The automatic semicolon insertion (ASI)

In C-like languages, the semicolon denotes the end of a statement. Unlike other C-like languages, JavaScript does not always enforce the use of a semicolon at the end of a statement. Rather, a carriage return may be "understood" as a missing semicolon, resulting in different semantics.

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.

Examples

return
2*a + 1;

return;
2*a + 1;

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

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

i
++;

i;
++;

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

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

References

1. ↑ cjihrig (2012-03-09). "The Dangers of JavaScript’s Automatic Semicolon Insertion" (in English) (HTML). cjihrig.com. Archived from the original on 2012-03-09. http://cjihrig.com/blog/the-dangers-of-javascripts-automatic-semicolon-insertion/. Retrieved 2015-04-03. 

See also

• Whitespaces & semicolons in JavaScript