Why do we need a Map, if we have Object?

The MDN Documentation has a pretty great comparison on this. For those who don't want to click through the link:

Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Because of this (and because there were no built-in alternatives), Objects have been used as Maps historically; however, there are important differences that make using a Map preferable in certain cases:

• An Object has a prototype, so there are default keys in the map that could collide with your keys if you're not careful. As of ES5 can be bypassed by using map = Object.create(null), but is seldom done.
• The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
• You can get the size of a Map easily with the size property, while the size of an Object must be determined manually.

This does not mean you should use Maps everywhere. If you're not sure which one to use, ask yourself the following questions:

• Are keys usually unknown until run time? Do you need to look them up dynamically?
• Do all values have the same type? Can they be used interchangeably?
• Do you need keys that aren't strings?
• Are key-value pairs frequently added or removed?
• Do you have an arbitrary (easily changing) number of key-value pairs?
• Is the collection iterated?

If you answered 'yes' to any of those questions, that is a sign that you might want to use a Map. Contrariwise, if you have a fixed number of keys, operate on them individually, or distinguish between their usage, then you probably want to use an Object.