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"]
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.
for ... in enumerates property names, which are arbitrary strings and therefore may or may not be an identifier.Items in an array are accessed using the syntax myArray[index].
The dot notation only allows you to access properties whose names are valid identifiers, and the index of an item in an array, being a number, does not qualify. Therefore, you have to use the more lenient bracket notation.
JavaScript identifiers cannot begin with a digit. The property that you are trying to access is an identifier inside the object.
You will find here https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Values,_variables,_and_literals this fragment:
Variables
You use variables as symbolic names for values in your application. The names of variables, called identifiers, conform to certain rules.
A JavaScript identifier must start with a letter, underscore (_), or dollar sign ($); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase).
Starting with JavaScript 1.5, you can use ISO 8859-1 or Unicode letters such as å and ü in identifiers. You can also use the \uXXXX Unicode escape sequences as characters in identifiers.
Some examples of legal names are Number_hits, temp99, and _name.
for...in, that is for objects only.