计算单个字母在一个单词中出现多少次

Question

I am very new to JavaScript, and I'm trying to figure out how to count how many times a single letter appears in a word. For example, how many times does 'p' appear in 'apple'

Here is what I have written so far but am having trouble figuring out where am I going wrong.

var letterInWord = function (letter, word) {
  var letter = 0;
  var word = 0;
  for (var i = 0; i < letter.charAt; i+= 1) {
    if (letter.charAt(i) === " " = true) {
        letter++;
        console.log('The letter `letter` occurs in `word` 1 time.');
    } 
  }
  return letter;
};

Answer 1

You've got a number of problems:

1. You're reusing parameter names as local variable names. Use different identifiers to track each bit of information in your function.
2. letter.charAt is undefined, if letter is a number, and is a function if letter is a string. Either way, i < letter.charAt makes no sense.
3. If you're searching for letter in word why do you want to look at letter.charAt(i) ? You probably want word.charAt(i) .
4. " " = true makes no sense at all.

Perhaps you meant something like this?

var letterInWord = function (letter, word) {
  var count = 0;
  for (var i = 0; i < word.length; i++) {
    if (word.charAt(i) === letter) {
        count++;
    } 
  }
  return count;
};

Answer 2

'apple'.match(/p/g).length // outputs 2

in other words:

var letterInWord = function (letter, word) {
  return (word.match( new RegExp(letter, 'g') ) || []).length;
};

FIDDLE

Answer 3

Here's a smaller function that also works with characters like $ or * (and since it's calling length on a string, there's no need to use || [] )

'apple'.replace(/[^p]/g,'').length // outputs 2


  
 
  
  
  
 
   
 
    
function charcount(c, str) {
  return str.replace(new RegExp('[^'+c+']','g'),'').length
}

console.log = function(x) { document.write(x + "<br />"); };
console.log( "'*' in '4*5*6' = " + charcount('*', '4*5*6') ) // outputs 2
console.log( "'p' in 'pineapples' = " + charcount('p', 'pineapples') )// outputs 3