var arr = [{name: "上海"},{name: "上海"},{name: "北京"},{name: "杭州"}]

最后显示上海有几个，北京几个，杭州几个

用 lodash.groupBy

function groupBy( array , f ) {
    let groups = {};
    array.forEach( function( o ) {
        let group = JSON.stringify( f(o) );
        groups[group] = groups[group] || [];
        groups[group].push( o );
    });
    return Object.keys(groups).map( function( group ) {
        return groups[group];
    });
}

function cityL(arr){
 for(var i=0,obj = {},n;i<arr.length;i++){
   n = arr[i].name;
   if(!obj[n]) obj[n] = 0
    obj[n]++;
   }
   return obj;
 }