Develop Reference

Find duplicate names within an array of different files

A bit of a different use case from the ones I was suggested above.
I need to loop through and check each file name within an array of files and push the files that have the same name into a new array so that I can upload them later separately.
This is my code so far, and surely I have a problem with my conditional checking, can somebody see what I am doing wrong?
filesForStorage = [
{id: 12323, name: 'name', ...},
{id: 3123, name: 'abc', ...},
{id: 3213, name: 'name', ...},
...
]
filesForStorage.map((image, index) => {
for (let i = 0; i < filesForStorage.length; i++) {
for (let j = 0; j < filesForStorage.length; j++) {
if (
filesForStorage[i].name.split(".", 1) ===. //.split('.', 1) is to not keep in consideration the file extension
filesForStorage[j].name.split(".", 1)
) {
console.log(
"----FILES HAVE THE SAME NAME " +
filesForStorage[i] +
" " +
filesForStorage[j]
);
}
}
}

Using map without returning anything makes it near on pointless. You could use forEach but that is equally pointless when you're using a double loop within - it means you would be looping once in the foreach (or map in your case) and then twice more within making for eye-wateringly bad performance.
What you're really trying to do is group your items by name and then pick any group with more than 1 element
const filesForStorage = [
{id: 12323, name: 'name'},
{id: 3123, name: 'abc'},
{id: 3213, name: 'name'}
]
const grouped = Object.values(
filesForStorage.reduce( (a,i) => {
a[i.name] = a[i.name] || [];
a[i.name].push(i);
return a;
},{})
);
console.log(grouped.filter(x => x.length>1).flat());

JavaScript has several functions which perform "hidden" iteration.
Object.values will iterate through an object of key-value pairs and collect all values in an array
Array.prototype.reduce will iterate through an array and perform a computation for each element and finally return a single value
Array.prototype.filter will iterate through an array and collect all elements that return true for a specified test
Array.prototype.flat will iterate through an array, concatenating each element to the next, to create a new flattened array
All of these methods are wasteful as you can compute a collection of duplicates using a single pass over the input array. Furthermore, array methods offer O(n) performance at best, compared to O(1) performance of Set or Map, making the choice of arrays for this kind of computation eye-wateringly bad -
function* duplicates (files) {
const seen = new Set()
for (const f of files) {
if (seen.has(f.name))
yield f
else
seen.add(f.name, f)
}
}
const filesForStorage = [
{id: 12323, name: 'foo'},
{id: 3123, name: 'abc'},
{id: 3213, name: 'foo'},
{id: 4432, name: 'bar'},
{id: 5213, name: 'qux'},
{id: 5512, name: 'bar'},
]
for (const d of duplicates(filesForStorage))
console.log("duplicate name found", d)
duplicate name found {
"id": 3213,
"name": "foo"
}
duplicate name found {
"id": 5512,
"name": "bar"
}

A nested loop can be very expensive on performance, especially if your array will have a lot of values. Something like this would be much better.
filesForStorage = [
{ id: 12323, name: 'name' },
{ id: 3123, name: 'abc' },
{ id: 3213, name: 'name' },
{ id: 3123, name: 'abc' },
{ id: 3213, name: 'name' },
{ id: 3123, name: 'random' },
{ id: 3213, name: 'nothing' },
]
function sameName() {
let checkerObj = {};
let newArray = [];
filesForStorage.forEach(file => {
checkerObj[file.name] = (checkerObj[file.name] || 0) + 1;
});
Object.entries(checkerObj).forEach(([key, value]) => {
if (value > 1) {
newArray.push(key);
}
});
console.log(newArray);
}
sameName();

get values from list of objects in javascript [duplicate]

I want to cycle through the objects contained in an array and change the properties of each one. If I do this:
for (var j = 0; j < myArray.length; j++){
console.log(myArray[j]);
}
The console should bring up every object in the array, right? But in fact it only displays the first object. if I console log the array outside of the loop, all the objects appear so there's definitely more in there.
Anyway, here's the next problem. How do I access, for example Object1.x in the array, using the loop?
for (var j = 0; j < myArray.length; j++){
console.log(myArray[j.x]);
}
This returns "undefined." Again the console log outside the loop tells me that the objects all have values for "x". How do I access these properties in the loop?
I was recommended elsewhere to use separate arrays for each of the properties, but I want to make sure I've exhausted this avenue first.
Thank you!

