callback functions – Crooked Code

Asynchronous JavaScript using Callbacks

I recently struggled when faced with a problem of using callbacks to handle the asynchronous flow of multiple API calls.

So I figured I’d do a deep dive into callbacks to refresh and solidify my knowledge. After all, if you want to truly understand something, teach it to someone else!

Functions as First Class Objects

Before we start talking about callbacks, we must first know that JavaScript functions are first-class objects.

Which means what? And why is that important to a discussion about callbacks?

Being a first-class object in JavaScript means that functions can have properties and methods just like any other object. Consider this:

function firstClass() {
    //DO SOMETHING
}

//ADD A PROPERTY TO THE FUNCTION OBJECT
firstClass.answer = 42;

console.log(firstClass.answer);  //OUTPUT = 42

function firstClass() {

//DO SOMETHING

}

//ADD A PROPERTY TO THE FUNCTION OBJECT

firstClass.answer = 42;

console.log(firstClass.answer); //OUTPUT = 42

It also means that functions can be assigned to a variable.

//ASSIGN A FUNCTION TO THE firstClass VARIABLE
let firstClass = function(answer) {
    console.log(answer);
}

console.log(firstClass(42));  //OUTPUT = 42

//ASSIGN A FUNCTION TO THE firstClass VARIABLE

let firstClass = function(answer) {

console.log(answer);

}

console.log(firstClass(42)); //OUTPUT = 42

More importantly for our discussion on callbacks, however, is that because functions are first-class objects, they can be passed to (and returned from) other functions.

//CREATE FUNCTION THAT TAKES A CALLBACK
function logCallback(answer) {
    console.log(answer());
}

//DEFINE THE CALLBACK
function firstClassFunction() {
    return 42;
}

//CALL THE ORIGINAL FUNCTION WITH OUR CALLBACK
logCallback(firstClassFunction);  //OUTPUT = 42

//CREATE FUNCTION THAT TAKES A CALLBACK

function logCallback(answer) {

console.log(answer());

}

//DEFINE THE CALLBACK

function firstClassFunction() {

return 42;

}

//CALL THE ORIGINAL FUNCTION WITH OUR CALLBACK

logCallback(firstClassFunction); //OUTPUT = 42

This last point, passing functions as arguments to other functions, is the foundation of callbacks.

A callback is simply a function that is passed to another function as an argument, that can then be used (executed) inside that other function.

The previous example used a named function (answer) as the callback. You can (and probably have) also use anonymous functions as callbacks. This is common when passing callbacks to the native prototype methods for arrays, objects and strings.

Consider the following:

let numArr = [1,2,3];

numArr.forEach((element) => { console.log(element * 2)});

//2
//4
//6

let numArr = [1,2,3];

numArr.forEach((element) => { console.log(element * 2)});

//2

//4

//6

Here we used an anonymous arrow function as our callback to output twice the value of each element.

Now that we know what callbacks are, let’s talk about their use in asynchronous JavaScript.

Understanding ASYNCHRONY

Image of the Police Album - Synchronicity

No, this isn’t what we’re talking about, but the word ‘asynchony’ makes me think of this album….. Yeah, I know, I’m old…

Before we start talking about controlling asynchronous flow we must first understand the nature of the JavaScript event loop and the fact that it is single threaded. Digging deep into the JavaScript engine is beyond the scope of this post, but understanding the single threaded nature of the event loop is critical to understanding why we need asynchronous code.

For now, think of the event loop as a queue (first in, first out) that executes snippets of your JS code one by one and in the order they were placed in the queue.

Now think of an Ajax request that we might use to get some data from an API. That Ajax request is costly (in time). If we place it in the event loop and run it synchronously it will block everything that comes after it while it waits for a response. This affects performance and causes people to leave your site!

In his You Don’t Know JS series, Kyle Simpson describes asynchrony as the difference between now and later, as opposed to parallel, which describes two processes that are executed independent of each other (but possibly at the same time).

