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 ---- ### `0` and `1` to `false` and `true` Double Negation operator works perfectly here: ```typescript return !!0; // Returns false return !!1; // Returns true ``` ---- ### `'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. ---- ### `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`)_. ---- ### 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); }); }); ``` ---- ### 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. ---- ## see Also * [JavaScript Recipe: Round to a number of decimal places stripping trailing zeros](https://bloomlab.blogspot.com/2017/06/javascript-recipe-round-to-number-of.html) * [React Recipe: Components Error Handling](https://bloomlab.blogspot.com/2018/04/react-recipe-components-error-handling.html) * [JavaScript Double Negation (!!) Trick or Trouble?](https://www.sitepoint.com/javascript-double-negation-trick-trouble/) * [What is the !! (not not) operator in JavaScript?](https://stackoverflow.com/questions/784929/what-is-the-not-not-operator-in-javascript)