Use forEach its a built-in array function. Array.forEach():
yourArray.forEach(function (arrayItem) {
var x = arrayItem.prop1 + 2;
console.log(x);
});

Some use cases of looping through an array in the functional programming way in JavaScript:
1. Just loop through an array
const myArray = [{x:100}, {x:200}, {x:300}];
myArray.forEach((element, index, array) => {
console.log(element.x); // 100, 200, 300
console.log(index); // 0, 1, 2
console.log(array); // same myArray object 3 times
});
Note: Array.prototype.forEach() is not a functional way strictly speaking, as the function it takes as the input parameter is not supposed to return a value, which thus cannot be regarded as a pure function.
2. Check if any of the elements in an array pass a test
const people = [
{name: 'John', age: 23},
{name: 'Andrew', age: 3},
{name: 'Peter', age: 8},
{name: 'Hanna', age: 14},
{name: 'Adam', age: 37}];
const anyAdult = people.some(person => person.age >= 18);
console.log(anyAdult); // true
3. Transform to a new array
const myArray = [{x:100}, {x:200}, {x:300}];
const newArray= myArray.map(element => element.x);
console.log(newArray); // [100, 200, 300]
Note: The map() method creates a new array with the results of calling a provided function on every element in the calling array.
4. Sum up a particular property, and calculate its average
const myArray = [{x:100}, {x:200}, {x:300}];
const sum = myArray.map(element => element.x).reduce((a, b) => a + b, 0);
console.log(sum); // 600 = 0 + 100 + 200 + 300
const average = sum / myArray.length;
console.log(average); // 200
5. Create a new array based on the original but without modifying it
const myArray = [{x:100}, {x:200}, {x:300}];
const newArray= myArray.map(element => {
return {
...element,
x: element.x * 2
};
});
console.log(myArray); // [100, 200, 300]
console.log(newArray); // [200, 400, 600]
6. Count the number of each category
const people = [
{name: 'John', group: 'A'},
{name: 'Andrew', group: 'C'},
{name: 'Peter', group: 'A'},
{name: 'James', group: 'B'},
{name: 'Hanna', group: 'A'},
{name: 'Adam', group: 'B'}];
const groupInfo = people.reduce((groups, person) => {
const {A = 0, B = 0, C = 0} = groups;
if (person.group === 'A') {
return {...groups, A: A + 1};
} else if (person.group === 'B') {
return {...groups, B: B + 1};
} else {
return {...groups, C: C + 1};
}
}, {});
console.log(groupInfo); // {A: 3, C: 1, B: 2}
7. Retrieve a subset of an array based on particular criteria
const myArray = [{x:100}, {x:200}, {x:300}];
const newArray = myArray.filter(element => element.x > 250);
console.log(newArray); // [{x:300}]
Note: The filter() method creates a new array with all elements that pass the test implemented by the provided function.
8. Sort an array
const people = [
{ name: "John", age: 21 },
{ name: "Peter", age: 31 },
{ name: "Andrew", age: 29 },
{ name: "Thomas", age: 25 }
];
let sortByAge = people.sort(function (p1, p2) {
return p1.age - p2.age;
});
console.log(sortByAge);
9. Find an element in an array
const people = [ {name: "john", age:23},
{name: "john", age:43},
{name: "jim", age:101},
{name: "bob", age:67} ];
const john = people.find(person => person.name === 'john');
console.log(john);
The Array.prototype.find() method returns the value of the first element in the array that satisfies the provided testing function.
References
Array.prototype.some()
Array.prototype.forEach()
Array.prototype.map()
Array.prototype.filter()
Array.prototype.sort()
Spread syntax
Array.prototype.find()

You can use a for..of loop to loop over an array of objects.
for (let item of items) {
console.log(item); // Will display contents of the object inside the array
}
One of the best things about for..of loops is that they can iterate over more than just arrays. You can iterate over any type of iterable, including maps and objects. Make sure you use a transpiler or something like TypeScript if you need to support older browsers.
If you wanted to iterate over a map, the syntax is largely the same as the above, except it handles both the key and value.
for (const [key, value] of items) {
console.log(value);
}
I use for..of loops for pretty much every kind of iteration I do in Javascript. Furthermore, one of the coolest things is they also work with async/await as well.

for (var j = 0; j < myArray.length; j++){
console.log(myArray[j].x);
}

Here's an example on how you can do it :)
var students = [{
name: "Mike",
track: "track-a",
achievements: 23,
points: 400,
},
{
name: "james",
track: "track-a",
achievements: 2,
points: 21,
},
]
students.forEach(myFunction);
function myFunction(item, index) {
for (var key in item) {
console.log(item[key])
}
}