This now and later concept is where callback functions come in. Rather than running that Ajax request from start to finish, blocking all other code execution while we wait, we pass it a callback.

In this scenario, the JS engine can make the Ajax call (now), continue with the event loop, and once the Ajax call returns (later) pass the callback, along with any data it needs from the response, back into the event loop.

Async in action

Let use an example to show what we’re talking about. For the remaining examples, I’m going to use setTimeout() to simulate async functions, but you can think of it as time spent making an Ajax request.

function first() {
    console.log("first");
}

function second() {
    console.log("second");
}

function delay(callback){
    window.setTimeout(callback, 500);
}

delay(first);
second();

//second
//first

function first() {

console.log("first");

}

function second() {

console.log("second");

}

function delay(callback){

window.setTimeout(callback, 500);

}

delay(first);

second();

//second

//first

This example shows asynchronous control of the event loop. Let’s dig deeper in case this isn’t clear.

We’ve defined three functions first, second and delay.

first and second merely log “first” and “second” to the console so we can tell when they’re executed.

delay takes a callback, which it passes to setTimeout(). setTimeout() sets a timer, then calls the function it was passed – in this case, it will call callback after 500 milliseconds.

After the three function definitions, we call delay(first), which puts the function delay() in our event loop. Upon execution, delay sets a timer for 500 ms, after which, it will call its callback (in this case first()).

Because the timer in setTimeout() is non-blocking, or asynchronous, the event loop is now free to move on to the next snippet of JS, which is our function call to second().

When second is executed, it logs “second” to the console, which is why we see “second” logged to the console before we see “first”, even though delay(first) was called before second().

And finally, after the 500 ms timer elapses, first() is placed in our event loop and executed, which is when “first” shows up in our console.

I’m hoping the example above was simple enough that you could figure it out on your own, otherwise, maybe my long winded explanation helped.

Fun with callbacks and closure

OK, now that we’re pros with callbacks, lets have a little fun with them!

We’re going to use callbacks and closure to set up a four lane race. Granted, it’s only a race in the sense that we’re randomizing the setTimout() time, but it’s fun nonetheless…

/*
    *WHEN EXECUTED, racer() WILL RETURN A FUNCTION THAT HAS CLOSURE OVER THE 'lane' VALUE 
    *THE RETURNED FUNCTION TAKES A CALLBACK AS AN ARGUMENT
    *THIS CALLBACK IS THEN EXECUTED WITH THE LANE VALUE AS AN ARGUMENT AT A RANDOM TIME BETWEEN 0 AND 5 SECONDS
    *WE'LL CALL THIS FUNCTION WITH 4 DIFFERENT LANE VALUES TO SIMULATE A 'RACE'
*/
function racer(lane) {
    return (place) => {
        window.setTimeout(() => {
            place(lane);
        }, Math.random() * 5000);
    };
}

function runRace(one, two, three, four, printResults) {

    let placeOrder = [0, 0, 0, 0];  //TO STORE RESULTS

    //START RACE BY EXECUTING FIRST 4 ARGS (FUNCTIONS) 
    one(finish);
    two(finish);
    three(finish);
    four(finish);

    //FUNCTION TO SERVE AS CALLBACK - SIMULATES THE RACER 'FINISHING'
    function finish(laneValue) {
        //PUT laneValue IN THE NEXT AVAILABLE SPOT IN placeOrder
        let i=0;
        while(placeOrder[i] != 0) {
            i++;
        }
        placeOrder[i] = laneValue; 

        //CHECK IF RACE IS OVER, IF SO, PRINT RESULTS
        if(i == placeOrder.length-1){
            printResults(...placeOrder);
        }
    }
}

function showResults(first, second, third, fourth) {
    console.log(`The race order was ${first}, ${second}, ${third}, ${fourth}`);
}

