let num = 123;
let str = String(num); // 顯式轉(zhuǎn)換
let str2 = num + '';   // 隱式轉(zhuǎn)換
console.log(str);  // "123"
console.log(str2); // "123"

let str = "456";
let num = Number(str); // 顯式轉(zhuǎn)換
let num2 = +str;       // 隱式轉(zhuǎn)換
console.log(num);  // 456
console.log(num2); // 456

let str = "3.14";
console.log(parseInt(str));  // 3
console.log(parseFloat(str)); // 3.14

console.log(Boolean(0));        // false
console.log(Boolean(""));       // false
console.log(Boolean(null));     // false
console.log(Boolean(undefined));// false
console.log(Boolean(NaN));      // false

console.log(Boolean(123));      // true
console.log(Boolean("hello"));  // true
console.log(Boolean({}));       // true

console.log(123 == "123"); // true，字符串"123"被轉(zhuǎn)換為數(shù)字123
console.log(123 === "123"); // false，類型不同

let result = "4" - 3; // "4" 被轉(zhuǎn)換為 4，結(jié)果為 1
console.log(result); // 1

result = "5" + 5; // 由于 + 運算符優(yōu)先進行字符串連接，結(jié)果為 "55"
console.log(result); // "55"

console.log(!!"hello"); // true，非空字符串被轉(zhuǎn)換為 true
console.log(!!0);       // false，0 被轉(zhuǎn)換為 false
console.log(null || "default"); // "default"，null 被轉(zhuǎn)換為 false

let result = Number("hello");
console.log(result); // NaN

console.log(Number(null));      // 0
console.log(Number(undefined)); // NaN

console.log("5" + 3); // "53"，字符串連接
console.log("5" - 3); // 2，算術(shù)運算