Looping through an array of objects is a pretty fundamental functionality. This is what works for me.
var person = [];
person[0] = {
firstName: "John",
lastName: "Doe",
age: 60
};
var i, item;
for (i = 0; i < person.length; i++) {
for (item in person[i]) {
document.write(item + ": " + person[i][item] + "<br>");
}
}

It's really simple using the forEach method since ES5+. You can directly change each property of each object in your array.
myArray.forEach(function (arrayElem){
arrayElem = newPropertyValue;
});
If you want to access a specific property on each object:
myArray.forEach(function (arrayElem){
arrayElem.nameOfYourProperty = newPropertyValue;
});

myArray[j.x] is logically incorrect.
Use (myArray[j].x); instead
for (var j = 0; j < myArray.length; j++){
console.log(myArray[j].x);
}

const jobs = [
{
name: "sipher",
family: "sipherplus",
job: "Devops"
},
{
name: "john",
family: "Doe",
job: "Devops"
},
{
name: "jim",
family: "smith",
job: "Devops"
}
];
const txt =
` <ul>
${jobs.map(job => `<li>${job.name} ${job.family} -> ${job.job}</li>`).join('')}
</ul>`
;
document.body.innerHTML = txt;
Be careful about the back Ticks (`)

this.data = [{name:"Rajiv", city:"Deoria"},{name:"Babbi", city:"Salempr"},{name:"Brijesh", city:"GKP"}];
for(const n of this.data) {
console.log(n.name)
}

This would work. Looping thorough array(yourArray) . Then loop through direct properties of each object (eachObj) .
yourArray.forEach( function (eachObj){
for (var key in eachObj) {
if (eachObj.hasOwnProperty(key)){
console.log(key,eachObj[key]);
}
}
});

Accepted answer uses normal function. So posting the same code with slight modification using arrow function on forEach
yourArray.forEach(arrayItem => {
var x = arrayItem.prop1 + 2;
console.log(x);
});
Also in $.each you can use arrow function like below
$.each(array, (item, index) => {
console.log(index, item);
});

Here's another way of iterating through an array of objects (you need to include jQuery library in your document for these).
$.each(array, function(element) {
// do some operations with each element...
});

Array object iteration, using jQuery,
(use the second parameter to print the string).
$.each(array, function(index, item) {
console.log(index, item);
});

var c = {
myProperty: [
{ name: 'this' },
{ name: 'can' },
{ name: 'get' },
{ name: 'crazy' }
]
};
c.myProperty.forEach(function(myProperty_element) {
var x = myProperty_element.name;
console.log('the name of the member is : ' + x);
})
This is one of the ways how I was able to achieve it.

I want to loop and deconstruction assignment at the same time, so code like this: config.map(({ text, callback })=>add_btn({ text, callback }))

This might help somebody. Maybe it's a bug in Node.
var arr = [ { name: 'a' }, { name: 'b' }, { name: 'c' } ];
var c = 0;
This doesn't work:
while (arr[c].name) { c++; } // TypeError: Cannot read property 'name' of undefined
But this works...
while (arr[c]) { c++; } // Inside the loop arr[c].name works as expected.
This works too...
while ((arr[c]) && (arr[c].name)) { c++; }
BUT simply reversing the order does not work. I'm guessing there's some kind of internal optimization here that breaks Node.
while ((arr[c].name) && (arr[c])) { c++; }
Error says the array is undefined, but it's not :-/ Node v11.15.0

I know it's been long but for anyone else encountering this issue, my problem is that I was looping through an array of arrays containing only one array. Like this:
// array snippet (returned from here)
} else {
callback([results])
}
And I was using the array like this
for(const result of results){
console.log(result.x)
}
As you can see, the array I wanted to iterate over was actually inside another array. removing the square brackets helped. Node JS and MySQL.

How do I get a specific object from an immutable js map by value?