  //////////////    START THE RACE  ////////////////////
/* 
    *CALL THE RACER FUNCTION WITH THE LANE NUMBER (1 THRU 4)
    *ARGS 1 THRU 4 IN runRace ARE NOW FUNCTIONS WITH CLOSURE OVER THEIR LANE NUMBER
*/
runRace(racer(1), racer(2), racer(3), racer(4), showResults);

*WHEN EXECUTED, racer() WILL RETURN A FUNCTION THAT HAS CLOSURE OVER THE 'lane' VALUE

*THE RETURNED FUNCTION TAKES A CALLBACK AS AN ARGUMENT

*THIS CALLBACK IS THEN EXECUTED WITH THE LANE VALUE AS AN ARGUMENT AT A RANDOM TIME BETWEEN 0 AND 5 SECONDS

*WE'LL CALL THIS FUNCTION WITH 4 DIFFERENT LANE VALUES TO SIMULATE A 'RACE'

function racer(lane) {

return (place) => {

window.setTimeout(() => {

place(lane);

}, Math.random() * 5000);

};

}

function runRace(one, two, three, four, printResults) {

let placeOrder = [0, 0, 0, 0]; //TO STORE RESULTS

//START RACE BY EXECUTING FIRST 4 ARGS (FUNCTIONS)

one(finish);

two(finish);

three(finish);

four(finish);

//FUNCTION TO SERVE AS CALLBACK - SIMULATES THE RACER 'FINISHING'

function finish(laneValue) {

//PUT laneValue IN THE NEXT AVAILABLE SPOT IN placeOrder

let i=0;

while(placeOrder[i] != 0) {

i++;

}

placeOrder[i] = laneValue;

//CHECK IF RACE IS OVER, IF SO, PRINT RESULTS

if(i == placeOrder.length-1){

printResults(...placeOrder);

}

function showResults(first, second, third, fourth) {

console.log(`The race order was ${first}, ${second}, ${third}, ${fourth}`);

}

////////////// START THE RACE ////////////////////

*CALL THE RACER FUNCTION WITH THE LANE NUMBER (1 THRU 4)

*ARGS 1 THRU 4 IN runRace ARE NOW FUNCTIONS WITH CLOSURE OVER THEIR LANE NUMBER

runRace(racer(1), racer(2), racer(3), racer(4), showResults);

Hopefully, the comments in the code clear up how the “race” is being run. If you have any questions, feel free to leave them below.

-Jeremy

For Loops vs. forEach()

I recently wrote a post here at Crooked Code discussing that, as a relative beginner to JavaScript, I’ve had a tendency to use loops exclusively when traversing an array.

Further, I wondered if it would be better if I were taking advantage of the methods built into the array prototype. Methods such as forEach(), reduce(), map() and filter().

Let’s take a look…

For reference, MDN is a great resource to learn more about arrays and the array prototype. They cover forEach() in far greater detail than I will here, so I encourage you to go read it.

How do you use forEach()?

Simply put, forEach() takes a callback function as an argument and executes that function on each element of the calling array.

The callback function takes 3 arguments – the element value, the element index and the array being traversed. Instances where the index and array are not needed in the callback function, you’ll see it invoked with only the element value.

A simple example of forEach():

var array = ['one', 'two', 'three', 'four'];

array.forEach(function(val){
  console.log(val);
});

//one
//two
//three
//four

var array = ['one', 'two', 'three', 'four'];

array.forEach(function(val){

console.log(val);

});

//one

//two

//three

//four

When should I use forEach vs. a for loop?

Researching the differences between a for loop and forEach() to write this post has been somewhat enlightening (there are some very opinionated people out there).

First, if you need to stop or break out of the loop before every element is visited, forEach() is not the right function. In such cases, use a for loop, or look into using every() or some().

Other than that, it seems to come down to performance vs. readability.

