ES6 substr() 字符串String 方法

var str = "Apples are round, and apples are juicy.";
console.log("(1,2): "    + str.substr(1,2));
console.log("(-2,2): "   + str.substr(-2,2));
console.log("(1): "      + str.substr(1));
console.log("(-20, 2): " + str.substr(-20,2));
console.log("(20, 2): "  + str.substr(20,2));