JavaScript Object

JavaScript Array
JavaScript Date Object
JavaScript Map and Set
JavaScript WeakMap
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JavaScript objects can be thought of as simple collections of name-value pairs. As such, they are similar to:

• Dictionaries in Python.
• Hashes in Perl and Ruby.
• Hash tables in C and C++.
• HashMaps in Java.
• Associative arrays in PHP.

There are two basic ways to create an empty object:

const obj = new Object();
const obj = {};

Object.keys，values，entries

对于普通对象，下列这些方法是可用的：

• Object.keys(obj) —— 返回一个包含该对象所有的键的数组。

• Object.values(obj) —— 返回一个包含该对象所有的值的数组。

• Object.entries(obj) —— 返回一个包含该对象所有 [key, value] 键值对的数组。

• Map.keys() 返回可迭代项

• Object.keys(obj) 返回“真正的”数组

Objects Vs Map

• The keys of an Object are Strings or Symbols, where they can be of any value for a Map.
• You can get the size of a Map easily, while you have to manually keep track of size for an Object.
• The iteration of maps is in insertion order of the elements.
• An Object has a prototype, so there are default keys in the map. (This can be bypassed using map = Object.create(null).)

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• Use maps over objects when keys are unknown until run time, and when all keys are the same type and all values are the same type.
• Use maps if there is a need to store primitive values as keys because object treats each key as a string whether it’s a number value, boolean value or any other primitive value.
• Use objects when there is logic that operates on individual elements.

Objects Prototype

Object prototypes - Learn web development | MDN
Every object in JavaScript has a built-in property, which is called its prototype. The prototype is itself an object, so the prototype will have its own prototype, making what’s called a prototype chain. The chain ends when we reach a prototype that has null for its own prototype.