JavaScript function to automatically count consecutive letters in a string

Question

I am attempting (unsuccessfully) to write JavaScript function LetterCount to count the consecutive letters in a string (and not the total number).

Ideally: LetterCount("eeeeeoooohhoooee") = [["e", 5],["o",3],["h",2],["o",3],["e",2]]

The following code attempts to count the number of consecutive letters in a string only when I already know what they are:

function LetterCount(str) {
for (var i=0; i<str.length;i++) {
    var arr1=[]; arr2=[]; arr3=[]; arr4=[]; arr5=[];
    var e=0; o=0; h=0; o2=0; e2=0;
    if(str[i]="e") {
        arr1 += "e";
        e++;
    }
    arr1.push(e);
    if(str[i]="o") {
        arr2 += "o";
        o++;
    }
    arr2.push(o);
    if(str[i]="h") {
        arr3 += "h";
        h++;
    }
    arr3.push(h);
    if(str[i]="o") {
        arr4 += "o";
        o2++;
    }
    arr4.push(o2);
    if(str[i]="e") {
        arr5 += "e";
        e2++;
    }
    arr5.push(e2);
}
return arr1.concat(arr2).concat(arr3).concat(arr4).concat(arr5);
}

In the code above, I need to first know what the letters in the string are, and how many of them are present, in what order.

INSTEAD: How do you write a function that will automatically recognize the letter themselves, and then return the count of consecutive letters. Would also be great if the answer is in the following format:

 LetterCount("eeeeeoooohhoooee") = [["e", 5],["o",3],["h",2],["o",3],["e",2]]

Any help is much appreciated!

Answer 1

You can use a regular expression to match any letter followed by zero or more instances of the same letter.

rx=/([a-zA-Z])\1*/g;

Your example matches ["eeeee","oooo","hh","ooo","ee"].

Using map, return the initial letter and the number of occurences in a new array for each index.

function letterCount(str){
    var s= str.match(/([a-zA-Z])\1*/g)||[];
    return s.map(function(itm){
        return [itm.charAt(0), itm.length];
    });
}

letterCount("eeeeeoooohhoooee")

returned value: (Array)

[["e",5],["o",4],["h",2],["o",3],["e",2]]

NOTES:

1. var s= str.match(/([a-zA-Z])\1*/g)||[];

returns an array of matches (repeated letters) or an empty array([]). Otherwise, if the string does not contain any letters an error would be thrown (from calling map on null).

2. \1* is used to allow matching instances of a single letter with any or no sequential repetition. '\1+' would not match a single unrepeated letter.

3. Array map expects a function and passes three arguments- the value at each index, the index number, and a reference to the entire array. In this case, only the value of each index is used, so we can ignore the other arguments.