---
title: Rule semi
layout: doc
---
<!-- Note: No pull requests accepted for this file. See README.md in the root directory for details. -->
# Enforce or Disallow Semicolons (semi)

JavaScript is unique amongst the C-like languages in that it doesn't require semicolons at the end of each statement. In many cases, the JavaScript engine can determine that a semicolon should be in a certain spot and will automatically add it. This feature is known as **automatic semicolon insertion (ASI)** and is considered one of the more controversial features of JavaScript. For example, the following lines are both valid:

```js
var name = "ESLint"
var website = "eslint.org";
```

On the first line, the JavaScript engine will automatically insert a semicolon, so this is not considered a syntax error. The JavaScript engine still knows how to interpret the line and knows that the line end indicates the end of the statement.

In the debate over ASI, there are generally two schools of thought. The first is that we should treat ASI as if it didn't exist and always include semicolons manually. The rationale is that it's easier to always include semicolons than to try to remember when they are or are not required, and thus decreases the possibility of introducing an error.

However, the ASI mechanism can sometimes be tricky to people who are using semicolons. For example, consider this code:

```js
return
{
    name: "ESLint"
};
```

This may look like a `return` statement that returns an object literal, however, the JavaScript engine will interpret this code as:

```js
return;
{
    name: "ESLint";
}
```

Effectively, a semicolon is inserted after the `return` statement, causing the code below it (a labeled literal inside a block) to be unreachable. This rule and the [no-unreachable](no-unreachable) rule will protect your code from such cases.

On the other side of the argument are those who says that since semicolons are inserted automatically, they are optional and do not need to be inserted manually. However, the ASI mechanism can also be tricky to people who don't use semicolons. For example, consider this code:

```js
var globalCounter = { }

(function () {
    var n = 0
    globalCounter.increment = function () {
        return ++n
    }
})()
```

In this example, a semicolon will not be inserted after the first line, causing a run-time error (because an empty object is called as if it's a function). The [no-unexpected-multiline](no-unexpected-multiline) rule can protect your code from such cases.

Although ASI allows for more freedom over your coding style, it can also make your code behave in an unexpected way, whether you use semicolons or not. Therefore, it is best to know when ASI takes place and when it does not, and have ESLint protect your code from these potentially unexpected cases. In short, as once described by Isaac Schlueter, a `\n` character always ends a statement (just like a semicolon) unless one of the following is true:

1. The statement has an unclosed paren, array literal, or object literal or ends in some other way that is not a valid way to end a statement. (For instance, ending with `.` or `,`.)
1. The line is `--` or `++` (in which case it will decrement/increment the next token.)
1. It is a `for()`, `while()`, `do`, `if()`, or `else`, and there is no `{`
1. The next line starts with `[`, `(`, `+`, `*`, `/`, `-`, `,`, `.`, or some other binary operator that can only be found between two tokens in a single expression.

## Rule Details

This rule is aimed at ensuring consistent use of semicolons. You can decide whether or not to require semicolons at the end of statements.

**Fixable:** This rule is automatically fixable using the `--fix` flag on the command line.

### Options

The rule takes one or two options. The first one is a string, which could be `"always"` or `"never"`. The default is `"always"`. The second one is an object for more fine-grained configuration when the first option is `"always"`.

You can set the option in configuration like this:

#### "always"

By using the default option, semicolons must be used any place where they are valid.

```json
semi: [2, "always"]
```

The following patterns are considered problems:

```js
/*eslint semi: 2*/

var name = "ESLint"          /*error Missing semicolon.*/

object.method = function() {
    // ...
}                            /*error Missing semicolon.*/
```

The following patterns are not considered problems:

```js
/*eslint semi: 2*/

var name = "ESLint";

object.method = function() {
    // ...
};
```

##### Fine-grained control

When setting the first option as "always", an additional option can be added to omit the last semicolon in a one-line block, that is, a block in which its braces (and therefore the content of the block) are in the same line:

```json
semi: [2, "always", { "omitLastInOneLineBlock": true}]
```

The following patterns are considered problems:

```js
/*eslint semi: [2, "always", { "omitLastInOneLineBlock": true}] */

if (foo) {
    bar()                   /*error Missing semicolon.*/
}

if (foo) { bar(); }         /*error Extra semicolon.*/
```

The following patterns are not considered problems:

```js
/*eslint semi: [2, "always", { "omitLastInOneLineBlock": true}] */

if (foo) { bar() }

if (foo) { bar(); baz() }
```

#### "never"

If you want to enforce that semicolons are never used, switch the configuration to:

```json
semi: [2, "never"]
```

Then, the following patterns are considered problems:

```js
/*eslint semi: [2, "never"]*/

var name = "ESLint";         /*error Extra semicolon.*/

object.method = function() {
    // ...
};                           /*error Extra semicolon.*/
```

And the following patterns are not considered problems:

```js
/*eslint semi: [2, "never"]*/

var name = "ESLint"

object.method = function() {
    // ...
}
```

Even in `"never"` mode, semicolons are still allowed to disambiguate statements beginning with `[`, `(`, `/`, `+`, or `-`:

```js
/*eslint semi: [2, "never"]*/

var name = "ESLint"

;(function() {
    // ...
})()
```

## When Not To Use It

If you do not want to enforce semicolon usage (or omission) in any particular way, then you can turn this rule off.

## Further Reading

* [An Open Letter to JavaScript Leaders Regarding Semicolons](http://blog.izs.me/post/2353458699/an-open-letter-to-javascript-leaders-regarding)
* [JavaScript Semicolon Insertion](http://inimino.org/~inimino/blog/javascript_semicolons)
* [Understanding Automatic Semicolon Insertion in JavaScript](http://jamesallardice.com/understanding-automatic-semi-colon-insertion-in-javascript/)

## Related Rules

* [no-extra-semi](no-extra-semi)
* [no-unexpected-multiline](no-unexpected-multiline)
* [semi-spacing](semi-spacing)

## Version

This rule was introduced in ESLint 0.0.6.

## Resources

* [Rule source](https://github.com/eslint/eslint/tree/master/lib/rules/semi.js)
* [Documentation source](https://github.com/eslint/eslint/tree/master/docs/rules/semi.md)