I created an immutable map (with Immutable-JS) from a list of objects:
var result = [{'id': 2}, {'id': 4}];
var map = Immutable.fromJS(result);
Now i want to get the object with id = 4.
Is there an easier way than this:
var object = map.filter(function(obj){
return obj.get('id') === 4
}).first();

Essentially, no: you're performing a list lookup by value, not by index, so it will always be a linear traversal.
An improvement would be to use find instead of filter:
var result = map.find(function(obj){return obj.get('id') === 4;});

The first thing to note is that you're not actually creating a map, you're creating a list:
var result = [{'id': 2}, {'id': 4}];
var map = Immutable.fromJS(result);
Immutable.Map.isMap(map); // false
Immutable.List.isList(map); // true
In order to create a map you can use a reviver argument in your toJS call (docs), but it's certainly not the most intuitive api, alternatively you can do something like:
// lets use letters rather than numbers as numbers get coerced to strings anyway
var result = [{'id': 'a'}, {'id': 'b'}];
var map = Immutable.Map(result.reduce(function(previous, current) {
previous[ current.id ] = current;
return previous;
}, {}));
Immutable.Map.isMap(map); // true
Now we have a proper Immutable.js map which has a get method
var item = Map.get('a'); // {id: 'a'}

It may be important to guarantee the order of the array. If that's the case:
Use an OrderedMap
Do a set method on the OrderedMap at each iteration of your source array
The example below uses "withMutations" for better performance.
var OrderedMap = Immutable.OrderedMap
// Get new OrderedMap
function getOm(arr) {
return OrderedMap().withMutations(map => {
arr.forEach(item => map.set(item.id, item))
})
}
// Source collection
var srcArray = [
{
id: 123,
value: 'foo'
},
{
id: 456,
value: 'bar'
}
]
var myOrderedMap = getOm(srcArray)
myOrderedMap.get(123)
// --> { id: 123, value: 'foo' }
myOrderedMap.toObject()
// --> { 123: {id: 123, value: 'foo'}, 456: {id: 456, value: 'bar'} }
myOrderedMap.toArray()
// --> [ {id: 123, value: 'foo'}, { id: 456, value: 'bar' } ]

When using fromJS for array, you'll get List not map. It will be better and easier if you create a map. The following code will convert the result into Immutable map.
const map = result.reduce((map, json) =>
map.set(json.id, Immutable.fromJS(json))
, Map());
Now, you can
map.get('2'); //{'id': 2}
Note, if the result has nested structure and if that has array, it will be a List with the above code.

With ES2015 syntax (and constants):
const result = map.find(o => o.get('id') === 4);

Is there already a way thats easier? I don't know. but you can write your own function. Something like this should work:
var myFunc = function(id){
var object = map.filter(function(obj){return obj.get('id') === id}).first();
return object;
}
Then you would just do:
var myObj = myFunc(4);

How to loop through an array containing objects and access their properties

I want to cycle through the objects contained in an array and change the properties of each one. If I do this:
for (var j = 0; j < myArray.length; j++){
console.log(myArray[j]);
}
The console should bring up every object in the array, right? But in fact it only displays the first object. if I console log the array outside of the loop, all the objects appear so there's definitely more in there.
Anyway, here's the next problem. How do I access, for example Object1.x in the array, using the loop?
for (var j = 0; j < myArray.length; j++){
console.log(myArray[j.x]);
}
This returns "undefined." Again the console log outside the loop tells me that the objects all have values for "x". How do I access these properties in the loop?
I was recommended elsewhere to use separate arrays for each of the properties, but I want to make sure I've exhausted this avenue first.
Thank you!

Use forEach its a built-in array function. Array.forEach():
yourArray.forEach(function (arrayItem) {
var x = arrayItem.prop1 + 2;
console.log(x);
});

