JS: Semicolon

Rule for Always Using Semicolon Style

• ALWAYS add semicolon at end of assignment.
• A redundant semicolon is a empty statement. Extra semicolon won't hurt.

need semicolon:

• Assignment needs semicolon let x = 3;
• Sequence of expressions, such as function call, need semicolon to separate them. e.g. f(3); f(4);.

no need semicolon:

• function f (x) {return x+1;}.
• for
• while
• if
• switch
• try

Rule for Omitting Semicolon Style

Do not add semicolon, except:

• A line should never start with left parenthesis () nor left square bracket [].
• If a line starts with parenthesis or square bracket, add a semicolon in the beginning of line (or end of previous line).

Advantage of Omitting Semicolon

• Less typing.
• Easier to edit, move expression in functional programing style code.

Disadvantage of Omitting Semicolon

• Code becomes layout dependent. If you join 2 lines into 1 line, the code won't run correctly.

Use Automatic Formatting Tool

Use Deno to autoformat code.

• Deno: Intro
• Deno: Auto Format JavaScript Code

deno adds semicolon automatically.

When Do You Need Semicolon by JavaScript Grammar?

The full spec is fairly complex.

Tips on Semicolon

Return Statement Must be in One-Line

function ff() {
  // bad
  return
  [3, 4, 5];
}

console.log(ff() === undefined);

Example of Bad Omitted Semicolon

/*
example of erro due to semicolon omission.

you intended to assign the first function, and eval the second.
but js compiler tried to feed the second function as arg to your first function
 */

const f = function () {
  return 3;
} // bad semicolon omission
(function () {
  return 4;
})();

/*

error: Uncaught TypeError: (intermediate value)(...) is not a function
})();
  ^

*/