JS How to create a function that count the number of hashtags

Question

I am new to coding and have been given this question to answer.

This question is;

Create a function that receives a string that will contain a number of mentions and hashtags as on Twitter.

   E.g. "So excited to start  @coding on Monday! #learntocode #codingbootcamp"

The function should return an object describing the number of hashtags and mentions found:

   { hashtags: 2, mentions: 1 }
 

The code that I have created is this;

function countHashtagsAndMentions(str) {
   let split = str.split(" ");
    let count = 0
    for (let i = 9; i < str.length; i++) {
        let hash = split.filter(hashtag => hashtag.match(/#/g))
        if (hash === 1) {
          count ++
         }
        let total = {
        hash = count,
      }
    return hash
   }
 }

My code is being run against this;

describe("countHashtagsAndMentions", () => {
  it("returns an object", () => {
    expect(typeof countHashtagsAndMentions("")).to.equal("object");
  });
  it("returns {hashtags: 0, mentions: 0} if it finds none", () => {
    expect(
      countHashtagsAndMentions(
        "hello this is a tweet guaranteed to get very little engagement"
      )
    ).to.eql({ hashtags: 0, mentions: 0 });
  });
  it("recognises no mentions", () => {
    expect(countHashtagsAndMentions("#yolo")).to.eql({
      hashtags: 1,
      mentions: 0
    });
  });
  it("recognises no hashtags", () => {
    expect(countHashtagsAndMentions("@yobo")).to.eql({
      hashtags: 0,
      mentions: 1
    });
  });
  it("finds multiple hashtags and mentions and returns that number", () => {
    expect(countHashtagsAndMentions("#yolo @bolo #golo")).to.eql({
      hashtags: 2,
      mentions: 1
    });
    expect(countHashtagsAndMentions("@boyo #goyo @loyo #zoyo")).to.eql({
      hashtags: 2,
      mentions: 2
    });
    expect(
      countHashtagsAndMentions(
        '"So excited to start at @northcoders on Monday! #learntocode #codingbootcamp"'
      )
    ).to.eql({ hashtags: 2, mentions: 1 });
  });
});

Dose anyone have any suggestion of haw to make my code work?

Answer 1

A simple loop does it. Since you're using ES2015+ syntax, a for-of would work nicely:

function countHashtagsAndMentions(str) {
  let hashtags = 0;
  let mentions = 0;
  for (const ch of str) {
    if (ch === "#") {
      ++hashtags;
    } else if (ch === "@") {
      ++mentions;
    }
  }
  return {hashtags, mentions};
}
let str = "So excited to start  @coding on Monday! #learntocode #codingbootcamp";
console.log(countHashtagsAndMentions(str));

That works because strings are iterable in ES2015+. The for-of loop implicitly uses the iterator from the string to walk through its characters. So within the loop, ch is each character from the string. Note that unlike str.split(), a strings iterator doens't separately the two halves of a character that requires a surrogate pair (like most emojis), which is normally what you want.

This:

for (const ch of str) {
    // ...
}

is effectively the same as

let it = str[Symbol.iterator]();
let rec;
while (!(rec = it.next()).done) {
    const ch = rec.value;
    // ...
}

but without the it and rec variables.

---

Alternately, you could use replace with a regular expression to replace all characters other than the ones you want to count. It sounds like it would be more expensive, but it's something the JavaScript engine can optimize:

function countHashtagsAndMentions(str) {
  return {
    hashtags: str.replace(/[^#]/g, "").length,
    mentions: str.replace(/[^@]/g, "").length
  };
}
let str = "So excited to start  @coding on Monday! #learntocode #codingbootcamp";
console.log(countHashtagsAndMentions(str));

Which you use probably depends in part on the length of the string. The replace option is nice and short, but does go through the string twice.