Typescript conversion to boolean

You can use double exclamation sign trick which Typescript does allow and which works fine in JavaScript:

foo(!!xxx);

Alternatively, cast it to any

foo(<any>xxx);

Here is my solution for "typescript": "^3.3.3":

function toBool(a: any) {
  return Boolean(a).valueOf();
}

export { toBool };

unit test:

import { toBool } from '../../utils/bool';

describe('bool', () => {
  describe('#toBool', () => {
    it('should convert string to boolean', () => {
      expect(toBool('false')).toBeTruthy();
      expect(toBool('')).toBeFalsy();
    });
    it('should convert number to boolean', () => {
      expect(toBool(1)).toBeTruthy();
      expect(toBool(0)).toBeFalsy();
      expect(toBool(-1)).toBeTruthy();
      expect(toBool(Infinity)).toBeTruthy();
      expect(toBool(-Infinity)).toBeTruthy();
    });
    it('should convert null to boolean', () => {
      expect(toBool(null)).toBeFalsy();
    });
    it('should convert undefined to boolean', () => {
      expect(toBool(undefined)).toBeFalsy();
    });
    it('should convert NaN to boolean', () => {
      expect(toBool(NaN)).toBeFalsy();
    });
    it('should convert object to boolean', () => {
      expect(toBool({})).toBeTruthy();
    });
    it('should convert array to boolean', () => {
      expect(toBool([])).toBeTruthy();
    });
  });
});

unit test results:

 PASS  src/__tests__/utils/bool.spec.ts
  bool
    #toBool
      ✓ should convert string to boolean (3ms)
      ✓ should convert number to boolean (1ms)
      ✓ should convert null to boolean (1ms)
      ✓ should convert undefined to boolean
      ✓ should convert NaN to boolean (1ms)
      ✓ should convert object to boolean (1ms)
      ✓ should convert array to boolean

Test Suites: 1 passed, 1 total
Tests:       7 passed, 7 total
Snapshots:   0 total
Time:        3.79s, estimated 4s

While you can't cast a number directly to a boolean, you can cast it to the wrapper Boolean class and immediately unwrap it. For example:

foo(<boolean><Boolean>xxx);

While clumsy, it avoids the type erasure of casting to <any>. It's also arguably less obscure & more readable than the !! approach (certainly so in the transpiled js code).

With TypeScript 2.0.2 you can do this:

type Falsey = '' | 0 | false | null | undefined;

function eatFruit(fruit: string | Falsey) { 
  if (fruit) {
    alert(`Ate ${fruit}`);
  } else {
    alert('No fruit to eat!');
  }
}

const fruits = ['apple', 'banana', 'pear'];
eatFruit(fruits[0]); // alerts 'Ate apple'
eatFruit(fruits[1]); // alerts 'Ate banana'
eatFruit(fruits[2]); // alerts 'Ate pear'
eatFruit(fruits[3]); // alerts 'No fruit to eat!'

const bestBeforeDay = 12;
let day = 11;
eatFruit(day < bestBeforeDay && 'peach'); // alerts 'Ate peach'
day += 1;
eatFruit(day < bestBeforeDay && 'peach'); // alerts 'No fruit to eat!'

let numMangos = 1;
eatFruit(numMangos && 'mango'); // alerts 'Ate Mango'
numMangos -= 1;
eatFruit(numMangos && 'mango'); // alerts 'No fruit to eat!'

Typescript conversion to boolean

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