if indexOf == -1 meaning in js code example

Example 1: javascript indexof with condition

a = [
  {prop1:"abc",prop2:"qwe"},
  {prop1:"bnmb",prop2:"yutu"},
  {prop1:"zxvz",prop2:"qwrq"}
];
    
index = a.findIndex(x => x.prop2 ==="yutu");

console.log(index);

Example 2: what indexof in javascript

//indexOf getting index of array element, returns -1 if not found
var colors=["red","green","blue"];
var pos=colors.indexOf("blue");//2

//indexOf getting index of sub string, returns -1 if not found
var str = "We got a poop cleanup on isle 4.";
var strPos = str.indexOf("poop");//9