countChars.js

function countChars(string, char){
    b = 0;
    for (i=0;i<string.length;i++){
        if (string.charAt(i) === char) b++
    }
    return b
};

countChars("BBbbbcccdsdhshBBzb", "b")

• Don't name your variables after primitives, this can cause problems later down the road. Try to use more descriptive names. In small samples like this it won't cause a problem. Suggested Names: s/string/stringToScan/ and s/char/charToSearchFor/. Or something like that.
• Don't use single letter variable names. Using an "i" for a for loop is the only exception. s/b/count/
• You forgot a ; after your return statement. In js I don't think similcolons are significant (ie your program won't crash if you missed them) so you should decide up front: do I want to use these? If so then use them at the end of all lines.
• I would recommend avoiding the use of inline if blocks. Always use the {}. It helps with readability and If you have to add another statement the {} is already there.