array unwind

function unwind(arr, expandProp,renameProp){
 'use strict';
 
 return arr
  .map((el) => { 
      return el[expandProp]
      .map((subProp) => { 
        var subEl = Object.assign({},el,{[renameProp]:subProp});
        delete subEl[expandProp];
        return  subEl;
      });
  }) 
  .reduce( (a,b) => {return a.concat(b); } )
};
var arr = [{tag:'a',list:[1,2,3]},{tag:'b',list:[1,2]}];
var expandProp = 'list';
var renameProp = 'value';
unwind(arr, expandProp, renameProp);