flow-union-refinement

flow-union-refinement.js

// @flow

// https://flow.org/en/docs/types/unions/#toc-unions-refinements
// flow can refine on the union. 
// and knows when the code is operating on a number here for eg.

// works
function toStringPrimitivesGood(value: number | boolean | string): string { // Error!
  if (typeof value === 'number') {
  	return value.toLocaleString([], { maximumSignificantDigits: 3 }); // Works!
  } else if (typeof value === 'boolean') {
    return String(value);
  } else {
  	return "foo";
  }
}

// fails, with an abstruse error. but still!
function toStringPrimitivesBad(value: number | boolean | string): string { // Error!
  if (typeof value === 'number') {
    return String(value);
  } else if (typeof value === 'boolean') {
  	return value.toLocaleString([], { maximumSignificantDigits: 3 }); // Works!
  } else {
  	return "foo";
  }
}

toStringPrimitivesGood(12);
toStringPrimitivesBad(true);

//////////////////////////////////////
// Error: src/main.js:23
//  23:   	return value.toLocaleString([], { maximumSignificantDigits: 3 }); // Works!
//        	                            ^^ unused function argument
//    56:     toLocaleString(): string;
//            ^^^^^^^^^^^^^^^^^^^^^^^^ function type expects no arguments. See lib: /private/tmp/flow/flowlib_32d44c1c/core.js:56
//
// Error: src/main.js:23
//  23:   	return value.toLocaleString([], { maximumSignificantDigits: 3 }); // Works!
//        	                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused function argument
//    56:     toLocaleString(): string;
//            ^^^^^^^^^^^^^^^^^^^^^^^^ function type expects no arguments. See lib: /private/tmp/flow/flowlib_32d44c1c/core.js:56


// Found 2 errors