For loop over an array to find a specific value in said array. instead i get the whole array

Question

var users = [
{firstName: 'Pete', lastName: 'Barrat', favoriteFood: 'Pizza', age: 30},
{firstName: 'Lisa', lastName: 'Jenkins', favoriteFood: 'Curry', age: 34},
{firstName: 'Bob', lastName: 'Yates', favoriteFood: 'Fish', age: 54},
{firstName: 'Claire', lastName: 'Smith', favoriteFood: 'Steak', age: 21},
{firstName: 'Adam', lastName: 'Johnson', favoriteFood: 'Pasta', age: 27}
];
 function valueMapper(){
 for(var i=0; i < users.length; i++)
 return users; //is this where I'm going wrong?
 };
 console.log(valueMapper('firstName'));

• Mouser: Leroy Jenkins would've have been funnier for the second entry.

Answer 1

If I understand you correctly you need to get firstName's of list of objects. You can get this using .map on array.

var users = [{
 firstName: 'Pete',
 lastName: 'Barrat',
 favoriteFood: 'Pizza',
 age: 30
}, {
 firstName: 'Lisa',
 lastName: 'Jenkins',
 favoriteFood: 'Curry',
 age: 34
}, {
 firstName: 'Bob',
 lastName: 'Yates',
 favoriteFood: 'Fish',
 age: 54
}, {
 firstName: 'Claire',
 lastName: 'Smith',
 favoriteFood: 'Steak',
 age: 21
}, {
 firstName: 'Adam',
 lastName: 'Johnson',
 favoriteFood: 'Pasta',
 age: 27
}];
function valueMapper(key) {
 return users.map(function(item) {return item[key]});
}
console.log(valueMapper('firstName'));

Answer 2

 var users = [
 {firstName: 'Pete', lastName: 'Barrat', favoriteFood: 'Pizza', age: 30},
 {firstName: 'Lisa', lastName: 'Jenkins', favoriteFood: 'Curry', age: 34},
 {firstName: 'Bob', lastName: 'Yates', favoriteFood: 'Fish', age: 54},
 {firstName: 'Claire', lastName: 'Smith', favoriteFood: 'Steak', age: 21},
 {firstName: 'Adam', lastName: 'Johnson', favoriteFood: 'Pasta', age: 27}
 ];
 function valueMapper(key){
 var resultArray = [];
 for(var i=0; i < users.length; i++) {
 resultArray.push(users[i][key])
 }
 return resultArray;
 };
 console.log(valueMapper('firstName'));

Answer 3

So i guess you want to map all users to the attribute you specified as parameter. Try this:

var users = [
{firstName: 'Pete', lastName: 'Barrat', favoriteFood: 'Pizza', age: 30},
{firstName: 'Lisa', lastName: 'Jenkins', favoriteFood: 'Curry', age: 34},
{firstName: 'Bob', lastName: 'Yates', favoriteFood: 'Fish', age: 54},
{firstName: 'Claire', lastName: 'Smith', favoriteFood: 'Steak', age: 21},
{firstName: 'Adam', lastName: 'Johnson', favoriteFood: 'Pasta', age: 27}
];
function valueMapper(attribute) {
 return users.map(u => u[attribute]);
};
console.log(valueMapper('firstName'));

• kidwon: Is he using a ES6 your suggestion may be false
• Mouser: It's not php mate: u => u[attribute])
• fafl: Do you guys mean the arrow function? I don't see the problem.
• Mouser: Yup we do, fails miserably on almost every browser.

Answer 4

You are just returning the whole users array instead of performing your value check and only returning one element. Also your valueMapper function takes no arguments with this definition. You should start with reading JS basics since there are a lot of problems in this example.

Answer 5

var users = [
{firstName: 'Pete', lastName: 'Barrat', favoriteFood: 'Pizza', age: 30},
{firstName: 'Lisa', lastName: 'Jenkins', favoriteFood: 'Curry', age: 34},
{firstName: 'Bob', lastName: 'Yates', favoriteFood: 'Fish', age: 54},
{firstName: 'Claire', lastName: 'Smith', favoriteFood: 'Steak', age: 21},
{firstName: 'Adam', lastName: 'Johnson', favoriteFood: 'Pasta', age: 27}
];
function valueMapper(key){
 //let's use array.prototype.map here:
 return users.map(function(element){
 return element[key];
 });
}
 console.log(valueMapper('firstName'));

The way to go here is Array.prototype.map. It iterates over all the elements and returns the results in an array.

---

More explanation here:

The array is filled with Objects ({..}). Every object has keys and values. These keys are addressable by using square brackets [..] since we want them to be variable. Users itself is an array, so we can use array functions like map on them. Map iterates over each array item and returns something when a condition is met. In this case there is no special condition, only that we want to return a specific key.