超类型 getAnimalName 和子类型 getDogName 的方法可以相互分配。如果 strictFunctionTypes 设置为 true,则 Typescript 的参数进行逆变比较。 <pre class="prettyprint hljs typescript" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); b...
// in this function, the parameter `value` automatically gets assigned the type `number` from the type `Negate` const negateFunction: Negate = (value) => value * -1; Try it Yourself » TypeScript Exercises Test Yourself With Exercises Exercise: Create a function that returns the string...
function getDirectionFirstLetter(direction: Direction) { return direction.substr(0, 1); } getDirectionFirstLetter("test"); // ❌ 类型“"test"”的参数不能赋给类型“Direction”的参数。 getDirectionFirstLetter("east"); 这个例子中使用四个字符串字面量类型组成了一个联合类型。这样在调用函数时,编译...
or even to get the return type of a function. We can use this to build a “FnReturnType” type, that will give us the return type of the function passed in as the generic parameter.
function getValue(param: string | number): string | number { if (typeof param === 'string') { return 'The parameter is a string'; } else { return 100; } } 在上面的示例中,函数getValue的参数param可以是string类型或number类型。根据参数的类型,函数返回不同的值。如果参数是string类型...
expression, we can use the “infer” keyword to either get the type of the elements of an array, or even to get the return type of a function. We can use this to build a “FnReturnType” type, that will give us the return type of the function passed in as the generic parameter....
typeDirection="north"|"east"|"south"|"west";functiongetDirectionFirstLetter(direction:Direction) {returndirection.substr(0,1); }getDirectionFirstLetter("test");// ❌ 类型“"test"”的参数不能赋给类型“Direction”的参数。getDirectionFirstLetter("east"); ...
This apply to callback function, not normal function However, it cannot use a parameter that doesn't exist in its definition, as this would result in an error. This is why we needed to delete the first two members of theCallbackTypeunion. ...
That parameter is defined using a function type that accepts two parameters (one of string, one of boolean) and returns a number. If you’re a C# developer, you might find that the syntax looks much like a lambda expression. A class that implements this interface would look something like...
复制代码declare function freeze<Type>(obj: Type): Readonly<Type>; 04.Record<Keys, Type> 作用:构造一个对象类型,其属性键为Keys,属性值为Type。 常用指数: ⭐️⭐️⭐️⭐️⭐️ 使用场景示例(创建具有一致性的字典): ts复制代码interface User { name: string age: number } ...