JavaScriptタイマー:あなたが知る必要があるすべて

数週間前、私はこのインタビューの質問をツイートしました。

***続行する前に、頭の中で質問に答えてください***

ツイートへの返信の約半分が間違っていました。答えはV8(または他のVM)ではありません!! 有名な「JavaScriptのタイマー」として知られているが、のような機能setTimeoutとは、setIntervalECMAScriptの仕様や、任意のJavaScriptエンジンの実装の一部ではありません。タイマー機能はブラウザによって実装されており、その実装はブラウザによって異なります。タイマーは、Node.jsランタイム自体によってネイティブに実装されます。

ブラウザでは、メインのタイマー関数はWindowインターフェイスの一部であり、他のいくつかの関数とオブジェクトがあります。このインターフェースにより、そのすべての要素がメインのJavaScriptスコープでグローバルに利用できるようになります。これがsetTimeout、ブラウザのコンソールで直接実行できる理由です。

Nodeでは、タイマーはglobalオブジェクトの一部であり、ブラウザのWindowインターフェイスと同様に動作します。Nodeのタイマーのソースコードはここで確認できます。

これは面接の悪い質問だと思う人もいるかもしれません—とにかくこの問題を知っているのはなぜですか?!JavaScript開発者として、これを知っていることが期待されていると思います。知らない場合は、V8(および他のVM)がブラウザーやノードとどのように相互作用するかを完全に理解していないことを示している可能性があります。

タイマー機能に関するいくつかの例と課題を見てみましょう。

更新:この記事は、私の「Node.jsの完全な紹介」の一部になりました。

更新されたバージョンはここで読むことができます。

関数の実行を遅らせる

タイマー関数は、他の関数(最初の引数として受け取る)の実行を遅延または繰り返すために使用できる高階関数です。

遅延の例を次に示します。

// example1.js setTimeout( () => { console.log('Hello after 4 seconds'); }, 4 * 1000 );

この例ではsetTimeout、グリーティングメッセージの印刷を4秒遅らせるために使用します。の2番目の引数setTimeoutは、遅延(ミリ秒単位)です。これが、4に1000を掛けて4秒にした理由です。

の最初の引数setTimeoutは、実行が遅延する関数です。

コマンドをexample1.js使用してファイルを実行するとnode、ノードは4秒間一時停止し、グリーティングメッセージを出力します(その後終了します)。

の最初の引数setTimeoutは単なる関数参照であることに注意してください。持っているようなインライン関数である必要はありませんexample1.js。インライン関数を使用しない同じ例を次に示します。

const func = () => { console.log('Hello after 4 seconds'); }; setTimeout(func, 4 * 1000);

引数の受け渡し

setTimeout実行を遅らせるために使用する関数が任意の引数を受け入れる場合、setTimeout(これまでに学習した2の後)それ自体の残りの引数を使用して、引数値を遅延関数に中継できます。

// For: func(arg1, arg2, arg3, ...) // We can use: setTimeout(func, delay, arg1, arg2, arg3, ...)

次に例を示します。

// example2.js const rocks = who => { console.log(who + ' rocks'); }; setTimeout(rocks, 2 * 1000, 'Node.js');

rocks2秒遅れる上記の関数はwho引数を受け入れ、setTimeout呼び出しはその引数として値「Node.js」を中継しwhoます。

コマンドを使用example2.jsして実行すると、2秒後にnodeNode.jsrocks」が出力されます。

タイマーチャレンジ#1

これまでに学んだことを使用してsetTimeout、対応する遅延の後に次の2つのメッセージを出力します。

  • 4秒後に「4秒後にこんにちは」というメッセージを出力します
  • 8秒後にHelloafter8seconds」というメッセージを出力します。

制約

ソリューションでは、インライン関数を含む1つの関数のみを定義できます。これは、多くのsetTimeout呼び出しでまったく同じ関数を使用する必要があることを意味します。

解決

この課題を解決する方法は次のとおりです。

// solution1.js const theOneFunc = delay => { console.log('Hello after ' + delay + ' seconds'); }; setTimeout(theOneFunc, 4 * 1000, 4); setTimeout(theOneFunc, 8 * 1000, 8);

私が作ったtheOneFunc受信delay議論をし、その値が使用されdelay印刷されたメッセージに引数を。このようにして、関数は、渡された遅延値に基づいてさまざまなメッセージを出力できます。

I then used theOneFunc in two setTimeout calls, one that fires after 4 seconds and another that fires after 8 seconds. Both of these setTimeout calls also get a 3rd argument to represent the delay argument for theOneFunc.

