获取数据类型的方式和typescriptis类型谓词

获取数据类型的三种方式

1. typeof

console.log(typeof 1); // number
console.log(typeof true); // boolean
console.log(typeof &＃39;mc&＃39;); // string
console.log(typeof Symbol) // function
console.log(typeof function(){}); // function
console.log(typeof console.log()); // undefined
console.log(typeof []); // object
console.log(typeof {}); // object
console.log(typeof null); // object
console.log(typeof undefined); // undefined


优点&＃xff1a;能够快速区分基本数据类型

缺点&＃xff1a;不能将Object、Array和Null区分&＃xff0c;都返回object

2.instanceof

console.log(1 instanceof Number); // false
console.log(true instanceof Boolean); // false
console.log(&＃39;str&＃39; instanceof String); // false
console.log([] instanceof Array); // true
console.log(function(){} instanceof Function); // true
console.log({} instanceof Object); // true


优点&＃xff1a;能够区分Array、Object和Function&＃xff0c;适合用于判断自定义的类实例对象

缺点&＃xff1a;Number&＃xff0c;Boolean&＃xff0c;String基本数据类型不能判断

3.Object.prototype.toString.call()

var toString &＃61; Object.prototype.toString;
console.log(toString.call(1)); //[object Number]
console.log(toString.call(true)); //[object Boolean]
console.log(toString.call(&＃39;mc&＃39;)); //[object String]
console.log(toString.call([])); //[object Array]
console.log(toString.call({})); //[object Object]
console.log(toString.call(function(){})); //[object Function]
console.log(toString.call(undefined)); //[object Undefined]
console.log(toString.call(null)); //[object Null]


优点&＃xff1a;精准判断数据类型

缺点&＃xff1a;写法繁琐不容易记&＃xff0c;推荐进行封装后使用

typeScript小结

any和unknown区别

任何属性都可以赋值unknow,unknow属性只能赋值unknown和any属性

is 类型谓词

const func &＃61; (str: any): boolean &＃61;> {return typeof str &＃61;&＃61;&＃61; &＃39;string&＃39;;
};const test &＃61; (foo: any) &＃61;> {if (func(foo)) {console.log(foo.length);console.log(foo.toExponential(2));}
};
test(&＃39;string&＃39;)


当设置func返回值为布尔型时&＃xff0c;运行时报错 TypeError: foo.toExponential is not a function&＃xff0c;原因是因为string类型并没有toExponential方法&＃xff0c;但是这种情况可以在编译时就可以发现问题并且规避&＃xff0c;这是就需要用到我们的is 类型谓词&＃xff0c;代码如下

const func &＃61; (str: any): str is string &＃61;> {return typeof str &＃61;&＃61;&＃61; &＃39;string&＃39;;
};const test &＃61; (foo: any) &＃61;> {if (func(foo)) {console.log(foo.length);console.log(foo.toExponential(2));}
};


通过is 类型谓词&＃xff0c;缩小变量的类型&＃xff0c;类型保护的作用域仅仅在 if 后的块级作用域中生效&＃xff0c;在if块级作用域下foo变量从any进一步约束为string类型&＃xff0c;从而能够在编译时直接发现错误