Improved Readability with forEach()

Let’s revisit the example above and compare the same functionality using a for loop.

var array = ['one', 'two', 'three', 'four'];

//log array using forEach()
array.forEach(function(val){
  console.log(val);
});

//log array using for loop
for(let i=0; i<array.length; i++){
  console.log(array[i]);
};

var array = ['one', 'two', 'three', 'four'];

//log array using forEach()

array.forEach(function(val){

console.log(val);

});

//log array using for loop

for(let i=0; i<array.length; i++){

console.log(array[i]);

};

I’ll let you be the judge as to which one is more readable…

Personally, I think the more you use forEach(), the more you’ll prefer it to a for loop for readability. Clearly, not using additional variables or worrying about ‘off by one errors’ is an added benefit too.

It’s also worth mentioning that ES6 syntax, with the arrow function, improves the readability even further. I’m still writing my functions the archaic way, which is what I did above, but the same function written in ES6 would look something like this:

//log array using forEach()
array.forEach((val)=>{console.log(val);});

1 2	//log array using forEach() array.forEach((val)=>{console.log(val);});

Performance

I used jsperf to run performance tests to compare for loops, while loops and forEach(). The tests were run in the Chrome browser.

The for and while loops were the clear winners (6,233 and 6,261 Ops/sec respectively). forEach() was 18% slower at 5,202 Ops/sec.

So, on a sheer performance basis, the loops were faster than the function call to forEach(). I was a little surprised, however, that forEach() was only 18% slower than the loops.

Bottom Line…

I think you could argue either way of which one, the for loop or forEach(), is better. Ultimately, I think it depends on the use case…

If you’re developing something that is very data intensive, and the extra performance of the for loop will make a difference, go with the for loop.

Otherwise, if you find forEach() to be more readable and aren’t iterating over huge data sets, use forEach()…

Either way, get out there and build something cool… That’s it for now, see you next time.

-Jeremy

Side Note

Finally, I need to mention that the forEach() method takes an optional second argument that provides the value to use as this when executing the callback.

Covering this in depth was beyond the scope of this blog post, but You Don’t Know JavaScript: this & Object Prototypes Ch. 2 has an excellent, easy to understand example of using the this argument.

When all you have is a hammer, everything looks like a nail…

I recently posted about an overly elaborate solution that I wrote to a CodeWars algorithm. That post has been lurking in the back of my head since I wrote it…

I’ve come to realize that I can’t be too hard on myself for coming up with the solution that I did. After all, the only tool I had available to me was a hammer, so I treated every problem like a nail.

Man Hammering Nail — An image of me writing code

My point is that up until then, any time I had to traverse an array, I had used a for loop rather than any of the methods native to the array prototype. Setting the conditions of the loop to something like (var i = 0; i < arr.length; i++) allows you to easily access and manipulate any element of the array using arr[i]. So that’s what I did…

While there’s (usually) nothing inherently wrong with that (in fact, it’s how I solved many of the algorithm challenges at Free Code Camp), loops aren’t always the most elegant solution to a problem, especially when you start to consider asynchronous options in your code. Besides, the point of this journey is to become the strongest developer I can, so let’s explore other options.

While going through the front end program at Free Code Camp, I was exposed to many of the methods in the array prototype, including forEach(), reduce(), map() and filter(). The problem was that those methods required callback functions, of which my understanding was tenuous, so I found it much easier at the time to use loops.

Working with Node.js these past few months has given me much more exposure using callback functions, so I’d like to revisit these methods and solidify my understanding of them. That way, when I encounter problems in the future, I’ll have more than one tool at my disposal.

Over the next couple weeks, I will write a post that will cover each of the four array methods mentioned above in detail. Hopefully, these posts will serve as good resources for others in my position and, at the same time, give me extra practice using each.

I’ll be sure to come back and provide links to the follow up posts once they’re written.

That’s it for now, thanks for reading.

-Jeremy

FCC Advanced Algorithm Scripting Challenge – ‘Symmetric Difference’

Let’s tackle another Free Code Camp advanced algorithm – ‘Symmetric Difference’. The challenge here is to ‘create a function that takes two or more arrays and returns an array of the symmetric difference (△ or ⊕) of the provided arrays.’

