What does this grammar mean

[myNameIsDu]

I found this grammar when I read the source code of babel-parser:

In the switch statement:

I found them meaningless and ignored, But why write it like this, Will the prompt terminate the while statement?

[nicolo-ribaudo]

Yes, without that label break would terminate the switch statement instead of the loop.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label