Add a Callback Function to a Ruby Array to Do Something When an Element Is Added

Add a callback function to a Ruby array to do something when an element is added

The following code only invokes the size_changed hook when the array size has changed and it is passed the new size of the array:

a = []

class << a
  Array.instance_methods(false).each do |meth|
    old = instance_method(meth)
    define_method(meth) do |*args, &block|
      old_size = size
      old.bind(self).call(*args, &block)
      size_changed(size) if old_size != size
    end if meth != :size
  end
end

def a.size_changed(a)
  puts "size change to: #{a}"
end

a.push(:a) #=> size change to 1
a.push(:b) #=> size change to 2
a.length 
a.sort!
a.delete(:a) #=> size change to 1

Add key value pair to all objects in array

The map() function is a best choice for this case

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tl;dr - Do this:

const newArr = [
  {name: 'eve'},
  {name: 'john'},
  {name: 'jane'}
].map(v => ({...v, isActive: true}))

The map() function won't modify the initial array, but creates a new one.
This is also a good practice to keep initial array unmodified.

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Alternatives:

const initialArr = [
      {name: 'eve'},
      {name: 'john'},
      {name: 'jane'}
    ]

const newArr1 = initialArr.map(v => ({...v, isActive: true}))
const newArr2 = initialArr.map(v => Object.assign(v, {isActive: true}))

// Results of newArr1 and newArr2 are the same

Add a key value pair conditionally

const arr = [{value: 1}, {value: 1}, {value: 2}]   
const newArr1 = arr.map(v => ({...v, isActive: v.value > 1}))

What if I don't want to add new field at all if the condition is false?

const arr = [{value: 1}, {value: 1}, {value: 2}]   
const newArr = arr.map(v => {
  return v.value > 1 ? {...v, isActive: true} : v 
})

Adding WITH modification of the initial array

const initialArr = [{a: 1}, {b: 2}]
initialArr.forEach(v => {v.isActive = true;});

This is probably not a best idea, but in a real life sometimes it's the only way.

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Questions

• Should I use a spread operator(...), or Object.assign and what's the difference?

Personally I prefer to use spread operator, because I think it uses much wider in modern web community (especially react's developers love it). But you can check the difference yourself: link(a bit opinionated and old, but still)

• Can I use function keyword instead of =>?

Sure you can. The fat arrow (=>) functions play a bit different with this, but it's not so important for this particular case. But fat arrows function shorter and sometimes plays better as a callbacks. Therefore the usage of fat arrow functions is more modern approach.

• What Actually happens inside map function: .map(v => ({...v, isActive: true})?

Map function iterates by array's elements and apply callback function for each of them. That callback function should return something that will become an element of a new array.
We tell to the .map() function following: take current value(v which is an object), take all key-value pairs away from v andput it inside a new object({...v}), but also add property isActive and set it to true ({...v, isActive: true}) and then return the result. Btw, if original object contains isActive filed it will be overwritten. Object.assign works in a similar way.

• Can I add more then one field at a time

Yes.

[{value: 1}, {value: 1}, {value: 2}].map(v => ({...v, isActive: true, howAreYou: 'good'}))

• What I should not do inside .map() method

You shouldn't do any side effects[link 1, link 2], but apparently you can.

Also be noticed that map() iterates over each element of the array and apply function for each of them. So if you do some heavy stuff inside, you might be slow. This (a bit hacky) solution might be more productive in some cases (but I don't think you should apply it more then once in a lifetime).

• Can I extract map's callback to a separate function?

Sure you can.

const arr = [{value: 1}, {value: 1}, {value: 2}]   
const newArr = arr.map(addIsActive)

function addIsActive(v) {
    return {...v, isActive: true}
}

• What's wrong with old good for loop?

Nothing is wrong with for, you can still use it, it's just an old-school approach which is more verbose, less safe and mutate the initial array. But you can try:

const arr = [{a: 1}, {b: 2}]
for (let i = 0; i < arr.length; i++) {
 arr[i].isActive = true
}

• What also i should learn

It would be smart to learn well following methods map(), filter(), reduce(), forEach(), and find(). These methods can solve 80% of what you usually want to do with arrays.

