類(lèi)型檢測(cè)typeof

主要用于原始數(shù)據(jù)類(lèi)型（string, number, boolean, undefined, symbol）檢測(cè)

// 原始類(lèi)型 
// undefined
console.log(typeof undefined);           // "undefined"
console.log(typeof undeclaredVariable);  // "undefined"

// boolean
console.log(typeof true);                // "boolean"
console.log(typeof false);               // "boolean"
console.log(typeof Boolean(1));          // "boolean"

// number
console.log(typeof 42);                  // "number"
console.log(typeof 3.14);                // "number"
console.log(typeof NaN);                 // "number"
console.log(typeof Infinity);            // "number"
console.log(typeof -Infinity);           // "number"

// string
console.log(typeof "hello");             // "string"
console.log(typeof 'world');             // "string"
console.log(typeof `template`);          // "string"

// bigint
console.log(typeof 123n);                // "bigint"
console.log(typeof BigInt(123));         // "bigint"

// symbol
console.log(typeof Symbol());            // "symbol"
console.log(typeof Symbol('description')); // "symbol"

// object
console.log(typeof {});                  // "object"
console.log(typeof []);                  // "object" (注意：數(shù)組也是對(duì)象)
console.log(typeof null);                // "object" (這是JavaScript的一個(gè)bug)
console.log(typeof new Date());          // "object"
console.log(typeof new RegExp());        // "object"
console.log(typeof new Error());         // "object"

// function
console.log(typeof function() {});       // "function"
console.log(typeof () => {});            // "function"
console.log(typeof class {});            // "function"

類(lèi)型檢測(cè)instanceof

檢查對(duì)象是否是某個(gè)構(gòu)造函數(shù)的實(shí)例

// 原始類(lèi)型 
console.log("hello" instanceof String);      // false
console.log(42 instanceof Number);           // false
console.log(true instanceof Boolean);        // false

// 檢查數(shù)組
const arr = [1, 2, 3];
console.log(arr instanceof Array);        // true
console.log(arr instanceof Object);      // true (數(shù)組也是對(duì)象)

// 檢查字符串
const str = "hello";
console.log(str instanceof String);      // false (原始類(lèi)型)
console.log(new String("hello") instanceof String); // true

// 檢查數(shù)字
const num = 42;
console.log(num instanceof Number);      // false (原始類(lèi)型)
console.log(new Number(42) instanceof Number); // true

// 檢查布爾值
const bool = true;
console.log(bool instanceof Boolean);    // false (原始類(lèi)型)
console.log(new Boolean(true) instanceof Boolean); // true

//檢查函數(shù)類(lèi)型
function myFunction() {}
const arrowFunction = () => {};

console.log(myFunction instanceof Function);      // true
console.log(arrowFunction instanceof Function);   // true
console.log(Array instanceof Function);           // true

項(xiàng)目常用寫(xiě)法

function getTypeInfo(value) {
    const type = typeof value;
    
    switch (type) {
        case 'undefined':
            return '未定義的值';
        case 'boolean':
            return '布爾值';
        case 'number':
            return isNaN(value) ? '非數(shù)字' : '數(shù)字';
        case 'string':
            return '字符串';
        case 'bigint':
            return '大整數(shù)';
        case 'symbol':
            return '符號(hào)';
        case 'object':
            if (value === null) return '空值';
            if (Array.isArray(value)) return '數(shù)組';
            if (value instanceof Date) return '日期';
            if (value instanceof RegExp) return '正則表達(dá)式';
            return '對(duì)象';
        case 'function':
            return '函數(shù)';
        default:
            return '未知類(lèi)型';
    }
}

console.log(getTypeInfo(42));            // "數(shù)字"
console.log(getTypeInfo("hello"));       // "字符串"
console.log(getTypeInfo([]));            // "數(shù)組"
console.log(getTypeInfo(null));          // "空值"
console.log(getTypeInfo(undefined));     // "未定義的值"


function validateParameters(name, age, isActive) {
    const errors = [];
    
    if (typeof name !== 'string') {
        errors.push('姓名必須是字符串');
    }
    
    if (typeof age !== 'number' || isNaN(age)) {
        errors.push('年齡必須是有效數(shù)字');
    }
    
    if (typeof isActive !== 'boolean') {
        errors.push('激活狀態(tài)必須是布爾值');
    }
    
    return errors;
}

console.log(validateParameters("張三", 25, true));     // []
console.log(validateParameters(123, "25", "true"));