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TIL: no-case-declarations in ESLint

Yesterday I ran into the no-case-declarations ESLint rule, which was very surprising to me! This rule bans lexical declarations like let and const in switch statements without wrapping them in blocks. In other words, this rule bans code like this:

switch (foo) {
    case 1:
        let x = 1;
        break;
    default:
        const y = 2;
        break;
}

in favor of code like this:

switch (foo) {
    case 1: {
        let x = 1;
        break;
    }
    default: {
        const y = 2;
        break;
    }
}

Why are the extra blocks necessary? Because apparently in JavaScript switch statements, declarations like let and const are visible in all switch cases, even though they’re not initialized until that case is reached! 😱

This does break code that relies on switch case fallthrough, but… you probably shouldn’t be using that anyway?

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