The symmetric difference of two sets of elements is the set that occurs in either one of the two sets, but not both (i.e. the symmetric difference of [1,2,3,4] and [3,4,5,6] is [1,2,5,6]). For more reading on symmetric difference, check out this wiki page.

Free Code camp gives us the following as a starting point:

function sym(args) {
  return args;
}

sym([1, 2, 3], [5, 2, 1, 4]);

function sym(args) {

return args;

}

sym([1, 2, 3], [5, 2, 1, 4]);

This is another case where we have to write a function for which we need to handle an unknown number of arguments. To accomplish this, we’ll be using the arguments object. I’ve covered the arguments object in a previous post, ‘Seek and Destroy’. If you are unfamiliar with or would like a refresher of the arguments object, please check it out.

One of the helpful hints FCC provides us with is a link to the Array.prototype.reduce() method. The arr.reduce method is a great way to cycle through and apply a function to each element of an array.

The arr.reduce method takes 2 arguments, a callback function (which will be applied to each element of the array) and an initial value (the element to provide as the first argument to the first callback function call – see here for more).

If you’re new to JavaScript, or somewhat unclear of what a callback function is, JavaScriptIsSexy.com has an excellent post on callbacks.

The callback takes a possibility of 4 arguments – previousValue, currentValue, currentIndex and array. This is important, as it means when we write our callback function, we will have access to these 4 values within the function (for further clarity on exactly what the four values are, check out the MDN link mentioned previously).

I think we should break down this challenge into two main problems. We need to:

Write a function that returns an array of the symmetric difference (⊕) between two given arrays
Use this function to handle an unknown number of arguments. For example:
- find (arg1 ⊕ arg2)
- if there’s an agr3, find arg3 ⊕ (arg1 ⊕ arg2)
- continue for argX

At this point, I’m going to start discussing my solution to the problem. If you’re working through FCC, I’d suggest attempting a solution on your own given what we’ve discussed above. Otherwise, proceed on…

Spoilers_ahead

Let’s start by writing the function we’ll use to calculate the symmetric difference between two arrays.

In other words, we need to write a function that receives two arrays as arguments, then returns an array of the numbers that are in one of the argument arrays, but not both.

One way to do this is to

create a new empty array (curDiff) to store the symmetric diff
use arr.reduce() to cycle through each array (need to compare each array against the other, not just one against the other)
in the callback function of arr.reduce
- check if an element is in the other array or in the curDiff array (to prevent duplicates in curDiff)
- if not – push element to curDiff
  - hint – arr.indexOf() will return -1 if the given element is not found
return curDiff

Here’s the JavaScript code:

  function findSymDiff(a, b){
    //array to hold diffence
    var curDiff = [];
    //compare a against b and curDiff
    a.reduce(function(prevVal, curVal){
      if(b.indexOf(curVal)==-1 && curDiff.indexOf(curVal)==-1)
        curDiff.push(curVal);
    }, 0);
    //check b against a and currentDiff
    b.reduce(function(prevVal, curVal){
      if(a.indexOf(curVal)==-1 && curDiff.indexOf(curVal)==-1)
        curDiff.push(curVal);
    }, 0);
    
    return curDiff;
  }

function findSymDiff(a, b){

//array to hold diffence

var curDiff = [];

//compare a against b and curDiff

a.reduce(function(prevVal, curVal){

if(b.indexOf(curVal)==-1 && curDiff.indexOf(curVal)==-1)

curDiff.push(curVal);

}, 0);

//check b against a and currentDiff

b.reduce(function(prevVal, curVal){

if(a.indexOf(curVal)==-1 && curDiff.indexOf(curVal)==-1)

curDiff.push(curVal);

}, 0);

return curDiff;

}

We now have a function (findSymDiff()) that we can use within our main function to get the symmetric difference between two arrays. We now have to set up the logic to handle an unknown number of parameters received by our main function.

