Disallow Case Statement Fallthrough (no-fallthrough)

The switch statement in JavaScript is one of the more error-prone constructs of the language thanks in part to the ability to "fall through" from one case to the next. For example:

switch(foo) {
    case 1:
        doSomething();

    case 2:
        doSomethingElse();
}

In this example, if foo is 1, then execution will flow through both cases, as the first falls through to the second. You can prevent this by using break, as in this example:

switch(foo) {
    case 1:
        doSomething();
        break;

    case 2:
        doSomethingElse();
}

That works fine when you don't want a fallthrough, but what if the fallthrough is intentional, there is no way to indicate that in the language. It's considered a best practice to always indicate when a fallthrough is intentional using a comment which matches the /falls?\s?through/i regular expression:

switch(foo) {
    case 1:
        doSomething();
        // falls through

    case 2:
        doSomethingElse();
}

switch(foo) {
    case 1:
        doSomething();
        // fall through

    case 2:
        doSomethingElse();
}

switch(foo) {
    case 1:
        doSomething();
        // fallsthrough

    case 2:
        doSomethingElse();
}

In this example, there is no confusion as to the expected behavior. It is clear that the first case is meant to fall through to the second case.

Rule Details

This rule is aimed at eliminating unintentional fallthrough of one case to the other. As such, it flags any fallthrough scenarios that are not marked by a comment.

Examples of incorrect code for this rule:

/*eslint no-fallthrough: "error"*/

switch(foo) {
    case 1:
        doSomething();

    case 2:
        doSomething();
}

Examples of correct code for this rule:

/*eslint no-fallthrough: "error"*/

switch(foo) {
    case 1:
        doSomething();
        break;

    case 2:
        doSomething();
}

function bar(foo) {
    switch(foo) {
        case 1:
            doSomething();
            return;

        case 2:
            doSomething();
    }
}

switch(foo) {
    case 1:
        doSomething();
        throw new Error("Boo!");

    case 2:
        doSomething();
}

switch(foo) {
    case 1:
    case 2:
        doSomething();
}

switch(foo) {
    case 1:
        doSomething();
        // falls through

    case 2:
        doSomething();
}

Note that the last case statement in these examples does not cause a warning because there is nothing to fall through into.

Options

This rule accepts a single options argument:

commentPattern

Examples of correct code for the { "commentPattern": "break[\\s\\w]*omitted" } option:

/*eslint no-fallthrough: ["error", { "commentPattern": "break[\\s\\w]*omitted" }]*/

switch(foo) {
    case 1:
        doSomething();
        // break omitted

    case 2:
        doSomething();
}

switch(foo) {
    case 1:
        doSomething();
        // caution: break is omitted intentionally

    default:
        doSomething();
}

When Not To Use It

If you don't want to enforce that each case statement should end with a throw, return, break, or comment, then you can safely turn this rule off.

Related Rules

Version

This rule was introduced in ESLint 0.0.7.

Resources