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  1. Core Server
  2. SERVER-34281

ES6 arrow functions (lambdas) do not work with map-reduce

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Details

    • Bug
    • Status: Closed
    • Major - P3
    • Resolution: Gone away
    • None
    • None
    • JavaScript, MapReduce
    • None
    • Query Execution
    • ALL
    • Hide 

      var db = db.getSiblingDB('test');
      		 
      db.lamdba_strangeness.insertMany([{a:1}, {a:2}, {a:3}]);
      		 
       
      		 
      var mapFunction = function() { emit(this.ns, Object.bsonsize(this)); };
      		 
      var mapLamdba = () => { emit(this.ns, Object.bsonsize(this)) };
      		 
       
      		 
      var reduce = function(key, values) {
      		 
        return Array.sum(values);
      		 
      };
      		 
       
      		 
      var options = {
      		 
        out: {inline:1}
      		 
      };
      		 
       
      		 
      print("Using function() {}:");
      		 
      printjson(db.lamdba_strangeness.mapReduce(mapFunction, reduce, options).results);
      		 
      print("Using () => {}:");
      		 
      printjson(db.lamdba_strangeness.mapReduce(mapLamdba, reduce, options).results);
      		 
       
      		 
      db.lamdba_strangeness.drop();

      Results:

      $ mongo lamdba_strangeness.js
      		 
      MongoDB shell version v3.6.3
      		 
      connecting to: mongodb://127.0.0.1:27017
      		 
      MongoDB server version: 3.6.3
      		 
      Using function() {}:
      		 
      [ { "_id" : null, "value" : 99 } ]
      		 
      Using () => {}:
      		 
      [ ]

      Show
      var db = db.getSiblingDB('test'); db.lamdba_strangeness.insertMany([{a:1}, {a:2}, {a:3}]);   var mapFunction = function() { emit(this.ns, Object.bsonsize(this)); }; var mapLamdba = () => { emit(this.ns, Object.bsonsize(this)) };   var reduce = function(key, values) { return Array.sum(values); };   var options = { out: {inline:1} };   print("Using function() {}:"); printjson(db.lamdba_strangeness.mapReduce(mapFunction, reduce, options).results); print("Using () => {}:"); printjson(db.lamdba_strangeness.mapReduce(mapLamdba, reduce, options).results);   db.lamdba_strangeness.drop(); Results: $ mongo lamdba_strangeness.js MongoDB shell version v3.6.3 connecting to: mongodb://127.0.0.1:27017 MongoDB server version: 3.6.3 Using function() {}: [ { "_id" : null, "value" : 99 } ] Using () => {}: [ ]

    Description

      Because ES6 arrow functions (aka lambdas) do not bind this (see Arrow functions), they do not work with the map-reduce framework in MongoDB.

      For many people, lamdbas are seen as a more concise syntax and do not realize they behave differently. Either we should improve map-reduce to support ES6 arrow functions or clearly document that they are not supported (and why) in the map portion of map-reduce.

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        Issue Links

          related to

          Improvement - An improvement or enhancement to an existing feature or task. SERVER-50614 JS 'Set' type does not serialize well to BSON

          • Major - P3 - Major loss of function.
          • Backlog

          Improvement - An improvement or enhancement to an existing feature or task. SERVER-46243 Support ES6 Map object in MongoDB shell

          • Minor - P4 - Minor loss of function, or other problem where easy workaround is present.
          • Backlog

          Improvement - An improvement or enhancement to an existing feature or task. SERVER-31551 Create a test which enumerates the objects available in server side JS environments

          • Major - P3 - Major loss of function.
          • Closed

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              backlog-query-execution Backlog - Query Execution
              james.kovacs@mongodb.com James Kovacs
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                Updated:
                Resolved: