JavaScript merging objects by id

This should do the trick:

var mergedList = _.map(a1, function(item){
    return _.extend(item, _.findWhere(a2, { id: item.id }));
});

This assumes that the id of the second object in a1 should be 2 rather than "2"

Short ES6 solution

const a3 = a1.map(t1 => ({...t1, ...a2.find(t2 => t2.id === t1.id)}))

Assuming IDs are strings and the order does not matter, you can

1. Create a hash table.
2. Iterate both arrays and store the data in the hash table, indexed by the ID. If there already is some data with that ID, update it with Object.assign (ES6, can be polyfilled).
3. Get an array with the values of the hash map.

var hash = Object.create(null);
a1.concat(a2).forEach(function(obj) {
    hash[obj.id] = Object.assign(hash[obj.id] || {}, obj);
});
var a3 = Object.keys(hash).map(function(key) {
    return hash[key];
});

In ECMAScript6, if the IDs are not necessarily strings, you can use Map:

var hash = new Map();
a1.concat(a2).forEach(function(obj) {
    hash.set(obj.id, Object.assign(hash.get(obj.id) || {}, obj))
});
var a3 = Array.from(hash.values());

ES6 simplifies this:

let merge = (obj1, obj2) => ({...obj1, ...obj2});

Note that repeated keys will be merged, and the value of the second object will prevail and the repeated value of the first object will be ignored.

Example:

let obj1 = {id: 1, uniqueObj1Key: "uniqueKeyValueObj1", repeatedKey: "obj1Val"};
let obj2 = {id: 1, uniqueObj2Key: "uniqueKeyValueObj2", repeatedKey: "obj2Val"};

merge(obj1, obj2)
// {id: 1, uniqueObj1Key: "uniqueKeyValueObj1", repeatedKey: "obj2Val", uniqueObj2Key: "uniqueKeyValueObj2"}
merge(obj2, obj1)
// {id: 1, uniqueObj2Key: "uniqueKeyValueObj2", repeatedKey: "obj1Val", uniqueObj1Key: "uniqueKeyValueObj1"}

Complete solution (with Lodash, not Underscore)

var a1 = [{ id : 1, name : "test"}, { id : 2, name : "test2"}]
var a2 = [{ id : 1, count : "1"}, {id : 2, count : "2"}]
var merge = (obj1, obj2) => ({...obj1, ...obj2});
_.zipWith(a1, a2, merge)
(2) [{…}, {…}]
   0: {id: 1, name: "test", count: "1"}
   1: {id: 2, name: "test2", count: "2"}

If you have an array of arrays to merge you can do it like this:

var arrayOfArraysToMerge = [a1, a2, a3, a4]; //a3 and a4 are arrays like a1 and a2 but with different properties and same IDs.
_.zipWith(...arrayOfArraysToMerge, merge)
(2) [{…}, {…}]
   0: {id: 1, name: "test", count: "1", extra1: "val1", extra2: 1}
   1: {id: 2, name: "test2", count: "2", extra1: "val2", extra2: 2}