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Vince Campanale
Vince Campanale

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Revisited: A Beginner's Solution to a Common Javascript Interview Challenge

"Confirm the Ending" is another great problem from Free Code Camp's algorithm scripting challenges. The goal is to check if a string (first argument, str) ends with the given target string (second argument, target).

As is common for coding exercises, this problem can be solved countless ways. Today, I'm going to walk through my solution from when I first started coding, improve it, and conclude with a description of the difference between .substr() and .substring().

Beginner Solution

When I had first ventured into learning to code, I took a stab at this problem and successfully solved it, however my solution is a little..."verbose"...

I used the .substr() method which takes two arguments. The first argument is the index in the string to start at and the second argument number of characters to extract from that index. So, say you want to get "lo" from "hello!"". In order to do this with .substr(), you would say "hello!".substr(3, 2). This will go to the third character in "hello"!, which is the first "l", then go for one more character before stopping at the fourth character in "hello!", which is "o". If you're thinking my counting skills need some work, you're probably right, but in Javascript, string indices are 0-based (like arrays). So the character "h" has index 0 in the string.

function confirmEnding(str, target) {
  var substr = str.substr(str.length - target.length, target.length);

  if (substr === target) return true;
  else return false;

Just to briefly recap what I did here: create a variable containing a string chopped off the end of the str argument that has the same length as the target string. If the string contained in that variable matches the target, return true. Otherwise, return false. Pretty simple solution to a pretty simple problem but this can be way better.

Better Solution

There's a neat trick you can use when calling .substr. If you give it a negative number as it's first and only argument, e.g. "string".substr(-3), you will get a string containing the characters starting at the given number from the end of the string. So "string".substr(-3) returns "ing". Let's employ this trick in our solution:

function confirmEnding(str, target) {
  return target === str.substr(-target.length);

In English, this function says: return a string with the same length as target from the end of str and compare the new string to target. If they are strictly equal, return true. Otherwise, return false. This will work with two equal signs as well, but it's typically good practice to use the triple equals.

We can now use ES6 syntax to make a very nice, compact, pure function:

const confirmEnding = (str, target) => target === str.substr(-target.length);

The Difference Between .susbtr() and .substring()

This feat can also be accomplished with .substring(), however I don't think the solution is quite as elegant. It's a good exercise though, and I'll leave it to you to try it out. These methods may appear to be aliases for one another, however they actually do different things.

From the MDN docs (aka the Javascript Bible):

.substr(): "returns the characters in a string beginning at the specified location through the specified number of characters".
.substring(): "returns a subset of a string between one index and another, or through the end of the string".

Perhaps an easier way to think about it is this:

.substr(): starting at index argument one, get argument two number of characters.
.substring(): starting at index argument one, get the characters up until argument two.

Yet another way to look at it:

.substr(from this index, for this many characters)
.substring(from this index, to this index)

Hope this was helpful! Reach out anytime with feedback, questions, tips, and especially compliments. If you want to follow along as I explore this wonderful language of the web, find me on Twitter or at my blog.

Vinny out. ✌️

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