Map value of an attribute in one object array to an attribute in another object array Javascript

Question

I have two arrays of objects:

courses = [ { _id: 999, courseCode: "Eng1" },
{ _id: 777, courseCode: "Sci1" },
{ _id: 666, courseCode: "Eng2" },
{ _id: 888, courseCode: "Sci2" }  ]

sectionCourses = [ { sectionCode: "1A", courseId: "999" },
{ sectionCode: "1A", courseId: "777" },
{ sectionCode: "2A", courseId: "666" }, 
{ sectionCode: "2A", courseId: "888" }  ]

I want to filter the courses array in such a way that it contains only the courses that are not in a section. For example if I select section with sectionCode: "2A", the courses array should only contain

courses = [ { _id: 999, courseCode: "Eng1" },
    { _id: 777, courseCode: "Sci1" },
    { _id: 888, courseCode: "Sci2" }  ]

I tried to do this way:

 courses = courses.filter(c => !(sectionCourses.includes(c._id)))

but I know this is incomplete because I can't figure out how to access courseId in sectionCourses.

Please help.

The question is not clear.Not able to understand how on selecting 2A will have new array.In new array i can see _id:888 — brk
– brk, Commented Dec 29, 2017 at 13:53
Do you specifically need to use the filter method? Why not just loop through? — Mackija
– Mackija, Commented Dec 29, 2017 at 13:55
includes() only works to compare whole elements. Use Array#some() or Array#find() — charlietfl
– charlietfl, Commented Dec 29, 2017 at 13:56

cнŝdk · Accepted Answer · 2017-12-29 14:05:54Z

You can't use .includes() method to find the whole object by its _id, includes compares the whole objects and doesn't search for a specific property.

What you can do here is to get an array of courseIds to be ignored based on the sectionCode you provided, and then filter the courses that their _id doesn't exist in this array of ids:

function getCourses(catCode) {

  var coursesIdstoIgnore = sectionCourses.filter(s => s.sectionCode === catCode).map(s => s.courseId);
  return courses.filter(c => coursesIdstoIgnore.indexOf(c["_id"].toString()) == -1);
}

Demo:

var courses = [{
    _id: 999,
    courseCode: "Eng1"
  },
  {
    _id: 777,
    courseCode: "Sci1"
  },
  {
    _id: 666,
    courseCode: "Eng2"
  },
  {
    _id: 888,
    courseCode: "Sci2"
  }
];

var sectionCourses = [{
    sectionCode: "1A",
    courseId: "999"
  },
  {
    sectionCode: "1A",
    courseId: "777"
  },
  {
    sectionCode: "2A",
    courseId: "666"
  },
  {
    sectionCode: "2A",
    courseId: "888"
  }
];

function getCourses(catCode) {
  var cousesIdstoIgnore = sectionCourses.filter(s => s.sectionCode === catCode).map(s => s.courseId);
  console.log(cousesIdstoIgnore);

  return courses.filter(c => cousesIdstoIgnore.indexOf(c["_id"].toString()) == -1);

}

var results = getCourses("2A");


console.log(results);

Francis Leigh · Accepted Answer · 2018-01-01 15:29:28Z

1

courses.filter(course => sectionCourses.find(section => +section.courseId === +course._id))

Note how i use the +operator before of the courseId and _id properties. this automatically turns a String typed number into a Number. e.g

+"1" = 1
+1 = 1

This is very useful for slight comparison gotchas when using ===

Note Array.find() doesn't work with IE

edited Jan 1, 2018 at 15:29

answered Dec 29, 2017 at 14:07

Francis Leigh

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