type assertion [as syntax | <> syntax

Type assertions in TypeScript [as syntax | <> syntax]

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• To understand type assertions well, you need to understand one sentence deeply: you will know more about the details of a value than TypeScript  .

• Type assertion, assertion assertion, as the name suggests, I determine what, if it is substituted into this sentence, I determine what the type is. Of course, this is our subjective thinking logic, and the program does not recognize it, so we need to tell the program: "Trust me, I know what I am doing"  .

In this way, everyone may still not understand it thoroughly enough. I will use two functions to give an example:

---

/**
 * @param d 日期
 * @param f 想要格式化的字符串
 */
function dateFormatter(d: Date | string, f?: string) {
    const date = new Date(d);
    if (f) {
        return `${date.getFullYear()}${f}${date.getMonth() + 1}${f}${date.getDate()}`
    } else {
        return new Date(d);
    }
}

The above is a small function for simple date formatting. f is a connector. If f has a value, then use f to connect the date and return it. If there is no value, it will directly return a new Date(d).

/**
 * @param d 日期字符串
 */
function dealDate(d: string) {
    return new Date(d).getTime();
}

dealDate()The function accepts a string of type string and returns the milliseconds of this time.

---

Next, we first call the first dateFormatter() function, receive the return value, and then pass it to the dealDate() function, which is what we usually call at this time.

const date = dealDate(dateFormatter('2020-7-28', '/'));

The problem with this call is that we know that  the dateFormatter() function  f must return a string type because of the existence of parameters.

But TS doesn't think so. In the derivation of TS, the dateFormatter() function may return a Date type value, so it will give you a red mark to remind you that the dealDate() function needs to pass a string type value, but you pass It may be string or Date , and the type does not match.

But it can be compiled.

• At this time, we meet the situation we mentioned earlier, and we can conclude that the passed string must be a string. At this point we can use:

  const date = dealDate(dateFormatter('2020-7-28','/') as string);
  
  // or use this
  const a = dealDate(<string>dateFormatter('2020-7-28', '/')); copy

This is type assertion in Typescript.