Executing the solution1.js file with the node command will print out the challenge requirements, the first message after 4 seconds, and the second message after 8 seconds.

Repeating the execution of a function

What if I asked you to print a message every 4 seconds, forever?

While you can put setTimeout in a loop, the timers API offers the setInterval function as well, which would accomplish the requirement of doing something forever.

Here’s an example of setInterval:

// example3.js setInterval( () => console.log('Hello every 3 seconds'), 3000 );

This example will print its message every 3 seconds. Executing example3.js with the node command will make Node print this message forever, until you kill the process (with CTRL+C).

Cancelling Timers

Because calling a timer function schedules an action, that action can also be cancelled before it gets executed.

A call to setTimeout returns a timer “ID” and you can use that timer ID with a clearTimeout call to cancel that timer. Here’s an example:

// example4.js const timerId = setTimeout( () => console.log('You will not see this one!'), 0 ); clearTimeout(timerId);

This simple timer is supposed to fire after 0 ms (making it immediate), but it will not because we are capturing the timerId value and canceling it right after with a clearTimeout call.

When we execute example4.js with the node command, Node will not print anything and the process will just exit.

By the way, in Node.js, there is another way to do setTimeout with 0 ms. The Node.js timer API has another function called setImmediate, and it’s basically the same thing as a setTimeout with a 0 ms but we don’t have to specify a delay there:

setImmediate( () => console.log('I am equivalent to setTimeout with 0 ms'), );

The setImmediate function is not available in all browsers. Don’t use it for front-end code.

Just like clearTimeout, there is also a clearInterval function, which does the same thing but for setInerval calls, and there is also a clearImmediate call as well.

A timer delay is not a guaranteed thing

In the previous example, did you notice how executing something with setTimeout after 0 ms did not mean execute it right away (after the setTimeout line), but rather execute it right away after everything else in the script (including the clearTimeout call)?

Let me make this point clear with an example. Here’s a simple setTimeout call that should fire after half a second, but it won’t:

// example5.js setTimeout( () => console.log('Hello after 0.5 seconds. MAYBE!'), 500, ); for (let i = 0; i < 1e10; i++) { // Block Things Synchronously }

Right after defining the timer in this example, we block the runtime synchronously with a big for loop. The 1e10 is 1 with 10 zeros in front of it, so the loop is a 10 Billion ticks loop (which basically simulates a busy CPU). Node can do nothing while this loop is ticking.

This of course is a very bad thing to do in practice, but it’ll help you here to understand that setTimeout delay is not a guaranteed thing, but rather a minimum thing. The 500 ms means a minimum delay of 500 ms. In reality, the script will take a lot longer to print its greeting line. It will have to wait on the blocking loop to finish first.

Timers Challenge #2

Write a script to print the message “Hello World” every second, but only 5 times. After 5 times, the script should print the message “Done” and let the Node process exit.

Constraints: You cannot use a setTimeout call for this challenge.

Hint: You need a counter.

Solution

Here’s how I’d solve this one:

let counter = 0; const intervalId = setInterval(() => { console.log('Hello World'); counter += 1; if (counter === 5) { console.log('Done'); clearInterval(intervalId); } }, 1000);

I initiated a counter value as 0 and then started a setInterval call capturing its id.

The delayed function will print the message and increment the counter each time. Inside the delayed function, an if statement will check if we’re at 5 times by now. If so, it’ll print “Done” and clear the interval using the captured intervalId constant. The interval delay is 1000 ms.

Who exactly “calls” the delayed functions?

