Create a function that returns which chapter is nearest to the page you're on. If two chapters are equidistant, return the chapter with the higher page number.
nearestChapter({
"Chapter 1" : 1,
"Chapter 2" : 15,
"Chapter 3" : 37
}, 10) ➞ "Chapter 2"
nearestChapter({
"New Beginnings" : 1,
"Strange Developments" : 62,
"The End?" : 194,
"The True Ending" : 460
}, 200) ➞ "The End?"
nearestChapter({
"Chapter 1a" : 1,
"Chapter 1b" : 5
}, 3) ➞ "Chapter 1b"