A useful function to give values to a template full of placeholders

replaceall.js

/** 
 * [ description]
 * @param  {string} tpl      the template
 * @param  {literal|function} obj  a literal for substitution or a function that will
 *                                    return the substitution given as parameter a string
 * @param  {string} start       optional- the opening placeholder delimitator (%)
 * @param  {string} end         optional- the closing placeholder delimitator (%)
 * @param  {string} fb          optional- a fallback value in case an element is not found
 * @return {string}          the resulting string with replaced values
 */
function replaceall(tpl, obj, start, end, fb) {
    start || (start = '%');
    end || (end = '%');

    var reg = new RegExp(start + '([A-z0-9-_]*)' + end, 'g'),
        straight = true,
        str, tmp;

    while (straight) {

        if (!(tpl.match(reg))) {
            return tpl;
        }
        tpl = tpl.replace(reg, function (str, $1) {
            switch (true) {
                // 
                case typeof obj === 'function' :
                    // avoid silly infiloops
                    tmp = obj($1);
                    return (tmp !== start + $1 + end) ? obj($1)  : $1;
                break;

                // the label matches a obj literal element
                // use it
                case $1 in obj : return obj[$1]; break;

                // not a function and not found in literal
                // use fallback if passed or get back the placeholder
                // switching off before returning
                default:
                    straight = false;
                    return typeof fb !== 'undefined' ?  fb : start + $1 + end;    
                break;
            }
        });
    }
    return tpl;
}

another simplified version

const replaceAll = (tpl, o, opts = {}) => {
  const delimStart = opts?.start || '%',
        delimEnd = opts?.end || '%',
        fb = opts?.fb,
        reg = new RegExp(
          `${delimStart}(\\\+)?([A-z0-9-_\.]*)${delimEnd}`,
          'gm'
        );
  return tpl.replace(reg, (str, enc, $1) => o[$1] || fb($1) || $1);
}


console.log(replaceAll(
   'kill &who% &howmany% (&cmt%)',
   {who: '\'em', cmt: 'u asked howmany right?'},
   {start:'&', fb: n => `all!`}
))