TypeScript Recipe: Elegant Parse Boolean

There are many ways to convert String to Boolean or Number to Boolean, etc.
Here we will do it all in one and stop worry about form of boolean like parameters we receive from outside.

Lets' Convert 1 '1' and 'true' to true and 0 '0' 'false' null and undefined to false

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0 and 1 to false and true

Double Negation operator works perfectly here:

TypeScript

return !!0; // Returns false
return !!1; // Returns true

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'0' and '1' strings to false and true

All we need to parse string representation to number and we already know that Double Negation operator works perfectly there:

TypeScript

return !!+'0'; // Returns false
return !!+'1'; // Returns true

Note: By using the + operator you we cast the strings to int.

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null and undefined to false

Not strict null check is the shortest way to do so:

TypeScript

return value != null; // Returns false for null add undefined

Or we could use Double Negation here as well:

TypeScript

return !!null; // Returns false
return !!undefined; // Returns true

Note: Double negation is definitely faster than two strict comparisons (one for null and one for undefined).

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Convert primitive to boolean

Convert 1 '1' and 'true'to true and 0 '0' 'false' null and undefined to false:

TypeScript

function primitiveToBoolean(value?: string | number | boolean | null): boolean {
  if (value == null) {
    return false;
  }

  if (value === 'true') {
    return true;
  }

  return typeof value === 'string'
    ? !!+value   // we parse string to number first
    : !!value;
}

Or if we do not worry about typeof call it might be even more elegant:

TypeScript

function primitiveToBoolean(value?: string | number | boolean | null): boolean {
  if (value === 'true') {
    return true;
  }

  return typeof value === 'string'
    ? !!+value   // we parse string to number first
    : !!value;
}

Here is Unit test to verify how it works:

TypeScript

describe('primitiveToBoolean', () => {
    it('should be true if its 1 / "1" or "true"', () => {
      expect(primitiveToBoolean(1)).toBe(true);
      expect(primitiveToBoolean('1')).toBe(true);
      expect(primitiveToBoolean('true')).toBe(true);
      expect(primitiveToBoolean(111)).toBe(true);
    });
    it('should be false if its 0 / "0" or "false"', () => {
      expect(primitiveToBoolean(0)).toBe(false);
      expect(primitiveToBoolean('0')).toBe(false);
      expect(primitiveToBoolean('false')).toBe(false);
      expect(primitiveToBoolean('bullshit')).toBe(false);
    });
    it('should be false if its null or undefined', () => {
      expect(primitiveToBoolean(null)).toBe(false);
      expect(primitiveToBoolean(undefined)).toBe(false);
    });
    it('should pass through booleans - useful for undefined checks', () => {
      expect(primitiveToBoolean(true)).toBe(true);
      expect(primitiveToBoolean(false)).toBe(false);
    });
  });

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Conclusion

If we've cared about all boolean like parameters within the proper context, we could forget all those miss-communication and miss-configuration problems with third-parties we face in production.

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see Also

• JavaScript Recipe: Round to a number of decimal places stripping trailing zeros
• React Recipe: Components Error Handling
• JavaScript Double Negation (!!) Trick or Trouble?
• What is the !! (not not) operator in JavaScript?