Some use cases of looping through an array in the functional programming way in JavaScript:
1. Just loop through an array
const myArray = [{x:100}, {x:200}, {x:300}];
myArray.forEach((element, index, array) => {
console.log(element.x); // 100, 200, 300
console.log(index); // 0, 1, 2
console.log(array); // same myArray object 3 times
});
Note: Array.prototype.forEach() is not a functional way strictly speaking, as the function it takes as the input parameter is not supposed to return a value, which thus cannot be regarded as a pure function.
2. Check if any of the elements in an array pass a test
const people = [
{name: 'John', age: 23},
{name: 'Andrew', age: 3},
{name: 'Peter', age: 8},
{name: 'Hanna', age: 14},
{name: 'Adam', age: 37}];
const anyAdult = people.some(person => person.age >= 18);
console.log(anyAdult); // true
3. Transform to a new array
const myArray = [{x:100}, {x:200}, {x:300}];
const newArray= myArray.map(element => element.x);
console.log(newArray); // [100, 200, 300]
Note: The map() method creates a new array with the results of calling a provided function on every element in the calling array.
4. Sum up a particular property, and calculate its average
const myArray = [{x:100}, {x:200}, {x:300}];
const sum = myArray.map(element => element.x).reduce((a, b) => a + b, 0);
console.log(sum); // 600 = 0 + 100 + 200 + 300
const average = sum / myArray.length;
console.log(average); // 200
5. Create a new array based on the original but without modifying it
const myArray = [{x:100}, {x:200}, {x:300}];
const newArray= myArray.map(element => {
return {
...element,
x: element.x * 2
};
});
console.log(myArray); // [100, 200, 300]
console.log(newArray); // [200, 400, 600]
6. Count the number of each category
const people = [
{name: 'John', group: 'A'},
{name: 'Andrew', group: 'C'},
{name: 'Peter', group: 'A'},
{name: 'James', group: 'B'},
{name: 'Hanna', group: 'A'},
{name: 'Adam', group: 'B'}];
const groupInfo = people.reduce((groups, person) => {
const {A = 0, B = 0, C = 0} = groups;
if (person.group === 'A') {
return {...groups, A: A + 1};
} else if (person.group === 'B') {
return {...groups, B: B + 1};
} else {
return {...groups, C: C + 1};
}
}, {});
console.log(groupInfo); // {A: 3, C: 1, B: 2}
7. Retrieve a subset of an array based on particular criteria
const myArray = [{x:100}, {x:200}, {x:300}];
const newArray = myArray.filter(element => element.x > 250);
console.log(newArray); // [{x:300}]
Note: The filter() method creates a new array with all elements that pass the test implemented by the provided function.
8. Sort an array
const people = [
{ name: "John", age: 21 },
{ name: "Peter", age: 31 },
{ name: "Andrew", age: 29 },
{ name: "Thomas", age: 25 }
];
let sortByAge = people.sort(function (p1, p2) {
return p1.age - p2.age;
});
console.log(sortByAge);
9. Find an element in an array
const people = [ {name: "john", age:23},
{name: "john", age:43},
{name: "jim", age:101},
{name: "bob", age:67} ];
const john = people.find(person => person.name === 'john');
console.log(john);
The Array.prototype.find() method returns the value of the first element in the array that satisfies the provided testing function.
References
Array.prototype.some()
Array.prototype.forEach()
Array.prototype.map()
Array.prototype.filter()
Array.prototype.sort()
Spread syntax
Array.prototype.find()

You can use a for..of loop to loop over an array of objects.
for (let item of items) {
console.log(item); // Will display contents of the object inside the array
}
One of the best things about for..of loops is that they can iterate over more than just arrays. You can iterate over any type of iterable, including maps and objects. Make sure you use a transpiler or something like TypeScript if you need to support older browsers.
If you wanted to iterate over a map, the syntax is largely the same as the above, except it handles both the key and value.
for (const [key, value] of items) {
console.log(value);
}
I use for..of loops for pretty much every kind of iteration I do in Javascript. Furthermore, one of the coolest things is they also work with async/await as well.

for (var j = 0; j < myArray.length; j++){
console.log(myArray[j].x);
}

Here's an example on how you can do it :)
var students = [{
name: "Mike",
track: "track-a",
achievements: 23,
points: 400,
},
{
name: "james",
track: "track-a",
achievements: 2,
points: 21,
},
]
students.forEach(myFunction);
function myFunction(item, index) {
for (var key in item) {
console.log(item[key])
}
}

Looping through an array of objects is a pretty fundamental functionality. This is what works for me.
var person = [];
person[0] = {
firstName: "John",
lastName: "Doe",
age: 60
};
var i, item;
for (i = 0; i < person.length; i++) {
for (item in person[i]) {
document.write(item + ": " + person[i][item] + "<br>");
}
}

It's really simple using the forEach method since ES5+. You can directly change each property of each object in your array.
myArray.forEach(function (arrayElem){
arrayElem = newPropertyValue;
});
If you want to access a specific property on each object:
myArray.forEach(function (arrayElem){
arrayElem.nameOfYourProperty = newPropertyValue;
});

myArray[j.x] is logically incorrect.
Use (myArray[j].x); instead
for (var j = 0; j < myArray.length; j++){
console.log(myArray[j].x);
}

const jobs = [
{
name: "sipher",
family: "sipherplus",
job: "Devops"
},
{
name: "john",
family: "Doe",
job: "Devops"
},
{
name: "jim",
family: "smith",
job: "Devops"
}
];
const txt =
` <ul>
${jobs.map(job => `<li>${job.name} ${job.family} -> ${job.job}</li>`).join('')}
</ul>`
;
document.body.innerHTML = txt;
Be careful about the back Ticks (`)

this.data = [{name:"Rajiv", city:"Deoria"},{name:"Babbi", city:"Salempr"},{name:"Brijesh", city:"GKP"}];
for(const n of this.data) {
console.log(n.name)
}

