Creating a data structure in JavaScript

Question

I am currently attempting a JavaScript challenge on FreeCodeCamp. I have been working on this challenge for a few days now and am stumped. I asked for help on the site's forum but the response I got wasn't very helpful or useful.

For this particular challenge, I must "write a function nextInLine which takes an array (arr) and a number (item) as arguments. Add the number to the end of the array, then remove the first element of array. The nextInLine function should then return the element that was removed."

Here is the code I have so far:

function nextInLine(arr, item) {
  // Your code here
  var nextInLine = ([], 1);
  /*var addToNextInLine = nextInLine.push();
  var removedFromNextInLine = nextInLine.shift();*/
  return nextInLine; 
}

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

// Display Code
console.log("Before: " + JSON.stringify(testArr));
console.log(nextInLine([], 1)); // Modify this line to test
console.log("After: " + JSON.stringify(testArr));

Here are the conditions I must satisfy in order to successfully complete this challenge:

• 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

I successfully completed the first requirement, but am stuck with figuring out how to complete the last three. Because I am asked to "add" and "remove" elements from the array, I took it to mean that push and shift functions were necessary to successfully complete this exercise.

But when I tested my code, I got a TypeError message that read "nextInLine.push is not a function". It seems to me that either I wrote the code for the push and shift functions incorrectly or those functions may not be necessary to complete this challenge. So, that being said, my question to all of you is did I write something in my code incorrectly, should I remove something from my code, or am I missing something else?

Please let me know if there is any additional information you would like to know in order to better understand my problem.

Answer 1

So I'm spotting a couple of problems:

• ([], 1) is an expression with a comma operator operating on [] and 1, the comma operator evaluates each operand in turn, and returns the last one. So this is essentially equivalent to var nextInLine = 1 without any further processing.
• .push() without an argument would push undefined into the array, that's not what you want.
• By defining var nextInLine inside of the function that's named the same way, you're preventing yourself from using recursion and a bunch of other things, it's generally a good thing to avoid.

Your general logic should look somewhat like this:

given arguments arr (the array) and el (the element to add)
If arr is empty, add el to it (arr.push(el)) and return el
else,
remove the first element from the array (with shift), and store the value somewhere.
then add el to the array
return the value you've stored from the array

That should accomplish all goals.

Answer 2

As someone who also got his feet wet with Free Code Camp, I agree with the commenter above that the most important thing you can learn in the course is how to research and how to ask questions. So, I'm not going to provide a working snippet for you. I'll point out some specific problems in your code, though, and that should point you in the right direction.

You have some syntax problems in your code, as well as a misunderstanding of how to handle push and shift.

First, syntax.

var nextInLine = ([], 1);

This line assigns the value of ([], 1) to nextInLine. The ([], 1) is not any javascript data structure of which I'm aware. As noted in the comment to your question, it simply returns 1.

Since push and shift are methods on the Array object, nextInLine should be an Array, leaving us with:

var nextInLine = []

Next, consider the return value of the shift method, from MDN:

[The return value is ] the removed element from the array; undefined if the array is empty.

So what you ought to be returning is the value returned by your call to shift().

Finally, push() requires the value you want it to push onto the array, which you're not currently providing.