ちなみに、第二引数の走査開始インデックスは省略可能です。
window.onload = function () {
if (!('contains' in String.prototype)) {
String.prototype.contains = function (targetStr, searchStartIndex) {
return -1 !== String.prototype.indexOf.call(this, targetStr, searchStartIndex);
};
}
};
function StringCheck() {
var target = "One day in summer, a boy met a girl.";
var result = target.contains("summer", 0);
}<input type="button" value="Contains test" onclick="StringCheck()" />
参考:String.contains