Array.prototype.indexOf()

var arr1 = ['tokyo','nagoya','osaka','fukuoka'];
document.write( '配列<br>'　+
 arr1.indexOf('nagoya') + '<br>' +
 arr1.indexOf('fukuoka') + '<br>' +
 arr1.indexOf('tokyo') + '<br>' +
 arr1.indexOf('osaka') + '<br>'
);
var arr2 = 'tokyo,nagoya,osaka,fukuoka';
document.write( '文字列<br>'　+
 arr2.indexOf('nagoya') + '<br>' +
 arr2.indexOf('fukuoka') + '<br>' +
 arr2.indexOf('tokyo') + '<br>' +
 arr2.indexOf('osaka') + '<br>'
);

See the Pen js: Array.prototype.indexOf() by nwstcode (@nwst) on CodePen.12804