JavaScript function to check for vowels

I know this is a basic questions, but I'm trying to work through a beginner's coding problem and I'm stuck on why this code doesn't work.

This is what I have so far:

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const myString = "Testing for vowels";

const vowels = ["a", "e", "i", "o", "u", "y"];

function countVowels(myString) {
  let count = 0;


  if (vowels.includes(myString)) {
    count++;
  }

  return count
}

const result = countVowels(myString);

console.log(result);

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When it logs out, it just stays 0. Clearly I'm missing something, but I'm not sure what it is at this point. I've set up what vowels are, I think I'm asking that, if the vowels I set in the array are in my string, increase the count, but it's just giving me 0. I know it must be obvious, but I'm stuck.

I've tried a for loop, then I thought it may need to be an if statement, now I have that in a function. I haven't been able to get the answer either way, so I don't think I'm properly telling the code what the vowels are, but I'm not sure how to do that.

you have to use a for loop in order to check if each character is included in the vowels array like this :

const myString = "Testing for vowels";

const vowels = ["a", "e", "i", "o", "u", "y"];


function countVowels(myString) {
  let count = 0;

  for(let i = 0 ;  i < myString.length ; i++) {
    if(vowels.includes(myString[i])) {
      count++ ; 
    }
  }

  return count
}

const result = countVowels(myString);

console.log(result);

A simple case-insensitive and globally flagged regex like ... /[aeiou]/gi ... which within a character set does list all vowels, applied via match, already does the job. The latter operation either returns an array or the null value. Thus, an expression like ...

'Testing for vowels'.match(/[aeiou]/gi)?.length ?? 0

... is all what's needed.

And in order to prevent a TypeError in case of a null match, where one can not access a length property, but still wants to come up with a meaningful result, one might utilize both the Optional Chaining Operator / ?. and the Nullish Coalescing Operator / ??.

Example code ...

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console.log(
  "'Testing for vowels'.match(/[aeiou]/gi) ...",
  'Testing for vowels'.match(/[aeiou]/gi)
);
console.log(
  "''.match(/[aeiou]/gi) ...",
  ''.match(/[aeiou]/gi)
);
console.log('\n');

console.log(
  "'Testing for vowels'.match(/[aeiou]/gi)?.length ?? 0 ...",
  'Testing for vowels'.match(/[aeiou]/gi)?.length ?? 0
);
console.log(
  "''.match(/[aeiou]/gi)?.length ?? 0 ...",
  ''.match(/[aeiou]/gi)?.length ?? 0
);

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.as-console-wrapper { min-height: 100%!important; top: 0; }

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The implementation of countVowels then is as easy as this ...

function countVowels(value) {
  return String(value).match(/[aeiou]/gi)?.length ?? 0;
}

... or that ...

function countVowels(value) {
  return String(value ?? '').match(/[aeiou]/gi)?.length ?? 0;
}

... where the latter could be tested like follows ...

<!-- begin snippet: js hide: false console: true babel: false -->

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function countVowels(value) {
  return String(value ?? '').match(/[aeiou]/gi)?.length ?? 0;
}

console.log(
  "countVowels('Testing for vowels') ...",
  countVowels('Testing for vowels')
);
console.log(
  "countVowels('') ...",
  countVowels('')
);
console.log('\n');

console.log(
  "countVowels() ...",
  countVowels()
);
console.log(
  "countVowels(null) ...",
  countVowels(null)
);
console.log('\n');

console.log(
  "countVowels('NULL') ...",
  countVowels('NULL')
);
console.log(
  "countVowels('UNDEFINED') ...",
  countVowels('UNDEFINED')
);

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.as-console-wrapper { min-height: 100%!important; top: 0; }

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<!-- begin snippet: js hide: false console: true babel: false -->

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const myString = "Testing for vowels";

const vowels = ["a", "e", "i", "o", "u", "y"];

function countVowels(myString) {
  let count = 0;

  for(i = 0; i < myString.length; i++){
    if (vowels.includes(myString.substring(i,i+1))) {
      count++;
    }
   }
  return count
}

const result = countVowels(myString);

console.log(result);

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