IndexOf element in array [duplicate]

Answer 1

It's because arr is a (live) NodeList not an array; what you want is, I think:

console.log([].indexOf.call(arr, selected[0]));

 var arr = document.getElementsByClassName('class');
 var selected = document.getElementsByClassName('selected');
 var selected_id = [].indexOf.call(arr, selected[0]);
 console.log(selected_id);

The line:

var selected_id = [].indexOf.call(arr, selected[0]);

Uses the native Array.prototype.indexOf() method, supplying the arr as the this (basically using arr as the array on which the method is called), supplying selected[0] as the argument to be passed to the method, so it sort of gets applied as you called it yourself:

var selected_id = arr.indexOf(selected[0]);

but does it in a way that recognises the indexOf() method isn't available to the NodeList object (calling it in a legitimate way, and therefore preventing the error).

References:

• Array.prototype.indexOf().
• document.getElementsByClassName().
• Function.prototype.call().