If a switch statement has duplicate test expressions in case clauses, it is likely that a programmer copied a case clause but forgot to change the test expression.

Rule Details

This rule disallows duplicate test expressions in case clauses of switch statements.

Examples of incorrect code for this rule:

:::incorrect

/*eslint no-duplicate-case: "error"*/

const a = 1,
    one = 1;

switch (a) {
    case 1:
        break;
    case 2:
        break;
    case 1:         // duplicate test expression
        break;
    default:
        break;
}

switch (a) {
    case one:
        break;
    case 2:
        break;
    case one:         // duplicate test expression
        break;
    default:
        break;
}

switch (a) {
    case "1":
        break;
    case "2":
        break;
    case "1":         // duplicate test expression
        break;
    default:
        break;
}

:::

Examples of correct code for this rule:

:::correct

/*eslint no-duplicate-case: "error"*/

const a = 1,
    one = 1;

switch (a) {
    case 1:
        break;
    case 2:
        break;
    case 3:
        break;
    default:
        break;
}

switch (a) {
    case one:
        break;
    case 2:
        break;
    case 3:
        break;
    default:
        break;
}

switch (a) {
    case "1":
        break;
    case "2":
        break;
    case "3":
        break;
    default:
        break;
}

:::

When Not To Use It

In rare cases where identical test expressions in case clauses produce different values, which necessarily means that the expressions are causing and relying on side effects, you will have to disable this rule.

switch (a) {
    case i++:
        foo();
        break;
    case i++: // eslint-disable-line no-duplicate-case
        bar();
        break;
}