String.polyfills.js

(function(SP) {

    SP.endsWith || (SP.endsWith = function(str, pos) {
        pos = (pos === undefined) ? this.length : Math.floor(+pos);
        str = str.toString();
        var start = Math.min(Math.max(pos, 0), this.length) - str.length;
        if (start < 0) return !1;
        var idx = ''.indexOf.call(this, str, start);
        return idx === start;
    });

})(String.prototype);

String.polyfills.test.js

function assertEq(fn, val) {
    console.log(fn === val);
}

assertEq("abc".endsWith("abc"), true);
assertEq("abcd".endsWith("bcd"), true);
assertEq("abc".endsWith("c"), true);
assertEq("abc".endsWith("abcd"), false);
assertEq("abc".endsWith("bbc"), false);
assertEq("abc".endsWith("b"), false);
assertEq("abc".endsWith("abc", 3), true);
assertEq("abc".endsWith("bc", 3), true);
assertEq("abc".endsWith("a", 3), false);
assertEq("abc".endsWith("bc", 3), true);
assertEq("abc".endsWith("a", 1), true);
assertEq("abc".endsWith("abc", 1), false);
assertEq("abc".endsWith("b", 2), true);
assertEq("abc".endsWith("d", 2), false);
assertEq("abc".endsWith("dcd", 2), false);
assertEq("abc".endsWith("a", 42), false);
assertEq("abc".endsWith("bc", Infinity), true); //*****
assertEq("abc".endsWith("a", Infinity), false);
assertEq("abc".endsWith("bc", undefined), true);
assertEq("abc".endsWith("bc", -43), false);
assertEq("abc".endsWith("bc", -Infinity), false);
assertEq("abc".endsWith("bc", NaN), false); //*****

// イマイチわかってない。取りあえず切り捨て。
assertEq("abc".endsWith("a", 1.1), true);
assertEq("abc".endsWith("a", "1.1"), true);