Cara menggunakan javascript implementation

Functional programming has been making quite a splash in the development world these days. And for good reason: Functional techniques can help you write more declarative code that is easier to understand at a glance, refactor, and test.

One of the cornerstones of functional programming is its special use of lists and list operations. Those things are exactly what they sound like: arrays of things and the stuff you do to them. But the functional mindset treats them a bit differently than you might expect.

How to Use Map, Filter, & Reduce in JavaScript

This article will take a close look at what I like to call the "big three" list operations: 326, 327, and 328. Wrapping your head around these three functions is an important step towards being able to write clean, functional code, and it opens the doors to the vastly powerful techniques of functional and reactive programming.

Curious? Let's dive in.

A Map From List to List

Often, we find ourselves needing to take an array and modify every element in it in exactly the same way. Typical examples of this are squaring every element in an array of numbers, retrieving the name from a list of users, or running a regex against an array of strings.

326 is a method built to do exactly that. It's defined on 330, so you can call it on any array, and it accepts a callback as its first argument. 

The syntax for 326 is shown below.

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 2 // return element for newArray, after executing something 3}[, thisArg]);

When you call 326 on an array, it executes that callback on every element within it, returning a new array with all of the values that the callback returned.

Under the hood, 326 passes three arguments to your callback:

1. the current item in the array
2. the array index of the current item
3. the entire array you called 326 on 

Let's look at some code.

326 in Practice

Suppose we have an app that maintains an array of your tasks for the day. Each 336 is an object, each with a 337 and 338 property:

1// Durations are in minutes 2const tasks = [ 3let newArray = arr.map(callback(currentValue[, index[, array]]) { 1let newArray = arr.map(callback(currentValue[, index[, array]]) { 2let newArray = arr.map(callback(currentValue[, index[, array]]) { 3let newArray = arr.map(callback(currentValue[, index[, array]]) { 4let newArray = arr.map(callback(currentValue[, index[, array]]) { 5let newArray = arr.map(callback(currentValue[, index[, array]]) { 6let newArray = arr.map(callback(currentValue[, index[, array]]) { 7let newArray = arr.map(callback(currentValue[, index[, array]]) { 8let newArray = arr.map(callback(currentValue[, index[, array]]) { 12021222324let newArray = arr.map(callback(currentValue[, index[, array]]) { 726let newArray = arr.map(callback(currentValue[, index[, array]]) { 12829 // return element for newArray, after executing something 0 // return element for newArray, after executing something 1 // return element for newArray, after executing something 2 // return element for newArray, after executing something 3 // return element for newArray, after executing something 4 // return element for newArray, after executing something 5

Let's say we want to create a new array with just the name of each task, so we can take a look at everything we've done today. Using a 339 loop, we'd write something like this:

1 // return element for newArray, after executing something 72 // return element for newArray, after executing something 9331let newArray = arr.map(callback(currentValue[, index[, array]]) { 233let newArray = arr.map(callback(currentValue[, index[, array]]) { 435let newArray = arr.map(callback(currentValue[, index[, array]]) { 6let newArray = arr.map(callback(currentValue[, index[, array]]) { 838

JavaScript also offers a 340 loop. It functions like a 339 loop, but manages all the messiness of checking our loop index against the array length for us:

1 // return element for newArray, after executing something 72 // return element for newArray, after executing something 93}[, thisArg]); 4let newArray = arr.map(callback(currentValue[, index[, array]]) { 2}[, thisArg]); 6let newArray = arr.map(callback(currentValue[, index[, array]]) { 4}[, thisArg]); 8let newArray = arr.map(callback(currentValue[, index[, array]]) { 6let newArray = arr.map(callback(currentValue[, index[, array]]) { 838

Using 326, we can simply write:

1132153}[, thisArg]); 8let newArray = arr.map(callback(currentValue[, index[, array]]) { 2let newArray = arr.map(callback(currentValue[, index[, array]]) { 4// Durations are in minutes 0

Here I included the 343 and  344 parameters to remind you that they're there if you need them. Since I didn't use them here, though, you could leave them out, and the code would run just fine.

