Check if an element contains a class in JavaScript?

Use element.classList .contains method:

element.classList.contains(class);

This works on all current browsers and there are polyfills to support older browsers too.

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Alternatively, if you work with older browsers and don't want to use polyfills to fix them, using indexOf is correct, but you have to tweak it a little:

function hasClass(element, className) {
    return (' ' + element.className + ' ').indexOf(' ' + className+ ' ') > -1;
}

Otherwise you will also get true if the class you are looking for is part of another class name.

DEMO

jQuery uses a similar (if not the same) method.

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Applied to the example:

As this does not work together with the switch statement, you could achieve the same effect with this code:

var test = document.getElementById("test"),
    classes = ['class1', 'class2', 'class3', 'class4'];

test.innerHTML = "";

for(var i = 0, j = classes.length; i < j; i++) {
    if(hasClass(test, classes[i])) {
        test.innerHTML = "I have " + classes[i];
        break;
    }
}

It's also less redundant ;)

The easy and effective solution is trying .contains method.

test.classList.contains(testClass);