JavaScript array.slice() behavior

Question

I am working on an exercise in learning JavaScript, and cannot figure out why my code is failing a test evaluation. And yes, I realize that I am redundantly (and messily) re-defining Underscore.js _.last. That is the purpose of the exercise.

_.last = function(array, n) {
  if (n === 0) {
    return [];
  }
  else {
    return n === undefined ? array.slice(-1) : array.slice(-n);
  };
}'

These are the results I get when running:

_.last([1,2,3], 2)
[2, 3]
_.last([1,2,3], 5)
[1, 2, 3]
_.last([1,2,3], 0)
[]
_.last([1,2,3])
[3]

The very last call _.last([1,2,3]) is the one failing this evaluation:

expect(_.last([1,2,3])).to.equal(3)

Can anyone tell why this expect is returning false? What am I overlooking

Thank you in advance.

EDIT: solved. else edited to return value if n===undefined rather than returning a single-value array:

return n === undefined ? array[array.length-1] : array.slice(-n);

So you're expecting it to return 3, but receive an array with the entry 3. i.d. [3]? — janfoeh
– janfoeh, Commented May 19, 2015 at 18:08
Ah, ok, that makes sense. I was actually wondering if that might be the case. Thanks :) — Wes Tyler
– Wes Tyler, Commented May 19, 2015 at 18:12

Oriol · Accepted Answer · 2015-05-19 18:12:41Z

1

Array.prototype.slice returns an array. And if you compare an array and a number, they will be different.

Instead, consider doing the same underscore does:

if (n == null /*|| guard*/) return array[array.length - 1];

answered May 19, 2015 at 18:12

Oriol

291k71 gold badges459 silver badges535 bronze badges

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

2 Comments

Wes Tyler Over a year ago

This worked perfectly! I had checked out the underscore docs, and the || guard was throwing me off. Thanks!

Wes Tyler Over a year ago

Also, you and @taxicala answered too quickly. I can't mark this as solved for another 4 minutes. :P

taxicala · Accepted Answer · 2015-05-19 18:10:02Z

0

Quoting array.slice description:

The slice() method returns the selected elements in an array, as a new array object.

Basically, your code is ok, and your test is not:

You should change:

expect(_.last([1,2,3])).to.equal(3)

to:

expect(_.last([1,2,3])).to.equal([3])

More info here http://www.w3schools.com/jsref/jsref_slice_array.asp

answered May 19, 2015 at 18:10

taxicala

21.8k7 gold badges41 silver badges67 bronze badges

Collectives™ on Stack Overflow

JavaScript array.slice() behavior

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related