Building a Javascript Regex from string

Question

I'm trying to return the number of times a letter appears in a word.

I'm passing the letter to a function like so

function getCount(letter)
{
    var pattern = '/' + letter + '/g';
    var matches = word.match(pattern);
    return matches.length;
}

Unfortunately matches is null so I'm unable to call length on it, I know the letter appears in the word as I've already checked that

word.indexOf(letter) > -1

I suspect the problem is with the way I'm building or evaluating pattern

Answer 1

Here's how you build a non literal regular expression :

var pattern = new RegExp(letter, 'g');

See the MDN on building a regular expression.

And here's a simpler solution to count the occurrences of the letter :

return word.split(letter).length-1;