TypeScript 并不能幫你解決下面的類型問題

function yell(str) {
    if (typeof str !== "string") {
        throw new TypeError("str should have been a string.")
    }
    // Error caught!
    return str.toUppercase();
}

不過現(xiàn)在可以這樣定義一個 Asserts 了

function assert(condition: any, msg?: string): asserts condition {
    if (!condition) {
        throw new AssertionError(msg)
    }
}

asserts condition says that whatever gets passed into the condition parameter must be true if the assert returns (because otherwise it would throw an error). That means that for the rest of the scope, that condition must be truthy. As an example, using this assertion function means we do catch our original yell example.

function yell(str) {
    assert(typeof str === "string");

    return str.toUppercase();
    //         ~~~~~~~~~~~
    // error: Property 'toUppercase' does not exist on type 'string'.
    //        Did you mean 'toUpperCase'?
}

function assert(condition: any, msg?: string): asserts condition {
    if (!condition) {
        throw new AssertionError(msg)
    }
}

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完整參考資料：https://www.typescriptlang.org/docs/handbook/release-notes/overview.html#assertion-functions