Declare a variable:let x: number; Do some complicated logic to possibly assign it If it wasn't assigned, provide a default value. Currently, we can't compile this function. We get an error for!xsayingVariable 'x' is used before being assigned. ...
patchValue(target[key]):patchValue;// Error: Variable 'patchValue' is used before being assigned.});returntarget;} Expected behavior: No error Actual behavior: Error: Variable 'patchValue' is used before being assigned. It goes away if I assertpatchValueas non-null: constpatchValue=patch[key]!
3、确定赋值断言 在TypeScript 2.7 版本中引入了确定赋值断言,即允许在实例属性和变量声明后面放置一个!号,从而告诉 TypeScript 该属性会被明确地赋值。为了更好地理解它的作用,我们来看个具体的例子: let x: number; initialize();//Variable 'x' is used before being assigned.(2454)console.log(2* x);/...
问带有async和await的Typescript :变量在为assigned.ts之前使用(2454)ENJavaScript ES7中的 async/await ...
2447 错误 The '{0}' operator is not allowed for boolean types. Consider using '{1}' instead. “{0}”运算符不允许用于布尔类型。请考虑改用“{1}”。 2448 错误 Block-scoped variable '{0}' used before its declaration. 声明之前已使用的块范围变量“{0}”。
// Variable 'x' is used before being assigned.(2454) console.log(2 * x); // Error function initialize() { x = 10; } 1. 2. 3. 4. 5. 6. 7. 很明显该异常信息是说变量 x 在赋值前被使⽤了,要解决该问题,我们可以使⽤确定赋值断⾔: ...
// Error! Variable 'x' is used before being assigned. function initialize() { x = 10; } 添加!修饰:let x!: number,则可以修复这个问题 我们也可以在表达式中使用!,类似variable as string和<string>variable: let x: number; initialize(); ...
// 例子 3leta:numberconsole.log(a)// Error - Variable 'n' is used before being assigned.letb!:numberconsole.log(b)// OK - `!` 表示,你会给 b 一个赋值,不用编译器关心 可选链?. ?.遇到null或undefined就可以立即停止某些表达式的运行,并返回undefined ...
console.log(name); // Error: [ts] Variable 'address' is used before being assigned. 对于可能是undefined的变量: 使用显式声明 提示编译错误:当使用一个可能为null的变量的时候。 使用前,需要确定不是undefined. 代码语言:javascript 代码运行次数:0 运行 AI代码解释 var address: string | undefined; /...
Note that narrowing analysis doesn’t kick in if the variable is assigned anywhere in a nested function. This is because there’s no way to know for sure whether the function will be called later. Copy functionprintValueLater(value: string | undefined) {if(value ===undefined) { ...