What is the use of the following colon in javascript

Question

Following code block executed successfully. I was wondering what would be the use of this labeling other than using for loops?

<script>

js:
{
   alert("x");  
}

</script>

Answer 1

The : has a few uses in javascript, that I know of anyway.

1. ternary operator - used to evaluate an if statement in a single line:

   var x = "yes" == "yes" ? true : false;
   

   The above line of code is functionally equivalent to:

   if("yes" == "yes"){
       var x = true;
   }
   else{
       var x = false;
   }
   

2. Mark the beginning of a code block - Move to a block of code

   begin:
   for(int i = 0; i < 10; i++){
       break begin;
   }
   

3. Object Literals - Thanks @Ian for the reminder

   var someObject= {
       item: 'some value',
       anotherItem: 2 // Can put any type of variable here
   };
   

   This type of notation is commonly seen when using JSON

Answer 2

It is called Labels ^[MDN] in javascript

myPrettyLabel:
{
   alert('testMe');
}

Provides a statement with an identifier that you can refer to using a break or continue statement.

For example, you can use a label to identify a loop, and then use the break or continue statements to indicate whether a program should interrupt the loop or continue its execution.

Same as goto in general programs

---

Please note

Avoid using labels as Labels are not very commonly used in JavaScript since they make programs harder to read and understand. As much as possible, avoid using labels and, depending on the cases, prefer calling functions or throwing an error.

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Reference: SO - What does the colon (:) in JavaScript represent?

Answer 3

It's a labeled statement. You can use labeled statements with a form of the break and continue statements:

outer: for (var i = 0; i < 1000; ++i) {
  for (var j = 0; j < 1000; j++) {
    if (somethingBad())
      break outer;
  }
}

It's (rarely) useful to be able to get out of an inner nested loop to an outer iteration level. I don't think I've ever used it across many thousands of lines of code. In the example code posted in the original question, the label has no apparent purpose.