Non-Null Assertion Operator

non-null assertion operator ! 前缀表达式

1、忽略 undefined | null 类型

function myFunc(maybeString: string | undefined | null) {
   const onlyString: string = maybeString; //compilation error: string | undefined | null is not assignable to string
   const ignoreUndefinedAndNull: string = maybeString!; //no problem
}

2、执行方法时忽略 undefined

type NumGenerator = () => number;

function myFunc(numGenerator: NumGenerator | undefined) {
   const num1 = numGenerator(); //compilation error: cannot invoke an object which is possibly undefined
   const num2 = numGenerator!(); //no problem
}

3、当断言失败，代码实际上执行js，有可能导致一些错误

const a: number | undefined = undefined;
const b: number = a!;

console.log(b); // prints undefined, although b’s type does not include undefined

ype NumGenerator = () => number;
function myFunc(numGenerator: NumGenerator | undefined) {
   const num1 = numGenerator!();
}

myFunc(undefined); // runtime error: Uncaught TypeError: numGenerator is not a function

:::warning 只有 strictNullChecks flag 打开时，这个操作符才有用。关闭时，编译器不检查undefined 和 null 类型。 :::