This would work. Looping thorough array(yourArray) . Then loop through direct properties of each object (eachObj) .
yourArray.forEach( function (eachObj){
for (var key in eachObj) {
if (eachObj.hasOwnProperty(key)){
console.log(key,eachObj[key]);
}
}
});

Accepted answer uses normal function. So posting the same code with slight modification using arrow function on forEach
yourArray.forEach(arrayItem => {
var x = arrayItem.prop1 + 2;
console.log(x);
});
Also in $.each you can use arrow function like below
$.each(array, (item, index) => {
console.log(index, item);
});

Here's another way of iterating through an array of objects (you need to include jQuery library in your document for these).
$.each(array, function(element) {
// do some operations with each element...
});

Array object iteration, using jQuery,
(use the second parameter to print the string).
$.each(array, function(index, item) {
console.log(index, item);
});

var c = {
myProperty: [
{ name: 'this' },
{ name: 'can' },
{ name: 'get' },
{ name: 'crazy' }
]
};
c.myProperty.forEach(function(myProperty_element) {
var x = myProperty_element.name;
console.log('the name of the member is : ' + x);
})
This is one of the ways how I was able to achieve it.

I want to loop and deconstruction assignment at the same time, so code like this: config.map(({ text, callback })=>add_btn({ text, callback }))

This might help somebody. Maybe it's a bug in Node.
var arr = [ { name: 'a' }, { name: 'b' }, { name: 'c' } ];
var c = 0;
This doesn't work:
while (arr[c].name) { c++; } // TypeError: Cannot read property 'name' of undefined
But this works...
while (arr[c]) { c++; } // Inside the loop arr[c].name works as expected.
This works too...
while ((arr[c]) && (arr[c].name)) { c++; }
BUT simply reversing the order does not work. I'm guessing there's some kind of internal optimization here that breaks Node.
while ((arr[c].name) && (arr[c])) { c++; }
Error says the array is undefined, but it's not :-/ Node v11.15.0

I know it's been long but for anyone else encountering this issue, my problem is that I was looping through an array of arrays containing only one array. Like this:
// array snippet (returned from here)
} else {
callback([results])
}
And I was using the array like this
for(const result of results){
console.log(result.x)
}
As you can see, the array I wanted to iterate over was actually inside another array. removing the square brackets helped. Node JS and MySQL.

underscore.js filter an array of objects, based on another

I am trying to filter an array of objects, based on another. The common property id id.
I am not sure filter + each is the best way to do it or map reduce. Anyway, below code doesn't work as out is empty list.
var aaa = [
{name: "AAA", id: 845},
{name: "BBB", id: 839},
{name: "CCC", id: 854}
];
var bbb = [
{id: 839},
{id: 854}
];
var out = _.filter(aaa, function(val){
return _.each(this, function(val2){
return val['id'] === val2['id']
});
}, bbb);

Just create a "set" of the valid ids and use that "set" to do the filtering:
var aaa = [
{name: "AAA", id: 845},
{name: "BBB", id: 839},
{name: "CCC", id: 854}
];
var bbb = [
{id: 839},
{id: 854}
];
var ids = {};
_.each(bbb, function (bb) { ids[bb.id] = true; });
var out = _.filter(aaa, function (val) {
return ids[val.id];
}, bbb);
Filling ids is fast, it's in n * amortized O(1), i.e O(n). Same holds for the filtering.
If you use each(…) in the inner loop, you will have O(n²). For bigger data sets this would become very slow. Also the additional nesting make the code more difficult to read/understand at first glance.
See that code snipped in action: http://jsfiddle.net/SMtX5/

you can use _.find to filter:
_.filter(aaa, function(a){
return _.find(bbb, function(b){
return b.id === a.id;
});
});

You can use _.some(list, [iterator], [context]).
It returns true if any of the values in the list pass the iterator truth test.
var out = _.filter(aaa, function(val){
return _.some(this,function(val2){
return val2['id'] === val['id'];
});
}, bbb);
Here is jsfiddle. http://jsfiddle.net/h98ej/

bbb = bbb.map(_ => _.id) && aaa.filter(_ => bbb.indexOf( _.id ) > -1)
You just need pure JS array functions to do that assuming your use case.