i dont understand this

Question

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

answer: 2

why answer is 2? what is foo?

Answer 1

Unlike other programming languages, JavaScript does not support arrays with named indexes (a.k.a. associative arrays/hashes) and always use numbered indexes.

JavaScript redefines the named indexes ("myArr.foo" in your example) to a standard object, and therefore "myArr.length" only accounts for the numbered indexes (myArr[0] & myArr[1] in your example) and returns the array length as 2.

Answer 2

foo is just a placeholder.