In the if statement, we check if the emp.salary property is not equal to undefined and assign the salary variable to the corresponding value. The "Type 'undefined' is not assignable to type" error occurs when a possibly undefined value is assigned to something that expects a different type....
assign(data, inject()) return data } getInjectData(injectUser, injectBook) // Argument of type '() => { book: string; }' is not assignable to parameter of type '() => { user: number; }' 原因是 TypeScript 会取第一个参数作为 injects 数组元素的类型。解决方案是: function getInjec...
exporttype BasicPrimitive=number|string|boolean;exportfunctiondoStuff(value:BasicPrimitive){if(Math.random()<0.5){returnundefined;}returnvalue;} 我们可以看看 在 TypeScript playground 中 会发生什么。虽然我们可能希望 TypeScript 显示doStuff的类型为BasicPrimitive | undefined,但它实际显示的是string | number |...
2.)Undefined type是其唯一值为undefined 值的类型 typeof undefined返回“undefined”字符串 console.log(typeof undefined) //字符串undefined console.log(typeof undefined ==="undefined") //true let a; typeof a=== "undefined"; // => true 3.)导致undefined的常见场景 (1).未初始化变量 myVariable...
type AssignNever<T, K> = K & {[B in Exclude<keyof T, keyof K>]?: never}; 它可以这样使用: type NewType = InterfacesUnion<AttachmentMessageNoContext | TextMessageNoContext> 它通过接受接口/类型的联合来运行,构建一个包含它们所有属性的单一接口,并返回相同的接口/类型的联合,其中每个接口都包含其...
if (firstName) { return firstName + ' ' + lastName; } else { return lastName; } } let tomcat = buildName('Tom', 'Cat'); let tom = buildName(undefined, 'Tom'); // index.ts(1,40): error TS1016: A required parameter cannot follow an optional parameter. ...
TypeScript 不仅知道在 if 分支里 pet 是 Fish 类型;它还清楚在 else 分支里,一定不是 Fish 类型,一定是 Bird 类型 待推断类型(infer) 可以用 infer P 来标记一个泛型,表示这个泛型是一个待推断的类型,并且可以直接使用 比如下面这个获取函数参数类型的例子: ...
and it's giving me the error. But oddly, if I change the tag name from my custom component to a built-in HTML component, e.g.<button>, the error goes away. Another remedy is to just specify a blank attribute (which I guess Vue treats asundefined?) like this:<CustomComponent @click...
基本类型:string, boolean, number, null, undefined, void, never, enum, unknown, any, object, 数组, 元组。(null, undefined是除never外其他类型的子类型) enum Color {R = 0, G = 1, B = 2} let a: string | number | boolean | void | never | unknown | any | string[] | object | ...
if (otherName !== undefined) { this.name = otherName; } } err() { this.name = "not ok"; //Cannot assign to 'name' because it is a read-only property. } } const g = new Greeter(); g.name = "also not ok"; //Cannot assign to 'name' because it is a read-only property...