JavaScript: why myArr.length = 2

Question

var myArr = [];
myArr[0] = [10];
myArr[1] = [20];
myArr.foo = 30;
console.log(myArr.length);

What is myArr.foo here? Looks like an object’s property but isn’t myArr an array?

Answer 1

That's because foo is not a valid array index and just is an ordinary property:

'''
A property name P (in the form of a String value) is an array index if and only if  ToString(ToUint32(P)) is equal to P.
'''

Answer 2

'length' only refers to the 'enumerable' parts of your object, not to all of them. So in this case only the indexes [0] and [1] are counted in.