Javascript"变量变量":如何根据另一个变量分配变量？

var variableName = "variable1";
var variable1 = "value1";
var variable2 = "value2";

var value = window[variableName];
console.log(value); // 输出 "value1"

var variableName = "variable1";
var variable1 = "value1";
var variable2 = "value2";

var value = eval(variableName);
console.log(value); // 输出 "value1"

var variables = {
  variable1: "value1",
  variable2: "value2"
};

var variableName = "variable1";
var value = variables[variableName];
console.log(value); // 输出 "value1"