left-join.js

// Frees the `[].slice` method to accept `this` as the first actual argument,
// rather than a special argument.
var toArray = Function.call.bind([].slice)

// To make searching efficient (O(1)), we switch the
// list of things to a HashMap, which allows we to
// retrieve an item by its name in constant time.
function indexBy(field, list) {
    return list.reduce(function(result, item) {
        result[item[field]] = item
        return result
    }, {})
}


// Finally, we want something that for every item in
// the input array, enriches the object with the information
// from the other array
function mergeOn(hashmap, field, list) {
    return list.reduce(function(result, item) {
        var key = item[field]
        return key in hashmap?  push(result, extend(hashmap[key], item))
        :      /* otherwise */  result
    }, [])

    function push(xs, x){ xs.push(x); return xs }
}


// This just merges a list of objects
function extend(/* ...objects */) {
    return toArray(arguments).reduce(function(result, source) {
        return Object.keys(source).reduce(function(result, key) {
            result[key] = source[key]
            return result
        }, result)
    }, {})
}

// Test
var arr1 = [
  { id: 1, parentId: 34},
  { id: 2, parentId: 35}  
];

var arr2 = [
  { id: 34, text: "foo" },
  { id: 35, text: "bar" },
  { id: 36 }
]

mergeOn(indexBy('id', arr2), 'parentId', arr1)
// => [
//      { id: 1, parentId: 34, text: "foo" },
//      { id: 2, parentId: 35, text: "bar" }
//    ]