Are array keys implicitly coerced to numbers?

Question

I'm in the book, This & Object Prototypes, Chapter 3: Objects.

In the arrays section, the author says if you add a property to an array, but the array looks like a number, it will end up as part of the array index:

myArray["3"] = "baz";
console.log(myArray.length);
console.log(myArray[3]);

It looks like JavaScript is implicitly coercing the "3" string into the number 3, then it's saying place "baz" in index 3 of the array.

Answer 1

Actually, it's the other way around. All keys in JavaScript objects are strings, and arrays are objects. That means myArray[3] is the same as myArray["3"] because all keys, if not already strings, are coerced into strings because all JavaScript object keys are strings. Per MDN:

An object property name can be any valid JavaScript string, or anything that can be converted to a string, including the empty string.

And:

Please note that all keys in the square bracket notation are converted to String type, since objects in JavaScript can only have String type as key type. For example, in the above code, when the key obj is added to the myObj, JavaScript will call the obj.toString() method, and use this result string as the new key.

For example:

const obj = {
  toString() {
    return 'foobar';
  }
};

const anotherObj = {};
anotherObj[obj] = 'baz';
//anotherObj.foobar is now 'baz'

Since obj is converted to a string implicitly, obj.toString is called which returns 'foobar', which is used as the key value. The same applies for arrays -- when using bracket notation. All keys are strings, and accessing using bracket notation coerces the expression in the brackets into a string.