Question

This code will alert all the properties names in the object a. 0,1,2 and hello.

Object.prototype.hello = {};

var a = [1,2,3];

for ( var number in a ) {
    alert( number)
}

My question is, I can access the property hello by this syntax:

a.hello

But why can’t I access a.0 which should be equal to 1. Isn’t the array decleration creating “real properties”?

I know that I can access the properties by a[0] and a[“hello”]

Answer 1

It’s a syntax limitation. In JavaScript, identifier must not start with number, so 0 is not a valid one. a.0 will then produce syntax error.