Disallow duplicate conditions in if-else-if chains (no-dupe-else-if)

if-else-if chains are commonly used when there is a need to execute only one branch (or at most one branch) out of several possible branches, based on certain conditions.

if (a) {
    foo();
} else if (b) {
    bar();
} else if (c) {
    baz();
}

Two identical test conditions in the same chain are almost always a mistake in the code. Unless there are side effects in the expressions, a duplicate will evaluate to the same true or false value as the identical expression earlier in the chain, meaning that its branch can never execute.

if (a) {
    foo();
} else if (b) {
    bar();
} else if (b) {
    baz();
}

In the above example, baz() can never execute. Obviously, baz() could be executed only when b evaluates to true, but in that case bar() would be executed instead, since it's earlier in the chain.

Rule Details

This rule disallows duplicate conditions in the same if-else-if chain.

Examples of incorrect code for this rule:

/*eslint no-dupe-else-if: "error"*/

if (isSomething(x)) {
    foo();
} else if (isSomething(x)) {
    bar();
}

if (a) {
    foo();
} else if (b) {
    bar();
} else if (c && d) {
    baz();
} else if (c && d) {
    quux();
} else {
    quuux();
}

if (n === 1) {
    foo();
} else if (n === 2) {
    bar();
} else if (n === 3) {
    baz();
} else if (n === 2) {
    quux();
} else if (n === 5) {
    quuux();
}

Examples of correct code for this rule:

/*eslint no-dupe-else-if: "error"*/

if (isSomething(x)) {
    foo();
} else if (isSomethingElse(x)) {
    bar();
}

if (a) {
    foo();
} else if (b) {
    bar();
} else if (c && d) {
    baz();
} else if (c && e) {
    quux();
} else {
    quuux();
}

if (n === 1) {
    foo();
} else if (n === 2) {
    bar();
} else if (n === 3) {
    baz();
} else if (n === 4) {
    quux();
} else if (n === 5) {
    quuux();
}

This rule can also detect some cases where the conditions are not identical, but the branch can never execute due to the logic of || and && operators.

Examples of additional incorrect code for this rule:

/*eslint no-dupe-else-if: "error"*/

if (a || b) {
    foo();
} else if (a) {
    bar();
}

if (a) {
    foo();
} else if (b) {
    bar();
} else if (a || b) {
    baz();
}

if (a) {
    foo();
} else if (a && b) {
    bar();
}

if (a && b) {
    foo();
} else if (a && b && c) {
    bar();
}

if (a || b) {
    foo();
} else if (b && c) {
    bar();
}

if (a) {
    foo();
} else if (b && c) {
    bar();
} else if (d && (c && e && b || a)) {
    baz();
}

Please note that this rule does not compare conditions from the chain with conditions inside statements, and will not warn in the cases such as follows:

if (a) {
    if (a) {
        foo();
    }
}

if (a) {
    foo();
} else {
    if (a) {
        bar();
    }
}

When Not To Use It

In rare cases where you really need identical test conditions in the same chain, which necessarily means that the expressions in the chain are causing and relying on side effects, you will have to turn this rule off.

Related Rules

Version

This rule was introduced in ESLint 6.7.0.

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