javascript count key value pairs

Question

I want to iterate over an object array and return another object array with count of records based on a particular property value kind of like a hashtable.

For example:

Array1 = [
  { "name": "Pam", "role": "ceo" },
  { "name": "Joel", "role": "engineer" },
  { "name": "Mary", "role": "ceo" },
  { "name": "Alice", "role": "analyst" },
  { "name": "John", "role": "analyst" },
  { "name": "Nick", "role": "engineer" },
  { "name": "Sam", "role": "analyst" }
]

Expected output:

Array2 = [
  { key: "ceo", count: 2 },
  { key: "engineer", count: 2 },
  { key: "analyst", count: 3 }
]

I need a code that generates Array2 from Array1. Really appreciate the help :)

Solved my problem, thanks

Answer 1

Should be fairly straightforward to understand. Check the code.

 let array1 = [ { name: "Pam", role: "ceo" }, { name: "Joel", role: "engineer" }, { name: "Mary", role: "ceo" }, { name: "Alice", role: "analyst" }, { name: "John", role: "analyst" }, { name: "Nick", role: "engineer" }, { name: "Sam", role: "analyst" } ]; let obj = {}; array1.forEach(entry => (obj[entry.role] = (obj[entry.role] || 0) + 1)); let array2 = []; for (role in obj) { array2.push({ key: role, count: obj[role] }); } console.log(array2); 

Answer 2

You can use reduce to group the array of objects into a single object. Use Object.values to convert the object back into an array.

 let Array1 = [{"name":"Pam", "role":"ceo"}, {"name":"Joel", "role":"engineer"}, {"name":"Mary", "role":"ceo"}, {"name":"Alice", "role":"analyst"}, {"name":"John", "role":"analyst"}, {"name":"Nick", "role":"engineer"}, {"name":"Sam", "role":"analyst"}] let Array2 = Object.values(Array1.reduce((c, {role}) => { c[role] = c[role] || {key: role,count: 0}; c[role].count++; return c; }, {})); console.log(Array2); 

Answer 3

You have to create a hashmap/object to keep track of the unique counts.

For example:

 let Array1=[{"name":"Pam", "role":"ceo"}, {"name":"Joel", "role":"engineer"}, {"name":"Mary", "role":"ceo"}, {"name":"Alice", "role":"analyst"}, {"name":"John", "role":"analyst"}, {"name":"Nick", "role":"engineer"}, {"name":"Sam", "role":"analyst"}] let hashMap = {} for(var employee of Array1){ //if that role exists if(employee.role in hashMap ){ //up the prev count hashMap[employee.role] = hashMap[employee.role] + 1; }else{ hashMap[employee.role] = 1; } } //now we will iterate through those keys of the Map and format it for Array 2 let outputArray = [] Object.keys(hashMap).forEach(key => { outputArray.push({ key, count: hashMap[key] }) }) console.log(outputArray) 

Answer 4

Here you have two variations, as object and as array. (ES6 required)

 let arr = [ {"name":"Pam", "role":"ceo"}, {"name":"Joel", "role":"engineer"}, {"name":"Mary", "role":"ceo"}, {"name":"Alice", "role":"analyst"}, {"name":"John", "role":"analyst"}, {"name":"Nick", "role":"engineer"}, {"name":"Sam", "role":"analyst"} ] let objResult = arr.reduce((acc, obj) => { acc[obj.role] = (acc[obj.role] || 0) + 1; return acc; }, {}); console.log(objResult); let arrResult = Object.keys(objResult).map(k => ({[k]: objResult[k]})); console.log(arrResult)