.. code-block:: bash + + % http http://adm:pass@localhost:5984/_node/_local/_versions | jq '.javascript_engine' + { + "version": "1.8.5", + "name": "spidermonkey" + } + +Spidermonkey version compatibility +================================== + +Depending on the CouchDB version and what's available on supported operating +systems, the Spidermonkey version may be any one of these: 1.8.5, 60, 68, 78, +86 or 91. Sometimes there are differences in supported features between +versions. Usually later versions only add features, so views will work on +version upgrades. However, there are a few exceptions to this. These are a few +known regression or discrepancies between versions: + +1. ``for each (var x in ...)`` + + Version ``1.8.5`` supports the ``for each (var x in ...)`` looping + expression. That's not a standard JavaScript syntax and is not supported in + later versions: + +.. code-block:: bash + + % js + js> for each (var x in [1,2]) {print(x)} + 1 + 2 + + % js91 + js> for each (var x in [1,2]) {print(x)} + typein:1:4 SyntaxError: missing ( after for: + typein:1:4 for each (var x in [1,2]) {print(x)} + typein:1:4 ....^ + +2. E4X (ECMAScript for XML) + + This is not supported in versions greater than ``1.8.5``. This feature may + be inadvertently triggered when inserting a ``.`` character between a + variable and ``(``. That would compile on ``1.8.5`` and throw a + ``SyntaxError`` on other versions: + +.. code-block:: bash + + % js + js> var xml = <root><x></x></root> + js> xml.(x) + <root> + <x/> + </root> + + % js91 + js> var xml = <root><x></x></root> + typein:1:11 SyntaxError: expected expression, got '<': + typein:1:11 var xml = <root><x></x></root> + typein:1:11 ...........^ + +3. ``toLocaleFormat(...)`` function. + + This ``Date`` function is not present in versions greater than ``1.8.5``: + +.. code-block:: bash + + % js + js> d = new Date("Dec 1, 2015 3:22:46 PM") + (new Date(1449001366000)) + js> d.toLocaleFormat("%Y-%m-%d") + "2015-12-01" + + % js91 + js> d = new Date("Dec 1, 2015 3:22:46 PM") + (new Date(1449001366000)) + js> d.toLocaleFormat("%Y-%m-%d") + typein:2:3 TypeError: d.toLocaleFormat is not a function + +4. Invalid expressions following ``function(){...}`` are not ignored any longer + and will throw an error. + + Previously, in versions less than or equal to ``1.8.5`` it was possible add + any expression following the main function definition and they were mostly + ignored: + +.. code-block:: bash + + $ http put $DB/db/_design/d4 views:='{"v1":{"map":"function(doc){emit(1,2);} if(x) a"}}' + HTTP/1.1 201 Created + { + "id": "_design/d4", + "ok": true, + "rev": "1-08a7d8b139e52f5f3df5bc27e20eeff1" + } + + % http $DB/db/_design/d4/_view/v1 + HTTP/1.1 200 OK + { + "offset": 0, + "rows": [ + { + "id": "doc1", + "key": 1, + "value": 2 + } + ], + "total_rows": 1 + } + + With higher versions of spidermonkey, that would throw a compilation error: + +.. code-block:: bash + + $ http put $DB/db/_design/d4 views:='{"v1":{"map":"function(doc){emit(1,2);} if(x) a"}}' + HTTP/1.1 400 Bad Request + { + "error": "compilation_error", + "reason": "Compilation of the map function in the 'v1' view failed: ..." + } + +5. Object key order. + + Object key order may change between versions, so any views which rely on + that order may emit different results depending on the engine version: + +.. code-block:: bash + + % js + js> r={}; ["Xyz", "abc", 1].forEach(function(v) {r[v]=v;}); Object.keys(r) + ["Xyz", "abc", "1"] + + % js91 + js> r={}; ["Xyz", "abc", 1].forEach(function(v) {r[v]=v;}); Object.keys(r) + ["1", "Xyz", "abc"] + +Using QuickJS +============= + +The QuickJS-based JavaScript engine is available as of CouchDB version 3.4. It +has to be explicitly enabled via ``[couchdb] js_engine = quickjs`` and +restarting the service. + +Generally, QuickJS engine is a bit faster, consumes less memory, and provides +slightly better isolation between contexts by re-creating the whole javascript +engine runtime on every ``reset`` command. + +To try building invidual views using QuickJS, even when the default engine is +SpiderMonkey, can use the ``"javascript_quickjs"`` as the view language, +instead of ``"javascript"``. Just that view will be rebuilt using the QuickJS +engine. However, when switching back to ``"javascript"`` the view will have to +be re-built again. + +QuickJS vs Spidermonkey incompatibilities +============================================ + +The QuickJS engine is quite compatible with Spidermonkey version 91. The same Review Comment: "SpiderMonkey" -- This is an automated message from the Apache Git Service. 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