const NAME: string = "TypeScript"; NAME= "Haha";//Uncaught TypeError: Assignment to constant variableconst obj ={ name:"TypeScript"}; obj.name= "Haha"; interface Info { readonly name: string; } const info: Info={ name:"TypeScript"}; info["name"] = "Haha";//Cannot assign to 'n...
The last example usednamed function syntaxto declarethe function, but JavaScript and TypeScript support at least five ways to do this: // Named functionfunctiongreet(name:string){return'hello '+name}// Function expressionletgreet2=function(name:string){return'hello '+name}// Arrow function expr...
functionprocess(variable:string|number){if(typeofvariable==='string'){console.log(variable.length);// 可以访问 length 属性}else{console.log(variable.toFixed(2));// 可以调用 toFixed 方法}} 上述代码展示了如何利用typeof进行类型判断,从而在使用联合类型时实现类型保护。 类型区分 使用类型区分(Type Gu...
Writing types can be optional in TypeScript, becausetype inferenceallows you to get much of this power without writing extra code. If TypeScript can determine the data type implicitly (for example, when you assign a value to a variable by usinglet age = 42), it automatically infers the dat...
off clean-up, along with arbitrary amounts of cleanup. ADisposableStackis an object that has several methods for keeping track ofDisposableobjects, and can be given functions for doing arbitrary clean-up work. We can also assign them tousingvariables because — get this —they’re also...
// Type 'string' is not assignable to type 'boolean'. } 这里使用 declare 声明了一个类型变量,然后通过类型变量里面的判定条件就能配合检查其他变量的类型了。 Any 和Unknown 的情形类似,我们还可以使用 any 来代表任意的类型,示例如下: declare function getValue(key: string): any; ...
TypeScript enforces the best practice of accessing internal fields (like id and fullName) through a reference to the class (this). Classes can also have constructor functions that include a feature C# has just adopted: automatic definition of fields. The constructor function in a TypeScript clas...
functionquadruple(x){if(isFinite(x)){console.log((x+x)*2);}}quadruple("1");// Still prints 22 This is becauseisFinitecoerces its argument to a numeric valuebeforeevaluating if it's finite or not. This is dangerous, because it can lead to unexpected results like the one above. ...
function fun(): undefined { console.log("this is TypeScript"); }; fun(); // Error never never类型表示的是那些永不存在的值的类型。 值会永不存在的两种情况: 如果一个函数执行时抛出了异常,那么这个函数永远不存在返回值(因为抛出异常会直接中断程序运行,这使得程序运行不到返回值那一步,即具有不可...
That is, isList<Dog>assignable toList<Animals>? The straightforward way to find out is to do a structural comparison of the types, member by member. Unfortunately, this can be very expensive. However, if we know enough aboutList<T>, we can reduce this assignability check to determining wh...