key vs keyCode vs which

key, keyCode and which can be used to determine which key is pressed. Following is a sample code that handles the keyPress event of a text box.
It checks if user presses the Enter key whose key code is 13:
textBoxElement.addEventListener('keydown', function(e) {
    if (e.keyCode === 13) {
    // Do something ...
    }
});

Difference

According to MDN documentations, both keyCode and which are deprecated and will be removed from the Web standards.
Apart from that, both properties are supported differently by browsers. Some browsers use keyCode, other use which.
It's common to see how they are normalized as following:
const key = 'which' in e ? e.which : e.keyCode;

// Or
const key = e.which || e.keyCode || 0;

Good practice

It's recommended to use the key property. The sample code above can be rewritten as
if (e.key === 'Enter') {
    // Enter is pressed
}
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