如何解决变量比较时在JavaScript中变得未定义的问题？

var x; // 未定义变量
var y = 10;

if (x === undefined) {
  console.log("x is undefined");
} else {
  console.log("x is defined");
}

if (y === undefined) {
  console.log("y is undefined");
} else {
  console.log("y is defined");
}

x is undefined
y is defined

var x; // 未定义变量
var y = 10;

if (typeof x === "undefined") {
  console.log("x is undefined");
} else {
  console.log("x is defined");
}

if (typeof y === "undefined") {
  console.log("y is undefined");
} else {
  console.log("y is defined");
}

x is undefined
y is defined

var x; // 未定义变量
var y = 10;

x = x || "default value";

console.log(x); // 输出 "default value"

if (y) {
  console.log("y is defined");
} else {
  console.log("y is undefined");
}

default value
y is defined