How to make a queue (FIFO) in JS using arrays?

Question

I am working a question on FreeCodeCamp. I need to add a number to the end of the array, then remove the first element of the array. The function called nextInLine should return the element that was removed from it.

I'm not completely new, but I'm not advanced in JS either. However, I'm new to algorithms. I've watched Youtube videos for assistance with Queues. I have this concept fairly understood as follows:

//array as queue (FIFO)
const arr = [];
arr.push(5); // [5]
arr.push(7); // [5,7]
arr.push(9); // [5,7,9]
console.log(arr.shift()); // [7,9]
console.log(arr.shift()); // [9]
console.log(arr.shift()); // []

However, when trying to implement this idea into my code, I seem to not maybe fully understand the presented problem in order to make it run. I am to modify the function nextInLine, and the comment that says //Display Code, I am to change that line as well.

function nextInLine(arr, item) {
  // Your code here
  arr.push(arr[0]);
  arr.shift();
  return console.log(arr, item);  // Change this line
}

// Test Setup
var testArr = [1,2,3,4,5];

// Display Code
console.log("Before: " + JSON.stringify(testArr));
console.log(nextInLine(testArr)); // Modify this line to test
console.log("After: " + JSON.stringify(testArr));

When running the code, this is what is showing for me.

nextInLine([], 5) should return a number.
nextInLine([], 1) should return 1
nextInLine([2], 1) should return 2
nextInLine([5,6,7,8,9], 1) should return 5
After nextInLine(testArr, 10), testArr[4] should be 10

My function is outputting /* 2,3,4,5,1 */ but it seems to be incorrect. Perhaps I am failing to put it into an array? I have the first number (1) at the beginning, but am I failing to simply put my answer in an array? Or am I not even producing the right numbers?

Samuel · Accepted Answer · 2019-07-18 03:58:03Z

7

You can do shift first and then push.

function nextInLine(arr, item) {
  var removedElement = arr.shift();
  arr.push(item);
  return removedElement;
}

answered Jul 18, 2019 at 3:58

Samuel

4623 silver badges11 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Maurice cooper Over a year ago

Ahh I see, you're creating a variable named 'removeElement' and assigning it the literal 'removed element', and you're also returning that. As well as pushing whatever 'item' you decide to push to the end. I seem to understand what I was missing, which was assign the shift method to a variable, then returning said variable. Thank you for the input @Samuel

Barmar · Accepted Answer · 2019-07-18 03:17:52Z

1

You should be pushing item onto the array. You're simply moving the first element of the array to the end, without adding the new element.

And console.log() prints its arguments, it doesn't return them. So your function is just returning undefined.

function nextInLine(arr, item) {
  arr.push(item); // add new item to end
  return arr.length > 1 ? arr.shift() : null; // remove first item and return it
}

The conditional in the return statement prevents it from shifting off the new element if it's the only thing in the array, i.e. the array was originally empty.

answered Jul 18, 2019 at 3:17

Barmar

789k57 gold badges554 silver badges669 bronze badges

1 Comment

Maurice cooper Over a year ago

Oh right! I completely forgot that console.log(); does not return anything and comes out as undefined. It will log the results but does not return it back to the function? If I said that correctly.

Julian W. · Accepted Answer · 2019-07-18 03:51:16Z

1

You can use Array.prototype.splice to remove the first element from an array.

function nextInLine(arr, item) {
  var firstElement = arr[0];
  arr.splice(0, 1);
  arr.push(item);
  return firstElement;
}

var arr = [];
console.log('nextInLine(arr, 5) should return undefined and arr should be [5]:');
console.log('return value:', nextInLine(arr, 5));
console.log('array:', arr);
console.log('--------------------------------------')

console.log('nextInLine(arr, 1) should return 5 and arr should be [1]:');
console.log('return value:', nextInLine(arr, 1));
console.log('array:', arr);
console.log('--------------------------------------')

arr = [5,6,7,8,9];
console.log('nextInLine(arr, 10) should return 5 and arr should be [6, 7, 8, 9, 10]:');
console.log('return value:', nextInLine(arr, 10));
console.log('array:', arr);
console.log('--------------------------------------')

answered Jul 18, 2019 at 3:51

Julian W.

1,5819 silver badges20 bronze badges

1 Comment

Maurice cooper Over a year ago

Awesome! Thank you for the input @pindev. I appreciate your response.

Maurice cooper · Accepted Answer · 2019-07-19 03:00:42Z

1

Update: With the help of everyone, I was not only able to solve, but understand WHY the solution was, what it was! I just had to switch around the following lines of code and that did it. Apparently they wanted you to push the item first, THEN shift on the array. Who would've knew LOL. Thanks everyone! I don't know how to close a forum post down, so I will leave it. Happy coding!

answered Jul 19, 2019 at 3:00

Maurice cooper

591 silver badge7 bronze badges

Comments

MohammadTausif Shaikh · Accepted Answer · 2019-07-18 09:06:08Z

0

You can try this also

function nextInLine(arr, item) {
       arr.push(item);
       return arr.shift();
    }
    
    
 var arr = [1,5,6]
 console.log("removed element is : ", nextInLine(arr,10))
 console.log("array is  : ", arr)

answered Jul 18, 2019 at 9:06

MohammadTausif Shaikh

5433 silver badges9 bronze badges

Comments

user11286287 · Accepted Answer · 2020-01-01 14:53:04Z

0

i did fifo function with two array method - concat slice

const nextInLine = (arr, item) => item ? arr.concat(item).slice(1) : arr;

answered Jan 1, 2020 at 14:53

user11286287

Collectives™ on Stack Overflow

How to make a queue (FIFO) in JS using arrays?

6 Answers 6

1 Comment

1 Comment

1 Comment

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

1 Comment

1 Comment

1 Comment

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related