An even more succinct way of writing 326 in modern JavaScript is with arrow functions.

1// Durations are in minutes 223// Durations are in minutes 5

Arrow functions are a short form for one-line functions that just have a 346 statement. It doesn't get much more readable than that.

There are a few important differences between the different approaches:

1. Using 326, you don't have to manage the state of the 339 loop yourself.
2. With 326, you can operate on the element directly, rather than having to index into the array.
3. You don't have to create a new array and 350 into it. 326 returns the finished product all in one go, so we can simply assign the return value to a new variable.
4. You do have to remember to include a 346 statement in your callback. If you don't, you'll get a new array filled with 353. 

Turns out, all of the functions we'll look at today share these characteristics.

The fact that we don't have to manually manage the state of the loop makes our code simpler and more maintainable. The fact that we can operate directly on the element instead of having to index into the array makes things more readable. 

Using a 340 loop solves both of these problems for us. But 326 still has at least two distinct advantages:

1. 340 returns 353, so it doesn't chain with other array methods. 326 returns an array, so you can chain it with other array methods.
2. 326 returns an array with the finished product, rather than requiring us to mutate an array inside the loop. 

Keeping the number of places where you modify state to an absolute minimum is an important tenet of functional programming. It makes for safer and more intelligible code.

Gotchas

The callback you pass to 326 must have an explicit 346 statement, or 326 will spit out an array full of 353. It's not hard to remember to include a 346 value, but it's not hard to forget. 

If you do forget, 326 won't complain. Instead, it'll quietly hand back an array full of nothing. Silent errors like that can be surprisingly hard to debug. 

Fortunately, this is the only gotcha with 326. But it's a common enough pitfall that I'm obliged to emphasize: Always make sure your callback contains a 346 statement!

Implementation

Reading implementations is an important part of understanding. So let's write our own lightweight 326 to better understand what's going on under the hood. If you want to see a production-quality implementation, check out Mozilla's polyfill at MDN.

1// Durations are in minutes 72// Durations are in minutes 93 // return element for newArray, after executing something 9let newArray = arr.map(callback(currentValue[, index[, array]]) { 223let newArray = arr.map(callback(currentValue[, index[, array]]) { 425let newArray = arr.map(callback(currentValue[, index[, array]]) { 627let newArray = arr.map(callback(currentValue[, index[, array]]) { 8 // return element for newArray, after executing something 920const tasks = [ 122const tasks = [ 3

This code accepts an array and a callback function as arguments. It then creates a new array, executes the callback on each element on the array we passed in, pushes the results into the new array, and returns the new array. If you run this in your console, you'll get the same result as before. 

While we're using a for loop under the hood, wrapping it up into a function hides the details and lets us work with the abstraction instead. 

That makes our code more declarative—it says what to do, not how to do it. You'll appreciate how much more readable, maintainable, and, erm, debuggable this can make your code.

Filter Out the Noise

The next of our array operations is 327. It does exactly what it sounds like: It takes an array and filters out unwanted elements.

The syntax for filter is:

1const tasks = [ 52const tasks = [ 73}[, thisArg]);

Just like 326, 327 passes your callback three arguments:

1. the current item 
2. the current index
3. the array you called 327 on

Consider the following example, which filters out any string which is less than 8 characters.

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 012let newArray = arr.map(callback(currentValue[, index[, array]]) { 033let newArray = arr.map(callback(currentValue[, index[, array]]) { 05

The expected result will be:

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 07

Let's revisit our task example. Instead of pulling out the names of each task, let's say I want to get a list of just the tasks that took me two hours or more to get done.

