Automatic Semicolon Insertion (ASI)

In C-like languages, the semicolon denotes the end of a statement. Unlike other C-like languages, JavaScript does not enforce the use of a semicolon at the end of a statement. Instead, the semicolon is optional, and the JavaScript interpreter will "intelligently" add them when it runs the code.

In JavaScript, a semicolon is automatically inserted when ^[1]

1. two statements are separated by a line terminator
2. two statements are separated by a closing brace ('}')
3. a line terminator follows a break, continue, return, or throw.

While ASI would make your code easier to write (no need to type all of those semicolons), in practice the lack of semicolons makes your program harder to debug. Because of this, it is universally recognized as a best practice to use semicolons at the end of statements, anyway. However, the existence of ASI can still create some bugs that are hard to troubleshoot if you don't know what to look for.

return
2*a + 1;

return;
2*a + 1;

return 2*a + 1;

function getObject() {
  return
  {
    // some lines
  };
}

function getObject() {
  return;
  {
    // some lines
  };
}

function getObject() {
  return {
    // some lines
  };
}

i
++;

i;
++;

i++;

if (i === 5)
  // assuming a semicolon here
else
  foo = 0;

if (i === 5)
  // no semicolon here!
else
  foo = 0;

if (i === 5){
  // code
  } else {
    foo = 0;
  }