您可以標記JavaScript`if`語句

Labels are a feature that have existed since the creation of JavaScript. They aren’t new! I don’t think all that many people know about them and I’d even argue they are a bit confusing. But, as we’ll see, labels can be useful in very specific instances.

But first: A JavaScript label should not be confused with an HTML , which is a completely different thing!

A JavaScript label is a way to name a statement or a block of code. Typically: loops and conditional statements. That allows you to break or continue the labeled statement from within. To apply a label to a statement, start the statement with label: and whatever you put as “label” will be the label you can reference later.

Here’s the basic syntax for a label:

let x = 0;
// Label a loop as "myLoop"
myLoop:
while (true) {
  if (x >= 10) {
    // This will cause "myLoop" to end.
    break myLoop;
  }
  x++;
}

Labels are only an internal reference to a statement and are not something that can be looked up, exported, or stored in a value. They also do not conflict with variable names, so if you really want to confuse people, you can have a loop and a variable be the same name! Please don’t do this —  future you, and anyone else that has to read your code will appreciate it. The use cases for labels are limited, but incredibly powerful in the right hands.

A brief intro to break and continue

Let’s back up a bit and talk about break and continue. A break statement will terminate the currently running loop or conditional statement. It is most commonly used in a switch statement to end a case, but it can also be used to end an if statement early, or also to cause a for or while loop to end and stop looping. It’s a great way to escape out of a conditional statement or end a loop early.

Here’s a basic example of break in use:

const x = 1;
switch(x) {
  case 1:
    console.log('On your mark!');
    break; // Doesn't check the rest of the switch statement if 1 is true
  case 2:
    console.log('Get set!');
    break; // Doesn't check the rest of the switch statement if 2 is true
  case 3:
    console.log('GO!');
    break; // Doesn't check the rest of the switch statement if 3 is true
}
// logs: 'On your mark!'

Similarly, the continue statement can be used with loops to end the current iteration early, and start the next run of the loop. This will only work inside of loops however.

Here’s continue in use:

for (let x = 0; x &< 10; x++) {
  if (x !== 5) continue; // If the number isn't five, go to the next pass of the loop.
  console.log(x);
}
// logs: 5

Using a label with break

Typically, a use case for labels comes up when you get into nested statements of any kind. Using them with break can stop a deeply nested loop or conditional and make it stop immediately.

Let’s get to that title of this blog post!

// Our outer if statement
outerIf: 
if (true) {
  // Our inner if statement
  innerIf:
  if (true) {
    break outerIf; // Immediately skips to the end of the outer if statement
  }
  console.log('This never logs!');
}

There you have it, you can label an if statement.

Using a label with continue

There have been times where I have made a nested loop and wanted to skip over some iterations of the outer loop while inside the inner loop. I usually wind up breaking the inner loop, then checking to see if I’m in the state I want to skip, then continue the outer loop. Being able to simplify that code down into an easier to read statement is great!

let x = 0;
outerLoop:
while (x < 10) {
  x++;
  for (let y = 0; y < x; y++) {
    // This will jump back to the top of outerLoop
    if (y === 5) continue outerLoop;
    console.log(x,y);
  }
  console.log('----'); // This will only happen if x < 6
}

Block statements and labels

Block statements in JavaScript are a way to scope your const and let variables to only a portion of your code. This can be useful if you want to localize some variables without having to create a function. The (big) caveat to this is that block statements are invalid in strict mode, which is what ES modules are by default.

Here’s a labeled block statement:

// This example throws a syntax error in an ES module
const myElement = document.createElement('p');
myConditionalBlock: {
  const myHash = window.location.hash;
  // escape the block if there is not a hash.
  if (!myHash) break myConditionalBlock;
  myElement.innerText = myHash;
}
console.log(myHash); // undefined
document.body.appendChild(myElement);

Real world usage

It took me a while to come up with a reason to use a label in everyday production code. This might be a bit of a stretch, but a place where a label in JavaScript might come in handy is to escape early from a loop while inside a switch statement. Since you can break while in a switch, being able to apply a label to a loop that ends it early could potentially make your code more efficient.

Here’s how we might use that in a calculator app:

const calculatorActions = [
  { action: "ADD", amount: 1 },
  { action: "SUB", amount: 5 },
  { action: "EQ" },
  { action: "ADD", amount: 10 }
];
    
let el = {};
let amount = 0;
calculate: while (el) {
  // Remove the first element of the calculatorActions array
  el = calculatorActions.shift();
  switch (el.action) {
    case "ADD":
      amount += el.amount;
      break; // Breaks the switch
    case "SUB":
      amount -= el.amount;
      break; // Breaks the switch
    case "EQ":
      break calculate; // Breaks the loop
    default:
      continue calculate; // If we have an action we don't know, skip it.
  }
}

This way, we can bail out of the calculate loop when a condition is matched rather than allowing the script to continue!

Conclusion

It’s rare that you will need to use a JavaScript label. In fact, youcan lead a very fulfilling career without ever knowing that this exists. But, on the offhand chance you find that one place where this syntax helps out, you’re now empowered to use it.

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