TypeScript中函数的类型防护

Question

I've defined a type Filter in TypeScript, which can be string , RegExp or Predicator , which is a function. Type defines are listed below.

export type Predicator = (input: string) => boolean;
export type SingleFilter = string | RegExp | Predicator;

There's a function taking SingleFilter as input. However, the type guards seems to be not working with Predicator , as long as RegExp presists. But the issue is gone if the type is string | Predicator string | Predicator only.

function singlePredicator(value: any) {
  if (typeof value === 'string') {
    // do something for string
  }
  else if (value instanceof RegExp) {
    // do something
  }
  else {
    // do something for function
    // Error TS2349: Cannot invoke an expression whose type lacks a call signature.
    value(foo);
  }
}

Is there any solution to make it work in my case? Thanks. I'm using tsc 1.5.3

Answer 1

The first change in the example below is that I have typed the value paramater as a SingleFilter rather than `any.

The second change is the type assertion in the last else statement to hint to the compiler that value is a Predicator .

export type Predicator = (input: string) => boolean;
export type SingleFilter = string | RegExp | Predicator;

function singlePredicator(value: SingleFilter) {
  if (typeof value === 'string') {
    // do something for string
    return value.charAt(0);
  } else if (value instanceof RegExp) {
    // do something
    value.test("Something");
  } else {
    // do something for function
    var foo = "SomeFoo";
    value(foo);
  }
}