Using 340, we'd write:

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 092 // return element for newArray, after executing something 93}[, thisArg]); 4let newArray = arr.map(callback(currentValue[, index[, array]]) { 2let newArray = arr.map(callback(currentValue[, index[, array]]) { 15let newArray = arr.map(callback(currentValue[, index[, array]]) { 4let newArray = arr.map(callback(currentValue[, index[, array]]) { 17let newArray = arr.map(callback(currentValue[, index[, array]]) { 6let newArray = arr.map(callback(currentValue[, index[, array]]) { 19let newArray = arr.map(callback(currentValue[, index[, array]]) { 8}[, thisArg]); 82022let newArray = arr.map(callback(currentValue[, index[, array]]) { 242426let newArray = arr.map(callback(currentValue[, index[, array]]) { 2728let newArray = arr.map(callback(currentValue[, index[, array]]) { 29 // return element for newArray, after executing something 0let newArray = arr.map(callback(currentValue[, index[, array]]) { 31

With 327, we can simply write:

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 33

Just like 326, 327 lets us:

• avoid mutating an array inside a 340 or 339 loop
• assign its result directly to a new variable, rather than push into an array we defined elsewhere

Gotchas

The callback you pass to 326 has to include a return statement if you want it to function properly. With 327, you also have to include a return statement (unless you're using arrow functions), and you must make sure it returns a boolean value.

If you forget your return statement, your callback will return 353, which 327 will unhelpfully coerce to 383. Instead of throwing an error, it will silently return an empty array! 

If you go the other route and return something that's isn't explicitly 384 or 383, then 327 will try to figure out what you meant by applying JavaScript's type coercion rules. More often than not, this is a bug. And, just like forgetting your return statement, it'll be a silent one. 

Always make sure your callbacks include an explicit return statement. And always make sure your callbacks in 327 return 384 or 383. Your sanity will thank you.

Implementation

Once again, the best way to understand a piece of code is... well, to write it. Let's roll our own lightweight 327. The good folks at Mozilla have an industrial-strength polyfill for you to read, too.

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 352 // return element for newArray, after executing something 93let newArray = arr.map(callback(currentValue[, index[, array]]) { 39let newArray = arr.map(callback(currentValue[, index[, array]]) { 2 // return element for newArray, after executing something 9let newArray = arr.map(callback(currentValue[, index[, array]]) { 4let newArray = arr.map(callback(currentValue[, index[, array]]) { 43let newArray = arr.map(callback(currentValue[, index[, array]]) { 6let newArray = arr.map(callback(currentValue[, index[, array]]) { 45let newArray = arr.map(callback(currentValue[, index[, array]]) { 8let newArray = arr.map(callback(currentValue[, index[, array]]) { 4720let newArray = arr.map(callback(currentValue[, index[, array]]) { 4922let newArray = arr.map(callback(currentValue[, index[, array]]) { 5124 // return element for newArray, after executing something 926let newArray = arr.map(callback(currentValue[, index[, array]]) { 5528 // return element for newArray, after executing something 9 // return element for newArray, after executing something 0const tasks = [ 3

The Reduce Method

The syntax for the 328 array method in JavaScript is:

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 612let newArray = arr.map(callback(currentValue[, index[, array]]) { 633let newArray = arr.map(callback(currentValue[, index[, array]]) { 65

326 creates a new array by transforming every element in an array individually. 327 creates a new array by removing elements that don't belong. 328, on the other hand, takes all of the elements in an array and reduces them into a single value.

Just like 326 and 327, 328 is defined on 330 and so is available on any array, and you pass a callback as its first argument. But it also takes a second argument: the value to start combining all your array elements into. 

328 passes your callback four arguments:

1. the current value
2. the previous value 
3. the current index
4. the array you called 328 on

Notice that the callback gets a previous value on each iteration. On the first iteration, there is no previous value. This is why you have the option to pass 328 an initial value: It acts as the "previous value" for the first iteration, when there otherwise wouldn't be one.

Finally, bear in mind that 328 returns a single value, not an array containing a single item. This is more important than it might seem, and I'll come back to it in the examples.

