详解JavaScript如何准确获取任意变量的数据类型

console.log(typeof ''); // string
console.log(typeof (() => {})); // function
console.log(typeof Symbol()); // symbol
console.log(typeof undefined); // undefined
console.log(typeof null); // object
console.log(typeof []); // object
console.log(typeof {}); // object

console.log({} instanceof Array); // false
console.log([] instanceof Array); // true
console.log({} instanceof Object); // true
console.log([] instanceof Object); // true

const o = new Object();
o.toString(); // [object Object]
const arr = [1,2,3]; // 数组也会继承自Object
arr.toString(); // '1,2,3'，因为数组重写了toString

console.log(Object.prototype.toString.call(null)); // [object Null]
console.log(Object.prototype.toString.call(undefined)); // [object Undefined]
console.log(Object.prototype.toString.call([])); // [object Array]
console.log(Object.prototype.toString.call({})); // [object Object]
console.log(Object.prototype.toString.call(() => {})); // [object Function]

function getType(obj) {
    if (typeof obj === 'object') {
        if (obj instanceof Array) return 'array';
        if (obj instanceof Map) return 'map';
        if (obj instanceof Set) return 'set';
        if (obj instanceof RegExp) return 'regexp';
        // 尽可能枚举类型，最后兜底返回object
        return 'object'
    }
    return (typeof obj);
}
console.log(getType(null)); // object
console.log(getType(undefined)); // undefined
console.log(getType('')); // string
console.log(getType([])); // array
console.log(getType(/a/)); // regexp
console.log(getType(new WeakMap())); // object
console.log(getType(new WeakSet())); // object

function getType(obj) {
    const originType = Object.prototype.toString.call(obj); // 获取类型。返回格式为[object type]，需要提取出type的值
    const firstSpaceIndex = originType.indexOf(' '); // 找到第一个空格所在下标
    const type = originType.slice(firstSpaceIndex + 1, -1); // 跳过空格，截取到倒数第一个字符（不含最后一个）
    return type.toLowerCase(); // 转换为小写
}
console.log(getType(null)); // null
console.log(getType(undefined)); // undefined
console.log(getType('')); // string
console.log(getType([])); // array
console.log(getType(/a/)); // regexp
console.log(getType(new WeakMap())); // weakmap
console.log(getType(new WeakSet())); // weakset

function getType(obj) {
    const originType = Object.prototype.toString.call(obj); // 获取类型。返回格式为[object type]，需要提取出type的值
    const tmpType = originType.split(' ')[1]; // 根据空格分割，并取第二个
    const type = tmpType.slice(0, -1); // 从0截取到倒数第一个字符（不含最后一个）
    return type.toLowerCase(); // 转换为小写
}
console.log(getType(null)); // null
console.log(getType(undefined)); // undefined
console.log(getType('')); // string
console.log(getType([])); // array
console.log(getType(/a/)); // regexp
console.log(getType(new WeakMap())); // weakmap
console.log(getType(new WeakSet())); // weakset