'/a/b/c/d'.match(/\/\w+/g).reduce((s,i,index)=>{
  if(index>0) s.push(`${s[index-1]}${i}`);
  else s.push(`${i}`);
  return s;
},[]);

再简化

'/a/b/c/d'.match(/\/\w+/g).reduce((s,i,index)=>{
  s.push(`${s[index-1]||''}${i}`);
  return s;
},[]);

再牺牲可读性

'/a/b/c/d'.match(/\/\w+/g)
    .reduce((s,i,index)=>(s.push(`${s[index-1]||''}${i}`),s), []);

const s = "/a/b/c/d";

function process(s) {
    const parts = s.split(/\//);
    const r = parts.reduce((m, part) => {
        m.t.push(part);
        m.r.push(m.t.join("/"));
        return m;
    }, { r: [], t: [] });
    r.r.shift();
    return r.r;
}

console.log(process(s));

给出一个比较一般的方法吧,水平有限.

    var str = '/a/b/c/d';
    function test(data) {
        var str2, arr1, arr2=[];
        arr1 = data.split('/').splice(1);
        for(var i=0;i<arr1.length;i++){
            str2 = '';
            for(var x=0;x<=i;x++){
                str2=str2+'/'+arr1[x];
            };
            arr2[i]=str2;
        };
        return arr2;
    }
    console.log(test(str));

var str = "/a/b/c/d";
var array = [];
for(var i = 2, len = str.length; i <= len; i += 2){
    array.push(str.substr(0, i));
}
console.log(array);

'/a/b/c/d'.match(/((((\/\w)\/\w)\/\w)\/\w)/).slice(1).reverse()