Semicolon usage in JavaScript

David Y.

The problem

Are semicolons required in JavaScript? How should I use them?

The solution

In JavaScript, statements are separated by semicolons. Statements include variable assignments and function calls, as well as control flows (e.g. if ... else) and iterations (e.g. for). The code below provides some example usage:

const name = "John Doe";
const age = 35;

function oldEnoughToBePresident(name, age) {
  if (age >= 35) {
    console.log(`${name} is old enough to run for president of the United States.`);
    return true;
  } else {
    console.log(`${name} is too young run for president of the United States.`);
    return false;
  }
}

oldEnoughToBePresident(name, age);

Note that while semicolons are used to separate statements inside the blocks provided to the if and else clauses of our if ... else statement, we do not place them after the if condition or the else itself. Doing this would cause JavaScript to interpret the if statement and our block as two separate statements, as below:

if (age >= 35); // if age >= 35, execute an empty statement (i.e. do nothing)
{ // this block is seen as separate from the if and will run regardless of the value of age
  console.log(name + " is old enough to run for president of the United States.");
  return true;
}

An interesting feature of JavaScript is that its semicolons do not always need to be provided by the programmer – the JavaScript interpreter has a process called automatic semicolon insertion (ASI) which it uses to insert semicolons where needed, according to other heuristics such as line endings.

As a result, different JavaScript developers, teams and projects have different conventions about semicolon usage – some explicitly include them at the end of all statements, whereas others only include them in places where ASI does not act correctly.

For most code, ASI will behave in predictable ways, usually adding semicolons at the end of lines.

const name = "John Doe" // ; inserted here by ASI
const age = 35 // ; inserted here by ASI

if (age >= 35) {
  console.log(`${name} is old enough to run for president of the United States.`) // ; inserted here by ASI
}

However, ASI considers only syntactical correctness, and may thus produce the wrong results in ambiguous situations. For example, lines starting with () will be interpreted as arguments for a function called on the previous line:

const age = 35
(age).toString() //; inserted here by ASI

// the code becomes:
const age = 35(age).toString(); // will invoke an error as 35 is not a function.

Similarly, lines starting with [] will be interpreted as accessing members of a variable in the previous line.

const name = "John Doe"
[1, 2, 3].forEach(console.log) //; inserted here by ASI

// the code becomes:
const name = "John Doe"[1, 2, 3].forEach(console.log); // will throw an error

The MDN web docs provide a comprehensive overview of ASI’s behavior. Developers wishing to avoid semicolons should keep these rules in mind.