当我的名字作为字符串时,如何执行JavaScript函数

var name = "John";
var functionName = "sayHello";
eval("var " + functionName + " = function() { console.log('Hello, ' + name + '!'); }");
sayHello();

var name = "John";
var functionName = "sayHello";
var func = new Function("console.log('Hello, " + name + "!');");
func.name = functionName;
window[functionName] = func;
sayHello();

var name = "John";
var functionName = "sayHello";
window[functionName] = function() { console.log('Hello, ' + name + '!'); };
sayHello();