Our logic (using the arguments object) can look something like this:

create new array (symDiff) to store the array returned by findSymDiff()
compare arguments[0] to arguments[1] using findSymDiff()
cycle through argument[]
- compare symDiff to arguments[x]

To accomplish this, we can set up a simple for loop.

If we set

symDiff = arguments[0];

1	symDiff = arguments[0];

we can call

symDiff = findSymDiff(symDiff, arguments[i]);

1	symDiff = findSymDiff(symDiff, arguments[i]);

in our for loop starting at i=1 and loop while i<arguments.length.

Then all we have to do is return symDiff!

Here’s how everything looks when we put it together:

function sym(args) {

  //function to find symmetric difference between 2 arrays
  function findSymDiff(a, b){
    //array to hold diffence
    var curDiff = [];
    //check 1st array against 2nd and currentDiff (to prevent duplicates from same array)
    a.reduce(function(prevVal, curVal){
      if(b.indexOf(curVal)==-1 && curDiff.indexOf(curVal)==-1)
        curDiff.push(curVal);
    }, 0);
    //check 2nd against 1st and currentDiff
    b.reduce(function(prevVal, curVal){
      if(a.indexOf(curVal)==-1 && curDiff.indexOf(curVal)==-1)
        curDiff.push(curVal);
    }, 0);
    
    return curDiff;
  }
  
  //variable to hold the symmetric difference
  //set first arg equal to symDiff to make 1st call easier
  var symDiff = arguments[0];
  
  //loop through all arguments
  for(var i=1; i<arguments.length; i++){
    symDiff = findSymDiff(symDiff, arguments[i]);
  }
 
  return symDiff;
}

function sym(args) {

//function to find symmetric difference between 2 arrays

function findSymDiff(a, b){

//array to hold diffence

var curDiff = [];

//check 1st array against 2nd and currentDiff (to prevent duplicates from same array)

a.reduce(function(prevVal, curVal){

if(b.indexOf(curVal)==-1 && curDiff.indexOf(curVal)==-1)

curDiff.push(curVal);

}, 0);

//check 2nd against 1st and currentDiff

b.reduce(function(prevVal, curVal){

if(a.indexOf(curVal)==-1 && curDiff.indexOf(curVal)==-1)

curDiff.push(curVal);

}, 0);

return curDiff;

}

//variable to hold the symmetric difference

//set first arg equal to symDiff to make 1st call easier

var symDiff = arguments[0];

//loop through all arguments

for(var i=1; i<arguments.length; i++){

symDiff = findSymDiff(symDiff, arguments[i]);

}

return symDiff;

}

This solution passes all of the FCC test cases.

Thanks for reading, feel free to leave comments or additional solutions below. And if you’re working through Free Code Camp, hopefully this helped straighten out the code.

-Jeremy

FCC ‘Seek and Destroy’

This post will work through ‘Seek and Destroy’, one of freeCodeCamp‘s basic algorithm scripting challenges. This challenge will help us introduce the Arguments object and callback functions.

The Seek and Destroy challenge states that ‘provided with an initial array (the first argument in the destroyer function), followed by one or more arguments’ we need to write a function that will remove the values of all the arguments given after the initial array, from the initial array.

Sounds confusing, I know… Let’s visualize it to clear things up.

function destroyer(arr){
   //write a function that can be called 
   //with any number of arguments provided 
   //(see destroyer call below)
   //and remove those arguments from the initial array(arr)
}

destroyer(arr, value1, value2, value3, value4);

function destroyer(arr){

//write a function that can be called

//with any number of arguments provided

//(see destroyer call below)

//and remove those arguments from the initial array(arr)

}

destroyer(arr, value1, value2, value3, value4);

One of the major problems we have lies with ‘followed by one or more arguments’… If we don’t know how many arguments there will be, how do we write a function to handle them?

