How do I make Array.indexOf() case insensitive?

ES2015 findIndex:

var array = ['I', 'hAve', 'theSe', 'ITEMs'],
    indexOf = (arr, q) => arr.findIndex(item => q.toLowerCase() === item.toLowerCase());

console.log(  indexOf(array, 'i')      ) // 0
console.log(  indexOf(array, 'these')  ) // 2
console.log(  indexOf(array, 'items')  ) // 3

In ECMA-262, 5th edition, you could use Array.prototype.some for this.

var array = [ 'I', 'hAve', 'theSe', 'ITEMs' ];
var query = 'these'.toLowerCase();
var index = -1;
array.some(function(element, i) {
    if (query === element.toLowerCase()) {
        index = i;
        return true;
    }
});
// Result: index = 2