Array.size() vs Array.length

What is the difference between the two?

So I know that array.size() is a function while array.length is a property. Is there a usecase for using one over the other? Is one more efficient? (I would imagine .length to be significantly faster as it is a property rather then a method call?) Why would one ever use the slower option? Are there some browsers that are incompatible with one or the other?

  var x = [];
  console.log(x.size());
  console.log(x.length);
  console.log(x.size()==x.length);
  x =[1,2,3];
  console.log(x.size());
  console.log(x.length);
  console.log(x.size()==x.length);

Will print:

  0, 0, true
  3, 3, true