var arr = { first: 123, second: 456 };
console.log(arr['first']); // 123
console.log(arr['second']); // 456
console.log(arr.first); // 123
console.log(arr['first', 'second']); // 456
Problem 1: Why arr['first', 'second'] will get last element?
Problem 2: What's different ['child'] and .child ?
Thx all!
In JS, objects are collections of key-value pairs (maps or dictionaries, if you want). An array is just another type of object. Your example however features a plain object.
Objects, as I said earlier, have keys, which are used to provide access to methods and properties.
var o = {
x: 12,
y: function () { console.log(this.x); }
};
o.y(); // 12
You can access object's members via 2 notations:
o.x, o,yo['x'], o['y']Note, in the second approach you must provide a string, which means
var name = 'x'; o[name] will work, while o[x] will not.
This is valid for arrays as well:
var arr = ['a', 'b', 'c'];
arr[0] // 'a'
arr['0'] // 'a'
Object.keys(arr) // ['0', '1', '2']
however, you can not do arr.0
Now, regarding your problem:
arr['first', 'second'] is equivalent with arr['second'] because you use the comma operator which evaluates each expression, from left to right, and returns the value of the most right one.
eg: 1, alert(0), 5 gives you 5
It's not an array but an object, but the first problem is actually interesting.
Getting object properties using [] notations means you need to pass a key - it may look like an array literal, but it isn't.
What you put in the square brackets is a statement.
For example you could do something like this: arr[ condition ? 'first' : 'second' ] - the statement is parsed and its return value is passed as the key.
In arr['first', 'second'] the inner statement can be read as ('first', 'second') which returns second, because in JS a statement consisting of many of expressions returns the value of the last one, for example (x=2, 4) returns 4.
It also sheds some light on object.child vs. object['child']. In the first example you need to use an actual key, while in the second one you pass a statement; in this case the statement evaluates to string, but it could be a variable name, or anything that would evaluate to something that can be used as a key (string or number).
arrnot an array, also see about Property accessors and comma operatorconsole.log(arr['first', 'second']);:-)