When you use the JavaScript this keyword inside a regular function, like this:

function whoCalledMe() { console.log('Caller is', this); }

The value inside the this keyword will represent the caller of the function. If you define the function above inside a Node REPL, the caller will be the global object. If you define the function inside a browser’s console, the caller will be the window object.

Let’s define the function as a property on an object to make this a bit more clear:

const obj = { id: '42', whoCalledMe() { console.log('Caller is', this); } }; // The function reference is now: obj.whoCallMe

Now when you call the obj.whoCallMe function using its reference directly, the caller will be the obj object (identified by its id):

Now, the question is, what would the caller be if we pass the reference of obj.whoCallMe to a setTimetout call?

// What will this print?? setTimeout(obj.whoCalledMe, 0);

Who will the caller be in that case?

The answer is different based on where the timer function is executed. You simply can’t depend on who the caller is in that case. You lose control of the caller because the timer implementation will be the one invoking your function now. If you test it in a Node REPL, you’d get a Timetout object as the caller:

Note that this only matters if you’re using JavaScript’s this keyword inside regular functions. You don’t need to worry about the caller at all if you’re using arrow functions.

Timers Challenge #3

Write a script to continuously print the message “Hello World” with varying delays. Start with a delay of 1 second and then increment the delay by 1 second each time. The second time will have a delay of 2 seconds. The third time will have a delay of 3 seconds, and so on.

Include the delay in the printed message. Expected output looks like:

Hello World. 1 Hello World. 2 Hello World. 3 ...

Constraints: You can only use const to define variables. You can’t use let or var.

Solution

Because the delay amount is a variable in this challenge, we can’t use setInterval here, but we can manually create an interval execution using setTimeout within a recursive call. The first executed function with setTimeout will create another timer, and so on.

Also, because we can’t use let/var, we can’t have a counter to increment the delay in each recursive call, but we can instead use the recursive function arguments to increment during the recursive call.

Here’s one possible way to solve this challenge:

const greeting = delay => setTimeout(() => { console.log('Hello World. ' + delay); greeting(delay + 1); }, delay * 1000); greeting(1);

Timers Challenge #4

Write a script to continuously print the message “Hello World” with the same varying delays concept as challenge #3, but this time, in groups of 5 messages per main-delay interval. Starting with a delay of 100ms for the first 5 messages, then a delay of 200ms for the next 5 messages, then 300ms, and so on.

Here’s how the script should behave:

  • At the 100ms point, the script will start printing “Hello World” and do that 5 times with an interval of 100ms. The 1st message will appear at 100ms, 2nd message at 200ms, and so on.
  • After the first 5 messages, the script should increment the main delay to 200ms. So 6th message will be printed at 500ms + 200ms (700ms), 7th message will be printed at 900ms, 8th message will be printed at 1100ms, and so on.
  • After 10 messages, the script should increment the main delay to 300ms. So the 11th message should be printed at 500ms + 1000ms + 300ms (18000ms). The 12th message should be printed at 21000ms, and so on.
  • Continue the pattern forever.

Include the delay in the printed message. The expected output looks like this (without the comments):

Hello World. 100 // At 100ms Hello World. 100 // At 200ms Hello World. 100 // At 300ms Hello World. 100 // At 400ms Hello World. 100 // At 500ms Hello World. 200 // At 700ms Hello World. 200 // At 900ms Hello World. 200 // At 1100ms ...

Constraints: You can use only setInterval calls (not setTimeout) and you can use only ONE if statement.

Solution

Because we can only use setInterval calls, we’ll need recursion here as well to increment the delay of the next setInterval call. In addition, we need an if statement to control doing that only after 5 calls of that recursive function.

Here’s one possible solution:

let lastIntervalId, counter = 5; const greeting = delay => { if (counter === 5) { clearInterval(lastIntervalId); lastIntervalId = setInterval(() => { console.log('Hello World. ', delay); greeting(delay + 100); }, delay); counter = 0; } counter += 1; }; greeting(100);

Thanks for reading.

If you’re just beginning to learn Node.js, I recently published a first-steps course at Pluralsight, check it out: