This is very similar to default as first option in switch statement? , but regarding JS rather than PHP.

I have a helper function that wraps various calling styles for another function. The wrapped function requires at least one, and can accept up to 4 parameters. What I am doing is fairly well described by this minimum code example, which works fine in my versions of Chrome and FF.

function testSwitch(definition) {
    var term, minLength, boundaryStart, boundaryEnd;
    switch (definition.length) {
        default:
    case 4:
        boundaryEnd = definition[3];
    case 3:
        boundaryStart = definition[2];
    case 2:
        minLength = definition[1];
    case 1:
        term = definition[0];
        break;
    case 0:
        console.log('fail')
        return;
    }
    console.log(term, minLength, boundaryStart, boundaryEnd);
}

(In the real use case they are passed to the actual function rather than console.log). So the idea is to fall through the cases, allowing too many parameters to be provided (but ignoring the extra ones), but failing if the definition provided is empty.

So I know from my testing that this appears to work in some browsers at least, can I rely on this working across reasonably recent browsers (eg iOS/Android devices, IE8+, Edge etc)? I know that excluding the break statement is well-defined behavior and used fairly broadly, but am not so sure about having the default case first and empty (eg. if some browsers JS engines would optimize out the empty default case then this doesn't work for me, as it would not work when called with an array of size 5, for example).

This is very similar to default as first option in switch statement? , but regarding JS rather than PHP.

I have a helper function that wraps various calling styles for another function. The wrapped function requires at least one, and can accept up to 4 parameters. What I am doing is fairly well described by this minimum code example, which works fine in my versions of Chrome and FF.

function testSwitch(definition) {
    var term, minLength, boundaryStart, boundaryEnd;
    switch (definition.length) {
        default:
    case 4:
        boundaryEnd = definition[3];
    case 3:
        boundaryStart = definition[2];
    case 2:
        minLength = definition[1];
    case 1:
        term = definition[0];
        break;
    case 0:
        console.log('fail')
        return;
    }
    console.log(term, minLength, boundaryStart, boundaryEnd);
}

(In the real use case they are passed to the actual function rather than console.log). So the idea is to fall through the cases, allowing too many parameters to be provided (but ignoring the extra ones), but failing if the definition provided is empty.

So I know from my testing that this appears to work in some browsers at least, can I rely on this working across reasonably recent browsers (eg iOS/Android devices, IE8+, Edge etc)? I know that excluding the break statement is well-defined behavior and used fairly broadly, but am not so sure about having the default case first and empty (eg. if some browsers JS engines would optimize out the empty default case then this doesn't work for me, as it would not work when called with an array of size 5, for example).

• javascript
• cross-browser
• switch-statement

• Your scenario works for me on Firefox 48. I guess, it should work on all JS engines, because I don’t think any JS engine would optimize away the default case because that would definitely change the control flow. – Sebastian Simon Commented Aug 3, 2016 at 0:22
• Personally I'd avoid having the default first, as my feeling is that it should always act as the fallback condition when none of the other cases are met. This is the conventional understanding of its usage and using it this way obviously creates consistency and easier prehension. I suggest that if you find yourself using default first, there's probably a clearer way to construct your solution. – Chris Halcrow Commented May 16, 2022 at 4:41

1 Answer 1

I've never seen someone use this approach before. It's clever. As a side note, clever is a two-edged blade; if this is code that anyone other than you will be touching, I strongly suggest a ment there to make it clear you did that on purpose or someone is going to alter it (because it 'looks wrong') and break it without realizing the intent.

The documentation indicates that default is not required to be the last clause.

If you find a browser were it does NOT work, you could modify it slightly thus and avoid the default:

function testSwitch(definition) {
    var term, minLength, boundaryStart, boundaryEnd;
    switch (definition.length > 4 ? 5 : definition.length) {
    case 5:
    case 4:
        boundaryEnd = definition[3];
    case 3:
        boundaryStart = definition[2];
    case 2:
        minLength = definition[1];
    case 1:
        term = definition[0];
        break;
    case 0:
        console.log('fail')
        return;
    }
    console.log(term, minLength, boundaryStart, boundaryEnd);
}