Maybe you have an autocomplete field and you want to filter a list to case insensitive matches.

case-insensitive-search

// collect a haystack
const countries = [
    'Afghanistan',
    'Åland Islands',
    'Albania',
    'Algeria',
    'American Samoa',
    'Andorra',
    'Angola',
    'Anguilla',
    'Antarctica',
    'Antigua and Barbuda',
    'Argentina',
    'Armenia',
    'Aruba',
    'Australia',
    'Austria',
    'Azerbaijan',
    'Bahamas',
    'Bahrain',
    'Bangladesh',
    'Barbados',
    'Belarus',
    'Belgium',
    'Belize',
    'Benin',
    'Bermuda',
    'Bhutan',
    'Bolivia, Plurinational State of',
    'Bonaire, Sint Eustatius and Saba',
    'Bosnia and Herzegovina',
    'Botswana',
    'Bouvet Island',
    'Brazil',
    'British Indian Ocean Territory',
    'Brunei Darussalam',
    'Bulgaria',
    'Burkina Faso',
    'Burundi',
    'Cambodia',
    'Cameroon',
    'Canada',
    'Cape Verde',
    'Cayman Islands',
    'Central African Republic',
    'Chad',
    'Chile',
    'China',
    'Christmas Island',
    'Cocos (Keeling) Islands',
    'Colombia',
    'Comoros',
    'Congo',
    'Congo, the Democratic Republic of the',
    'Cook Islands',
    'Costa Rica',
    'Côte d\'Ivoire',
    'Croatia',
    'Cuba',
    'Curaçao',
    'Cyprus',
    'Czech Republic',
    'Denmark',
    'Djibouti',
    'Dominica',
    'Dominican Republic',
    'Ecuador',
    'Egypt',
    'El Salvador',
    'Equatorial Guinea',
    'Eritrea',
    'Estonia',
    'Ethiopia',
    'Falkland Islands (Malvinas)',
    'Faroe Islands',
    'Fiji',
    'Finland',
    'France',
    'French Guiana',
    'French Polynesia',
    'French Southern Territories',
    'Gabon',
    'Gambia',
    'Georgia',
    'Germany',
    'Ghana',
    'Gibraltar',
    'Greece',
    'Greenland',
    'Grenada',
    'Guadeloupe',
    'Guam',
    'Guatemala',
    'Guernsey',
    'Guinea',
    'Guinea-Bissau',
    'Guyana',
    'Haiti',
    'Heard Island and McDonald Islands',
    'Holy See (Vatican City State)',
    'Honduras',
    'Hong Kong',
];

// define a needle
const searchTerm = 'can';

// sprinkle bonus sauce on the needle
const lowerCaseSearchTerm = searchTerm.toLowerCase().trim();

// define predicate function
const hasMatchForSearchTerm = c => (c && (c.toString().toLowerCase().indexOf(searchTerm.toLowerCase()) >= 0));

// collect filtered haystack
const filteredCountries = countries.filter(hasMatchForSearchTerm);