How to test function with React Testing Library-CodePudding

I want to test this function with React Testing Library this error

export default function capitalizeFirstLetter(string: string) 
{
return string.charAt(0).toUpperCase()   string.slice(1); 
}

but I got

TypeError: Cannot read properties of undefined (reading 'charAt')

CodePudding user response：

One problem I see is that your function will fail when empty/null/undefined value is passed and you need a guard for that. And that is why you need a test for.

As an example, I'd test this function as follows

// Should be in another file
function capitalizeFirstLetter(string: string) {
  if (!string) return '';

  return string.charAt(0).toUpperCase()   string.slice(1);
}

describe('capitalizeFirstLetter', () => {
  it('should return "Hello"', () => {
    expect(capitalizeFirstLetter('hello')).toBe('Hello');
  });

  it('should return "Hi"', () => {
    expect(capitalizeFirstLetter('Hi')).toBe('Hi');
  });

  it('should not fail when undefined/null or empty value is passed', () => {
    expect(capitalizeFirstLetter(undefined)).toBe('');
    expect(capitalizeFirstLetter(null)).toBe('');
    expect(capitalizeFirstLetter('')).toBe('');
  });
});

Or you can use describe.each with the list of payloads.