The answer to that comes in the form of the Arguments object. JavaScript provides a local variable ‘arguments’ that acts like an array with an element for each argument passed to the function. You access the arguments just like you would an element of any other array like so:

arguments[0] //contains the initial array
arguments[1] //contains the first value to delete from the initial array

1 2	arguments[0] //contains the initial array arguments[1] //contains the first value to delete from the initial array

The arguments object makes solving this algorithm challenge much easier. Looking forward at ways to solve this problem, we need to a) access the arguments we need to delete from the initial array and b) somehow remove those arguments from the array.

Storing arguments for easy access

The first thing we’ll do is create a new array and store all the arguments (other than the initial array) in it.

  var argArr = [];
  
  for (var i = 1; i < arguments.length; i++){
    argArr.push(arguments[i]);
  }

var argArr = [];

for (var i = 1; i < arguments.length; i++){

argArr.push(arguments[i]);

}

As you can see, arguments can be treated like any other array… Here, we used the length property and looped through arguments (starting at element 1 so as to avoid the initial array, which will be passed in as the first argument, or arguments[0]). Setting it up this way will handle any number of arguments passed to the destroyer function.

The push() method in JavaScript is used to ‘push’ new elements on to the end of the calling array.

Tools we’ll need to remove arguments from the array

In order to remove the arguments from the initial array, we will use the filter() method. The filter method introduces a concept we haven’t talked about here at CrookedCode: the callback function.

A Quick note on functions in JavaScript

The power of functions, and what they allow you to do, are core to successful JavaScript programming. Functions can be passed to and returned from other functions. They can also be stored as variables.

A function that accepts another function as an argument is called a higher-order function, whereas, the function that is passed as the argument is called a callback function. The Eloquent JavaScript chapter on functions is a great tutorial on the use of functions in JavaScript.

Eloquent JavaScript also devotes an entire chapter to higher-order functions. I highly recommend reading and working through the examples in both of these chapters. If you’re looking for something a little quicker, JavaScriptissexy.com has an excellent article on understanding callbacks and higher order functions.

The filter() method accepts a callback function and performs that callback on every element in the calling array. If the callback returns true for a given element, that element is kept. If the callback returns false, obviously, that element is not kept.

As stated above, filter() calls the callback function on every element of the calling array, but it does not mutate that array. Therefore, you must save the kept elements as a new array.

The one final tool we’ll need before we try to solve this algorithm is the array.indexOf() method. indexOf() will return the first index at which a given value is found. If the given value is not found, indexOf() returns -1.

Putting it all together…. (finally)

We’ve already created argArr[] to hold the values that we need to delete from the initial array. We now need to write a callback function for filter() that will return true if we want the element to stay and false if we want to delete that element.

We know that indexOf() will return -1 if a given value is not found. We also know that we want to keep elements of the initial array that aren’t found in argArr[]. So, for each element of the initial array, lets call argArr.indexOf(), and return true for values of -1. Therefore, we’ll be returning true for (and keeping) values that aren’t in argArr[].

Like so:

function destroyer(arr) {
  // create array to store arguments to be deleted
  var argArr = [];
  
  //cycle through the arguments object and store in argArr
  for (var i = 1; i < arguments.length; i++){
    argArr.push(arguments[i]);
  }
  
  //filter out values found in argArr
  //if value is not found, -1 will be returned from indexOf() 
  //therefore the callback will return true and the element will be kept
  arr = arr.filter(function(val){
    return argArr.indexOf(val) === -1;
 });
  
  //return new array
  return arr;
}

function destroyer(arr) {

// create array to store arguments to be deleted

var argArr = [];

//cycle through the arguments object and store in argArr

for (var i = 1; i < arguments.length; i++){

argArr.push(arguments[i]);

}

//filter out values found in argArr

//if value is not found, -1 will be returned from indexOf()

//therefore the callback will return true and the element will be kept

arr = arr.filter(function(val){

return argArr.indexOf(val) === -1;

});

//return new array

return arr;

}

Wow… This post covered a bunch of stuff we haven’t previously gone over here at CrookedCode, hopefully this helped straighten out the code for you. See you next time…

-Jeremy