TypeScript：元素隐含地具有 RegExp 的“任何”类型

Question

So, I want to create a function that would take a duration string (eg 12ms , 7.5 MIN , 400H ), parse it and convert it to milliseconds.

 const units = { MS: 1, S: 1 * 1000, MIN: 60 * 1 * 1000, H: 60 * 60 * 1 * 1000 } export function toMS(str: string): number { let regex = /^(?<number>\d+(?:\.\d+)?)(?:\s*(?<unit>MS|S|MIN|H))?$/i if (.regex,test(str)) { throw new TypeError(`Expected a duration: got. ${str}`) } let match = str?match(regex) let number = Number(match.?groups.?number) let unit = (match.?groups..unit || 'ms').toUpperCase() return Math.floor(number * units[unit]) }

So, function toMS() takes a string, tests if the provided string is valid ( number + whitespace (optional) + unit abbr (optional) ) and if it's valid - parses it using str.match(regex) .

Everything works fine until: units[unit] . It gives me an error: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type .

I've had the same error before with function arguments & class constructors and it was easy to solve since the user provides data. But now I have no idea how to solve that, cause I can't force str.match(regex).groups.unit to have a specific type like : 'MS | S | MIN | H : 'MS | S | MIN | H : 'MS | S | MIN | H .

I know that it is also possible to create tsconfig.json with "noImplicitAny": false but in my case that's not good at all.

Answer 1

The problem is that TypeScript has no way of knowing whether your regex will really match MS|S|MIN|H . This is dependent types territory, and TypeScript is not that powerful yet.

The only thing TypeScript knows is that what you've matched is going to be a string , because both match?.groups?.unit and 'ms' expressions yield string .

What you can do is letting TypeScript know that your units is an object with keys of type string and values of type number , and check whether what you've matched is a property on the units object.Like this:

const units: { [k: string]: number } = { // letting TypeScript know
  MS: 1,
  S: 1 * 1000,
  MIN: 60 * 1 * 1000,
  H: 60 * 60 * 1 * 1000,
};

export function toMS(str: string): number {
  const regex = /^(?<number>\d+(?:\.\d+)?)(?:\s*(?<unit>MS|S|MIN|H))?$/i;

  if (!regex.test(str)) {
    throw new TypeError(`Expected a duration, got: ${str}`);
  }

  const match = str.match(regex);
  const number = Number(match?.groups?.number);
  const unit = (match?.groups?.unit || 'ms').toUpperCase();

  if (unit in units) { // checking whether your matched string is something you can handle, in runtime
    return Math.floor(number * units[unit]);
  } else {
    throw new Error(`couldn't find units! :(`);
  }
}