JavaScript: Remove everything after certain characters

Using  JavaScript, how can I remove everything after the last one of these characters?
.  ?  '  !  "  

var str = 'This is a test.  It is a'; // should become: This is a test.
var str = 'This is a test!  Hello'; // should become: This is a test!
var str = 'This is a "test."  123'; // should become: This is a "test."

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