DEV Community

You are JS noob without knowing these methods

Milan Mohapatra on December 06, 2023

Most important methods of your JavaScript codebase. In day-to-day development with JavaScript in the form of frontend framework or backe...

Read full post

Jon Randy 🎖️ • Dec 7 '23 • Edited

Your map example is poor as it is modifying the content of the original array... something that map is designed to help you avoid. It may confuse people into believing the the original array is always modified.

Milan Mohapatra • Dec 7 '23

Check out the new modifications. Thanks

Milan Mohapatra • Dec 7 '23

No, it might looks like muteble but but will change it. Thankyou

Jon Randy 🎖️ • Dec 7 '23 • Edited

Believe me, it definitely will. Run the code, then check the original array. I guarantee it will have changed.

To do what your example's introduction describes, you need to do something like this - which will leave the original array untouched:

students.map(({fee, ...student}) => ({...student, fee: fee-100}))

Or if you prefer something closer to your original:

students.map(student => {
  const newStudent = {...student}
  newStudent .fee -= 100
  return newStudent 
})

In your example, the newly created array contains the same objects as the original array, and you have modified those. To leave the original array untouched, you need to make copies of the student objects and work on those.

Tracy Gilmore • Dec 7 '23

Hi Milan, In addition to Jon's comment, the second point you made is not quite correct. Instead of "Return the array with the same number of elements present.", it would be more accurate to state "Returns a new array with the same number of elements as the original." but this is not being demonstrated by your example.

To confirm this, you could check what is returned by the map operation against the original array (after performing the operation).

const studentsWithReducedFee = students.map(({fee, ...student}) => ({...student, fee: fee-100}));

console.table(students);
console.table(studentsWithReducedFee);

Sorry to be picky but I also think you mean "transform" rather than "transfer" in your points about map.

Milan Mohapatra • Dec 7 '23

Thanks for commenting, love to unlearn and learn the correct way.

Milan Mohapatra • Dec 7 '23

check out the new modifications. thanks

Tracy Gilmore • Dec 8 '23 • Edited

Hi Milan, The example is far more representative of a typical use case. I still think the first two bullet points are not quite right and the following might be better:

To transform the elements of the input array to produce a new array.
Returns a new array...

The reduce example needs a little work and should present the return value.

const totalFees = students.reduce((acc, next) => acc += next.fee, 0);

Please accept my comments in the spirit of friendship and knowledge sharing.

Dusan Petkovic • Dec 8 '23

I think its fine if content like this already exists on dev.to already, from my perspective dev.to is more about opening discussions on some topics and also for people to offer feedback to content creators and also help them improve, so this is a good start!

Milan Mohapatra • Dec 8 '23

Thanks @dsaga

Ganesh Patil • Dec 6 '23

Agree!
Array methods are essentials.

Milan Mohapatra • Dec 7 '23

Agree too.

Julien Dephix • Dec 7 '23 • Edited

Hi!

Array methods FTW!

You can shorten your code a tad more:

students.find((student) => {
    return student.rank == 5;
  })

becomes (notice strict comparison as well)

students.find((student) => student.rank === 5)

Your reduce example becomes:

students.reduce((acc, next) => acc + next.fee, 0)

Also, under entries you wrote :

return a 2D array which is converted to an Object.

What do you mean by converted to an object? Are you referring to Object.fromEntries which creates an object from an array of entries?

And lastly, it'd be more interesting if you showed what students variable holds. It would allow "noobs", to quote you, to play with the code at home.

Full doc on arrays.

Milan Mohapatra • Dec 7 '23

Thanks for commenting. I wrote this to make a type of notes of what I studied. I will consider your thoughts on my upcoming notes. check out the correction I made on entries(). Thanks for your time.