Why is the < comparison operator working, but the <= comparison operator creating an error?

/***I know  <  and  <= are different but they're similar and from my understanding
 if one works the other should also work, I'm confused as to why the bottom function isn't working?

the bottom code isn't working but if I replace the <=with < it works?***/

function Animal(age, name){ 
    this.age=age; 
    this.name=name; 
    } 
var animals=[
     new Animal(4, 'spike'),
     new Animal(2, 'jack'), 
     new Animal(8, 'Penelope'), 
     new Animal(10, 'speedy')
];

function getIndexNames(arrayName){

    var names=[];

for(var i=0; i <= arrayName.length; i++){
   names.push(arrayName[i].name);
    }

return names;
}

var returnedArr = getIndexNames(animals);

console.log(returnedArr);