1.将String类型转换为布尔类型

只要字符串中有内容(空格也算内容)都会转换为true,只有字符串中没有内容才会转换为false

// 有内容(空格也算内容)就会转换为true
let str = " ";
let bool = Boolean(str);
console.log(bool);  // true
console.log(typeof bool);  // boolean

// 没内容就会转换为false
let str = "";
let bool = Boolean(str);
console.log(bool);  // false
console.log(typeof bool);  // boolean

2.将Number类型转换为布尔类型

只有数值为0和NaN时才会转为false,其他的都会转为true

// 只有数值为0和NaN时才会转为false
let num = 0;
let num1 = NaN;
let bool = Boolean(num);
let bool1 = Boolean(num1);
console.log(bool);  // false
console.log(bool1);  // false
console.log(typeof bool);    // boolean
console.log(typeof bool1);    // boolean

3.将undefined类型转换为布尔类型

undefined会转换为false

let num = undefined;
let bool = Boolean(num);
console.log(bool);  // false
console.log(typeof bool);    // boolean

4.将null类型转换为布尔类型

null会转换为false

let num = null;
let bool = Boolean(num);
console.log(bool);  // false
console.log(typeof bool);    // boolean