328 in Practice

Let's say you want to find the sum of a list of numbers. Using a loop, it would look like this:

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 672let newArray = arr.map(callback(currentValue[, index[, array]]) { 693let newArray = arr.map(callback(currentValue[, index[, array]]) { 71let newArray = arr.map(callback(currentValue[, index[, array]]) { 2let newArray = arr.map(callback(currentValue[, index[, array]]) { 73let newArray = arr.map(callback(currentValue[, index[, array]]) { 4let newArray = arr.map(callback(currentValue[, index[, array]]) { 75let newArray = arr.map(callback(currentValue[, index[, array]]) { 6}[, thisArg]); 8let newArray = arr.map(callback(currentValue[, index[, array]]) { 820let newArray = arr.map(callback(currentValue[, index[, array]]) { 80

While this isn't a bad use case for 340, 328 still has the advantage of allowing us to avoid mutation. With 328, we would write:

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 822let newArray = arr.map(callback(currentValue[, index[, array]]) { 843let newArray = arr.map(callback(currentValue[, index[, array]]) { 86let newArray = arr.map(callback(currentValue[, index[, array]]) { 2let newArray = arr.map(callback(currentValue[, index[, array]]) { 88

First, we call 328 on our list of numbers. We pass it a callback, which accepts the previous value and current value as arguments, and returns the result of adding them together. Since we passed }[, thisArg]); 08 as a second argument to 328, it'll use that as the value of }[, thisArg]); 10 on the first iteration.

With arrow functions, we would write it like this:

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 902let newArray = arr.map(callback(currentValue[, index[, array]]) { 92

If we take it step by step, it looks like this:

If you're not a fan of tables, run this snippet in the console:

1let newArray = arr.map(callback(currentValue[, index[, array]]) { 942let newArray = arr.map(callback(currentValue[, index[, array]]) { 963let newArray = arr.map(callback(currentValue[, index[, array]]) { 98let newArray = arr.map(callback(currentValue[, index[, array]]) { 2200let newArray = arr.map(callback(currentValue[, index[, array]]) { 4202let newArray = arr.map(callback(currentValue[, index[, array]]) { 6204let newArray = arr.map(callback(currentValue[, index[, array]]) { 8let newArray = arr.map(callback(currentValue[, index[, array]]) { 8620 // return element for newArray, after executing something 92221024 // return element for newArray, after executing something 9

To recap: 328 iterates over all the elements of an array, combining them however you specify in your callback. On every iteration, your callback has access to the previous value, which is the total-so-far, or accumulated value; the current value; the current index; and the entire array, if you need them.

Let's turn back to our tasks example. We've gotten a list of task names from 326, and a filtered list of tasks that took a long time with... well, 327. 

What if we wanted to know the total amount of time we spent working today?

Using a 340 loop, you'd write:

12142let newArray = arr.map(callback(currentValue[, index[, array]]) { 713}[, thisArg]); 4let newArray = arr.map(callback(currentValue[, index[, array]]) { 2220let newArray = arr.map(callback(currentValue[, index[, array]]) { 4222let newArray = arr.map(callback(currentValue[, index[, array]]) { 6224let newArray = arr.map(callback(currentValue[, index[, array]]) { 8}[, thisArg]); 820 // return element for newArray, after executing something 92223024 // return element for newArray, after executing something 926234

With 328, that becomes:

12362238

That's almost all there is to it. Almost, because JavaScript provides us with one more little-known method, called }[, thisArg]); 16. In the examples above, 328 started at the first item in the array, iterating from left to right:

124022423244let newArray = arr.map(callback(currentValue[, index[, array]]) { 2}[, thisArg]); 8let newArray = arr.map(callback(currentValue[, index[, array]]) { 4248let newArray = arr.map(callback(currentValue[, index[, array]]) { 6250

}[, thisArg]); 16 does the same thing, but in the opposite direction:

124022543244let newArray = arr.map(callback(currentValue[, index[, array]]) { 2}[, thisArg]); 8let newArray = arr.map(callback(currentValue[, index[, array]]) { 4 // return element for newArray, after executing something 9let newArray = arr.map(callback(currentValue[, index[, array]]) { 6262

I use 328 every day, but I've never needed }[, thisArg]); 16. I reckon you probably won't, either. But in the event you ever do, now you know it's there.

