Use a Map
Existing answers are using Object.groupBy to create a map of departments grouped by id.
This is inappropriate as there is only a one to one relationship between department id and departments. Object.groupBy creates groups using arrays to hold members of each group. One to many.
Using a more appropriate Map to get departments by id will simplify the code a little as there is no need to do the additional array indexing when locating a department.
Don`tDon't duplicate data
NoNor can I see why you duplicate the department_id as employee_department_id which is redundant information.
const employees = [{name: 'Alice', department_id: 12}, {name: 'Bob', department_id: 13}, {name: 'Chris', department_id: 13}, {name: 'Dan', department_id: 14}, {name: 'Eve', department_id: null}];
const departments = [{department_id: 12, name: 'Sales'}, {department_id: 13, name: 'Marketing'}, {department_id: 14, name: 'Engineering'}, {department_id: 15, name: 'Accounting'}, {department_id: 16, name: 'Operations'}];
console.log(innerJoinEmpDep(departments, employees));
function innerJoinEmpDep(deps, emps) {
const depsById = new Map(deps.map(dep=> ([dep.department_id, dep])));
const innerJoin = [];
for (const emp of emps) {
const dep = depsById.get(emp.department_id);
if (dep) {
innerJoin.push({
employ: emp.name,
department: dep.name,
depId: dep.department_id
});
}
}
return innerJoin;
}
