undefined:表示一个还未定义(声明)或者定义了还没有初始化的变量(对象)。 这里要注意,如果一个变量尚未声明,使用if语句检测该变量会报错,而使用typeof将会返回undefined。undefined和null的布尔值都是false。因此,未声明的变量应该用typeof来检测,而不是直接使用if语句。 另外,undefined是从null派生出来的,因此undefined...
这包括基本类型,如字符串、数字和布尔值: let name: string; name = "Marius"; // OK name = null; // OK name = undefined; // OK let age: number; age = 24; // OK age = null; // OK age = undefined; // OK let isMarried: boolean; isMarried = true; // OK isMarried = false...
//juggleif(x ==null) {} vara:number;varb:number=null;functioncheck(x, name){if(x ==null) {console.log(name +' == null');}if(x ===null) {console.log(name +' === null');}if(typeofx ==='undefined') {console.log(name +' is undefin...
function padLeft(value: string, padding: string | number) { if (typeof padding === "number") { return Array(padding + 1).join(" ") + value; } if (typeof padding === "string") { return padding + value; } throw new Error(`Expected string or number, got '${padding}'.`); }...
number = null; function check(x, name) { if (x == null) { console.log(name + ' == null'); } if (x === null) { console.log(name + ' === null'); } if (typeof x === 'undefined') { console.log(name + ' is undefined'); } } check(a, 'a'); check(b, 'b');...
functionvalidateQuantity(target:any,propertyKey:string){letvalue=target[propertyKey];constgetter=function(){returnvalue;};constsetter=function(newValue:number){if(newValue<0){thrownewError("商品数量不能为负数。");}value=newValue;};Object.defineProperty(target,propertyKey,{get:getter,set:setter,enumer...
function sanitizeFoo(checker: any) { if ( typeof checker.number != "number" || typeof checker.boolean != "boolean" || (checker.maybeString != undefined && typeof checker.maybeString != "string") || !sanitizeBar(checker.bar) ) { return false; } return true; } function sanitizeBar(...
hasInstance](val: unknown): val is PointLike { return !!val && typeof val === "object" && "x" in val && "y" in val && typeof val.x === "number" && typeof val.y === "number"; } } function f(value: unknown) { if (value instanceof Point) { // Can access both of ...
{suit: string; card: number; }[]): number; function pickCard(x: number): {suit: string; card: number; }; function pickCard(x): any { // Check to see if we're working with an object/array // if so, they gave us the deck and we'll pick the card if (typeof x == "...
Exception: new Map<string, number>() is OK. Using the types Function and Object is almost never a good idea. In 99% of cases it's possible to specify a more specific type. Examples are (x: number) => number for functions and { x: number, y: number } for objects. If there is...