Gotchas

The three big gotchas with 328 are:

1. forgetting to 346
2. forgetting an initial value
3. expecting an array when 328 returns a single value

Fortunately, the first two are easy to avoid. Deciding what your initial value should be depends on what you're doing, but you'll get the hang of it quickly.

The last one might seem a bit strange. If 328 only ever returns a single value, why would you expect an array?

There are a few good reasons for that. First, 328 always returns a single value, not always a single number. If you reduce an array of arrays, for instance, it will return a single array. If you're in the habit of reducing arrays, it would be fair to expect that an array containing a single item wouldn't be a special case.

Second, if 328 did return an array with a single value, it would naturally play nice with 326 and 327, and other functions on arrays that you're likely to be using with it. 

Implementation

Time for our last look under the hood. As usual, Mozilla has a bulletproof polyfill for reduce if you want to check it out.

126422663268let newArray = arr.map(callback(currentValue[, index[, array]]) { 2270let newArray = arr.map(callback(currentValue[, index[, array]]) { 4272let newArray = arr.map(callback(currentValue[, index[, array]]) { 6let newArray = arr.map(callback(currentValue[, index[, array]]) { 51let newArray = arr.map(callback(currentValue[, index[, array]]) { 82682027822const tasks = [ 3

Two things to note here:

1. This time, I used the name }[, thisArg]); 29 instead of }[, thisArg]); 10. This is what you'll usually see in the wild.
2. I assign }[, thisArg]); 29 an initial value if a user provides one, and default to }[, thisArg]); 08 if not. This is how the real 328 behaves, as well.

Putting It Together: Map, Filter, Reduce, and Chainability

At this point, you might not be that impressed. Fair enough: 326, 327, and 328, on their own, aren't awfully interesting. After all, their true power lies in their chainability. 

Let's say I want to do the following:

1. Collect two days' worth of tasks.
2. Convert the task durations to hours instead of minutes.
3. Filter out everything that took two hours or more.
4. Sum it all up.
5. Multiply the result by a per-hour rate for billing.
6. Output a formatted dollar amount.

First, let's define our tasks for Monday and Tuesday:

128222843286let newArray = arr.map(callback(currentValue[, index[, array]]) { 2288let newArray = arr.map(callback(currentValue[, index[, array]]) { 4290let newArray = arr.map(callback(currentValue[, index[, array]]) { 6284let newArray = arr.map(callback(currentValue[, index[, array]]) { 8294202962229824 // return element for newArray, after executing something 0026 // return element for newArray, after executing something 928 // return element for newArray, after executing something 04 // return element for newArray, after executing something 0284 // return element for newArray, after executing something 2 // return element for newArray, after executing something 08 // return element for newArray, after executing something 4 // return element for newArray, after executing something 10 // return element for newArray, after executing something 11290 // return element for newArray, after executing something 13284 // return element for newArray, after executing something 15 // return element for newArray, after executing something 16 // return element for newArray, after executing something 17288 // return element for newArray, after executing something 19290 // return element for newArray, after executing something 21284 // return element for newArray, after executing something 23 // return element for newArray, after executing something 24 // return element for newArray, after executing something 25 // return element for newArray, after executing something 26 // return element for newArray, after executing something 27298 // return element for newArray, after executing something 29 // return element for newArray, after executing something 00 // return element for newArray, after executing something 31 // return element for newArray, after executing something 32 // return element for newArray, after executing something 33 // return element for newArray, after executing something 34

And now, our lovely-looking transformation:

