1  import * as fc from 'fast-check';
 2  
 3  ///*bug*/ const contains = (pattern, text) => text.substr(1).indexOf(pattern) !== -1;
 4  const contains = (pattern, text) => text.indexOf(pattern) !== -1;
 5  
 6  test('The concatenation of a, b and c always contains b', () => {
 7    fc.assert(
 8      fc.property(fc.string(), fc.string(), fc.string(), (a, b, c) => {
 9        return contains(b, a + b + c);
10      })
11    );
12  });
13  test('Also works with expect', () => {
14    fc.assert(
15      fc.property(fc.string(), fc.string(), fc.string(), (a, b, c) => {
16        expect(contains(b, a + b + c)).toBe(true);
17      })
18    );
19  });