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$(document).one('someTrigger', function(){
	console.log('Do something');
});

$(window).on('scroll', function(){
	var distanceUntilVisible = $('.some-element').position().top - (window.scrollY + window.innerHeight);
	console.log(distanceUntilVisible);
	if( distanceUntilVisible <= 400 ){
		$(document).trigger('someTrigger');
	}
});