Join JavaScript arrays (like a DB join)

joinJavaScriptArrays.js

//sort of inner join

//improved version where we directly obtain the alias from the parameter name in the selector functions
function joinCollectionsImproved(collection1, collection2, idSelectorFn1, idSelectorFn2){
    let getAlias = selectorFn => selectorFn.toString().split("=>")[0].trim();
    let [alias1, alias2] = [idSelectorFn1, idSelectorFn2].map(getAlias);

    let result = [];
    collection1.forEach(it1 => {
        let id1 = idSelectorFn1(it1);
        collection2.forEach(it2 =>{
            if (id1 === idSelectorFn2(it2))
                result.push({
                    [alias1]: it1,
                    [alias2]: it2
                });sa
        });
    });
    return result;
}

//old version where we were providing the alias as parameters
function joinCollections_old(collection1, collection2, idSelectorFn1, idSelectorFn2, alias1, alias2){
    let result = [];
    collection1.forEach(it1 => {
        let id1 = idSelectorFn1(it1);
        collection2.forEach(it2 =>{
            if (id1 === idSelectorFn2(it2))
                result.push({
                    [alias1]: it1,
                    [alias2]: it2
                });
        });
    });
    return result;
}



let joinCollections = joinCollectionsImproved;

let employees = [
    { 
        firstName: "Terry", 
        lastName: "Adams", 
        role: "FrontEnd Developer"
    },
    { 
        firstName:"Charlotte", 
        lastName:"Weiss", 
        role: "Systems Administrator" 
    },
    { 
        firstName:"Magnus", 
        lastName:"Hedland", 
        role: "Psycologist"
    }, 
    { 
        firstName:"Vernette", 
        lastName:"Price", 
        role: "Shop Assistant"
    }
];

var students = [
    { 
        firstName:"Vernette", 
        lastName:"Price", 
        studies: "Philosophy"
    },
    { 
        firstName:"Terry", 
        lastName:"Earls", 
        studies: "Computer Engineering"
     },
     { 
         firstName:"Terry", 
         lastName:"Adams", 
         studies: "Computer Engineering"
    } 
];

let salaries = [
    {
        job: "FrontEnd Developer",
        amount: 40000 
    },
    {
        job: "Shop Assistant",
        amount: 20000 
    }
];

let persons = joinCollections(employees, 
    students, 
    employee => `${employee.firstName}-${employee.lastName}`, 
    student => `${student.firstName}-${student.lastName}`,
    "employee",
    "student"
).map(ob => {
    return {
        fullName: `${ob.employee.firstName} ${ob.employee.lastName}`,
        job: ob.employee.role,
        studies: ob.student.studies
    };
});

console.log("-- persons:\n " + JSON.stringify(persons, null, "\t"));

let persons2 = joinCollections(employees, 
    students, 
    employee => `${employee.firstName}-${employee.lastName}`, 
    student => `${student.firstName}-${student.lastName}`,
    "employee",
    "student"
)
.filter(ob => ob.employee.role === "FrontEnd Developer")
.map(ob => {
    return {
        fullName: `${ob.employee.firstName} ${ob.employee.lastName}`,
        job: ob.employee.role,
        studies: ob.student.studies
    };
});

console.log("-- persons2:\n " + JSON.stringify(persons2, null, "\t"));

let personsWithSalaries = joinCollections(
    persons,
    salaries,
    person => person.job, 
    salary => salary.job,
    "person",
    "salary"
).map(ob => {
    return {...ob.person, ...ob.salary};
});
    
console.log("-- personsWithSalaries:\n " + JSON.stringify(personsWithSalaries, null, "\t"));