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TypeScript 3.7 RC & Assertion Functions

assertion functions, assert

https://devblogs.microsoft.com/typescript/announcing-typescript-3-7-rc/#assertion-functions

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)
    }
}

function assertIsString(val: any): asserts val is string {
    if (typeof val !== "string") {
        throw new AssertionError("Not a string!");
    }
}

// Here asserts val is string ensures that after any call to assertIsString, any variable passed in will be known to be a string.

function yell(str: any) {
    assertIsString(str);

    // Now TypeScript knows that 'str' is a 'string'.

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

type predicate signatures

function isString(val: any): val is string {
    return typeof val === "string";
}

function yell(str: any) {
    if (isString(str)) {
        return str.toUppercase();
    }
    throw "Oops!";
}

assertion signatures

function assertIsDefined<T>(val: T): asserts val is NonNullable<T> {
    if (val === undefined || val === null) {
        throw new AssertionError(
            `Expected 'val' to be defined, but received ${val}`
        );
    }
}

refs

https://www.cnblogs.com/xgqfrms/tag/TypeScript 3.7 RC/

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