使用点表示法访问数组或类似数组的对象的元素

Question

It would be array-like object,but why we can't access it with dot notation instead of bracket notation?

function testArray (rat){
    return typeof arguments;
}

console.log(testArray("test")); // object

function testArray (rat){
    return arguments.0; //[0] is work
}

console.log(testArray("test")); // error

Answer 1

Your question seems to be about why we can't access array and array-like elements using the dot notation like this

var v = a.0;

Then, it's described in the ECMAScript specification :

The dot notation is explained by the following syntactic conversion:

MemberExpression . IdentifierName

And identifiers may not start with a digit as described here :

IdentifierName ::

IdentifierStart

IdentifierName IdentifierPart

IdentifierStart ::

UnicodeLetter

$

_

\\ UnicodeEscapeSequence

As for the reasoning, having identifier names just being made of digits would have made it difficult to write number literals. An exception could probably have been designed just for array access but that would have made the language more complex and departing from the common C family syntax without any real gain.

Answer 2

You can:

var arr = [];
arr.foo = 'foo';

console.log(arr.foo); // => 'foo'