1 // return element for newArray, after executing something 362 // return element for newArray, after executing something 383 // return element for newArray, after executing something 40let newArray = arr.map(callback(currentValue[, index[, array]]) { 2 // return element for newArray, after executing something 42let newArray = arr.map(callback(currentValue[, index[, array]]) { 4 // return element for newArray, after executing something 44let newArray = arr.map(callback(currentValue[, index[, array]]) { 6 // return element for newArray, after executing something 46let newArray = arr.map(callback(currentValue[, index[, array]]) { 8 // return element for newArray, after executing something 4820 // return element for newArray, after executing something 5022 // return element for newArray, after executing something 5224 // return element for newArray, after executing something 5426 // return element for newArray, after executing something 5628 // return element for newArray, after executing something 58 // return element for newArray, after executing something 0 // return element for newArray, after executing something 60 // return element for newArray, after executing something 2 // return element for newArray, after executing something 62 // return element for newArray, after executing something 4 // return element for newArray, after executing something 64 // return element for newArray, after executing something 11 // return element for newArray, after executing something 66

If you've made it this far, this should be pretty straightforward. There are two bits of weirdness to explain, though. 

First, on line 10, I have to write:

1 // return element for newArray, after executing something 682 // return element for newArray, after executing something 703 // return element for newArray, after executing something 72let newArray = arr.map(callback(currentValue[, index[, array]]) { 2 // return element for newArray, after executing something 74

Two things to explain here:

1. The plus signs in front of }[, thisArg]); 29 and }[, thisArg]); 38 coerce their values to numbers. If you don't do this, the return value will be the rather useless string, }[, thisArg]); 39.
2. If you don't wrap that sum in brackets, 328 will spit out a single value, not an array. That would end up throwing a }[, thisArg]); 41, because you can only use 326 on an array! 

The second bit that might make you a bit uncomfortable is the last 328, namely:

1 // return element for newArray, after executing something 682 // return element for newArray, after executing something 783 // return element for newArray, after executing something 80let newArray = arr.map(callback(currentValue[, index[, array]]) { 2 // return element for newArray, after executing something 82let newArray = arr.map(callback(currentValue[, index[, array]]) { 4 // return element for newArray, after executing something 84let newArray = arr.map(callback(currentValue[, index[, array]]) { 6}[, thisArg]); 8

That call to 326 returns an array containing a single value. Here, we call 328 to pull out that value.

Finally, let's see how our friend the 340 loop would get it done:

1 // return element for newArray, after executing something 882 // return element for newArray, after executing something 903 // return element for newArray, after executing something 92let newArray = arr.map(callback(currentValue[, index[, array]]) { 2 // return element for newArray, after executing something 94let newArray = arr.map(callback(currentValue[, index[, array]]) { 4248let newArray = arr.map(callback(currentValue[, index[, array]]) { 6 // return element for newArray, after executing something 98let newArray = arr.map(callback(currentValue[, index[, array]]) { 830020let newArray = arr.map(callback(currentValue[, index[, array]]) { 71223042430626let newArray = arr.map(callback(currentValue[, index[, array]]) { 1928}[, thisArg]); 8 // return element for newArray, after executing something 0248 // return element for newArray, after executing something 2314 // return element for newArray, after executing something 4316 // return element for newArray, after executing something 11}[, thisArg]); 8 // return element for newArray, after executing something 13 // return element for newArray, after executing something 15 // return element for newArray, after executing something 17 // return element for newArray, after executing something 19323 // return element for newArray, after executing something 21 // return element for newArray, after executing something 9

Tolerable, but noisy.

Conclusion and Next Steps

In this tutorial, you've learned how 326, 327, and 328 work; how to use them; and roughly how they're implemented. You've seen that they all allow you to avoid mutating state, which using 339 and 340 loops requires, and you should now have a good idea of how to chain them all together. 

By now, I'm sure you're eager for practice and further reading. For a masterclass in functional programming in JavaScript, check out our online course.

Bagaimana cara menjalankan JavaScript Coba Anda jelaskan?

Langkah kerja JavaScript?

Bagaimana cara yang benar untuk menggunakan file JavaScript di HTML?

Apakah JavaScript cocok untuk pemula?