In .filter() how do I make a callback function call another function?

Yes, you can certainly call any other functions inside a filter callback.

The only thing you're missing is that the callback needs to return a value. Your callback doesn't have a return statement. It calls the filterCreatures() function, but then it discards the value returned by that function and by default returns undefined.

Compare with the original version of your code:

creatures.filter(filterCreatures);

In this code, filter() calls the filterCreatures function directly and then uses its return value to decide whether to include the element. You don't see this explicitly in the code, but filter() is looking for that return value.

Here's an exercise to help clarify this. Take your broken code and move the inline filter callback out to a named function. We'll call it filterCreaturesWrapper. So instead of this:

filterCreatures = function(v) {
  return v.species === 'Zombie';
}

zombieCreatures = creatures.filter(function(v) {
  filterCreatures(v);
});

we have this:

filterCreatures = function(v) {
  return v.species === 'Zombie';
}

filterCreaturesWrapper = function(v) {
  filterCreatures(v);
}

zombieCreatures = creatures.filter(filterCreaturesWrapper);

If you study that, you'll see it's exactly the same code as before, we just moved it around a little. filterCreaturesWrapper is the same function that was inline inside the .filter() call. And now the problem should jump out at us: filterCreatures has a return statement and filterCreaturesWrapper doesn't.

Going back to the original broken code, it's as if we had written:

zombieCreatures = creatures.filter(function(v) {
  filterCreatures(v);
  return undefined;  // .filter treats this as 'false'
});

So just add a return in your callback and it works:

'use strict';
var creatures = [], zombieCreatures = [];
var filterCreatures;
creatures = [  {species: 'Zombie', hitPoints: 90},  {species: 'Orc', hitPoints: 40},  {species: 'Skeleton', hitPoints: 15},  {species: 'Zombie', hitPoints: 85}]; filterCreatures = function(v) {  return v.species === 'Zombie';} zombieCreatures = creatures.filter(function(v) {  return filterCreatures(v);  // <-- added "return"});
console.log(zombieCreatures);

Update if exists or add new element to array of objects - elegant way in javascript + lodash

You can use an object instead of an array:

var hash = {
  '1': {uid: 1, name: "bla", description: "cucu"},
  '2': {uid: 2, name: "smth else", description: "cucarecu"}
};

The keys are the uids. Now your function addOrReplace is simple like this:

function addOrReplace(hash, object) {
    hash[object.uid] = object;
}

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UPDATE

It's also possible to use an object as an index in addition to the array.

This way you've got fast lookups and also a working array:

var arr = [],
    arrIndex = {};

addOrReplace({uid: 1, name: "bla", description: "cucu"});
addOrReplace({uid: 2, name: "smth else", description: "cucarecu"});
addOrReplace({uid: 1, name: "bli", description: "cici"});

function addOrReplace(object) {
    var index = arrIndex[object.uid];
    if(index === undefined) {
        index = arr.length;
        arrIndex[object.uid] = index;
    }
    arr[index] = object;
}

Take a look at the jsfiddle-demo (an object-oriented solution you'll find here)

Is there a ruby equivalent to javascript's Array.prototype.every method?

I would use all?.

my_array.all? { |element| element >= 10 }

You pass in a block of code which is functionally equivalent to passing your function in JavaScript.

Arrays: conditionally mapping elements

There is no such native function because you can easily implement it yourself:

const mapWhen = (p, f) => xs => xs.map(x => p(x) ? f(x) : x);
const users = [  {id: 1, removed: true},  {id: 2, removed: true},  {id: 3, removed: true}];
console.log(  mapWhen(    ({id}) => id === 2,    user => Object.assign({}, user, {removed: false})  ) (users));

Magic First and Last Indicator in a Loop in Ruby/Rails?

You could grab the first and last elements and process them differently, if you like.

first = array.shift
last = array.pop
process_first_one
array.each { |x| process_middle_bits }
process_last_one

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