为何JS中带数字键的对象及数组无法使用点表示法访问？

Great question—this is a common point of confusion when learning how JavaScript handles objects and arrays. Let’s break this down step by step, using your examples as a guide.

First, let’s start with the core rule that governs property access in JavaScript: dot notation requires property names to be valid identifiers, while bracket notation accepts any string (or value that can be coerced to a string) as a property name.

Let’s walk through each of your code snippets:

• console.log(arr[0]);
  This works because when you pass a number inside square brackets, JavaScript automatically coerces it to a string (0 → "0"). Arrays are just specialized objects where "indexes" are actually string property names matching integer values (from 0 to 2^32 - 1). The engine optimizes these properties for array-like behavior (like auto-updating the length property), but at their core, they’re still regular object properties.
• console.log(arr['0']);
  This is functionally identical to the first example—you’re directly using the string property name that the number 0 gets converted to. This confirms that array indexes are just string keys under the hood.
• console.log(arr.0); & console.log(arr.'0');
  Both throw syntax errors for the same reason: dot notation doesn’t allow property names that aren’t valid identifiers. A valid JavaScript identifier can’t start with a number, and you can’t place a string literal directly after a dot. Even though "0" is a valid property name for the array object, the syntax rules of dot notation block you from using it this way.

What About the "Everything is an Object" Idea? #

You’re totally right that arrays are objects in JavaScript—but that doesn’t mean all property access rules go out the window. Objects have two distinct ways to access properties:

1. Dot notation: For valid identifiers (e.g., obj.username, obj._item123—note that underscores are allowed at the start)
2. Bracket notation: For any property name (including those that aren’t valid identifiers, like "0", "user preferences", or even Symbol() keys)

When you try to simulate an array with a custom object (like const fakeArr = { 0: 'apple', 1: 'banana', length: 2 }), you’ll hit the exact same restriction: fakeArr.0 will throw an error, but fakeArr['0'] works perfectly. This aligns with how real arrays behave—they’re just objects with extra optimizations for integer-like string keys.

Quick Recap #

• Array indexes are string property names in disguise, and JS coerces numbers to strings when using bracket notation.
• Dot notation is limited to valid identifiers, so numeric property names (like "0") can’t be accessed this way.
• The "everything is an object" principle holds true—arrays follow the same property access rules as regular objects, they just have special handling for index-like keys.

内容的提问来源于stack exchange，提问作者Ankit Kumar Ojha