library is an object, not an array. You push things onto arrays. Unlike PHP, Javascript makes a distinction.
Your code tries to make a string that looks like the source code for a key-value pair, and then "push" it onto the object. That's not even close to how it works.
What you want to do is add a new key-value pair to the object, where the key is the title and the value is another object. That looks like this:
library[title] = {"foregrounds" : foregrounds, "backgrounds" : backgrounds};
"JSON object" is a vague term. You must be careful to distinguish between an actual object in memory in your program, and a fragment of text that is in JSON format.