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The onblur event occurs when an object loses focus. The onblur event is most often used with form validation code (e.g. when the user leaves a form field). The onblur event is the opposite of the onfocus event, being thus similar to the onfocusout event, the difference being that the onblur event does not bubble.
https://www.w3schools.com/jsref/event_onblur.asp