walkit

'use strict';
var out=new Map();
var util=require("util");
function walkit(item) {
    let a = item.key.split("|");
    let iG, iP, iS;
    iG = a[0] || null;
    iP = a[1] || null;
    iS = a[2] || null;
    if (iG && !out.has(iG)) { //first time this gp is seen so create a new entry
        out.set(iG, {category:item.category,subcategories:new  Map()});
    }
    if (iP && !out.get(iG).subcategories.get(iP)){ //parent not found for this gp so set a new entry
        out.get(iG).subcategories.set(iP,{subcategory:item.subcategory,items: []});
    }

    if(iG && iP && iS){
        out.get(iG).subcategories.get(iP).items.push( item);
    }

}
    let input=[
        {key:'1',category:"Mens"},
        {key:'1|1',category:"Mens",subcategory:"Coats"},
        {key:'1|1|1',category:"Mens",subcategory:"Coats",item:"Bombers Jacket",description:"A really cool bomber"},
        {key:'1|1|2',category:"Mens",subcategory:"Coats",item:"Submarine Jacket",description:"Keeps the water out"},
        {key:'1|1|3',category:"Mens",subcategory:"Coats",item:"Parka",description:"Perfect for snow"},
        {key:'1|2',category:"Mens",subcategory:"Shirts"},
        {key:'1|2|4',category:"Mens",subcategory:"Shirts",item:"T Shirt",description:"Your standard T Shirt"},
        {key:'1|2|5',category:"Mens",subcategory:"Shirts",item:"Muscle Shirt",description:"For th buff"},
        {key:'1|2|6',category:"Mens",subcategory:"Shirts",item:"Coverup",description:"For the wimp"},
        {key:'2',category:"Womens"},
        {key:'2|1',category:"Womens",subcategory:"Jackets"},
        {key:'2|1|1',category:"Womens",subcategory:"Jackets",item:"Parka",description:"A godsend in snow"},
        {key:'2|1|2',category:"Womens",subcategory:"Jackets",item:"Ski Jacket",description:"Hit the slopes"},
        {key:'2|1|3',category:"Womens",subcategory:"Jackets",item:"Shell",description:"When it is breezy"},
        {key:'2|2',category:"Womens",subcategory:"Shirts"},
        {key:'2|2|4',category:"Womens",subcategory:"Shirts",item:"T Shirt",description:"V Neck"},
        {key:'2|2|5',category:"Womens",subcategory:"Shirts",item:"See Thru Shirt",description:"Not available in large sizes"},
        {key:'2|2|6',category:"Womens",subcategory:"Shirts",item:"Bikini Coverup",description:"Need we say more"}

    ]
input.forEach(function(item){
    walkit(item);
});
out.forEach(function(value,key,mp){
    console.log("Category:",value.category);
    value.subcategories.forEach(function(val2,key2,mp2){
        console.log("    ","SubCategory:",val2.subcategory);
        val2.items.forEach(function(item){
            console.log("               ",item.item,"--",item.description)
        });
    });
});

Output
Category: Mens
     SubCategory: Coats
                Bombers Jacket -- A really cool bomber
                Submarine Jacket -- Keeps the water out
                Parka -- Perfect for snow
     SubCategory: Shirts
                T Shirt -- Your standard T Shirt
                Muscle Shirt -- For th buff
                Coverup -- For the wimp
Category: Womens
     SubCategory: Jackets
                Parka -- A godsend in snow
                Ski Jacket -- Hit the slopes
                Shell -- When it is breezy
     SubCategory: Shirts
                T Shirt -- V Neck
                See Thru Shirt -- Not available in large sizes
                Bikini Coverup -- Need we say more