why we use -1 in javascript for loop?

Question

why we use -1 in javascript for loop

the example code here

var arr = [1,2,2,3,4,5,5,5,6,7,7,8,9,10,10];
function squash(arr){
 var tmp = [];
 for(var i = 0; i < arr.length; i++){
 if(tmp.indexOf(arr[i]) == -1){
 tmp.push(arr[i]);
 }
 }
 return tmp;
}
console.log(squash(arr));

• Sergio Tulentsev: You use it for indexOf, not the loop. Look up the documentation on it.

Answer 1

The indexOf function returns -1 if the item is not found in the desired array.

document.write([1, 2, 3].indexOf(1)+" "); //Exists
document.write([1, 2, 3].indexOf(0)); //Does not exist