when passed as parameter to

• <any> [when] passed as parameter to [constructor] <function> <assertion?>
• <any> [when] passed as parameter to [async] <function> <assertion?>

Apply a function to the subject, then delegate the return value to another assertion.

function increment(n) {
  return n + 1;
}
 
expect(1, 'when passed as parameter to', increment, 'to equal', 2);

In case of a failing expectation you get the following output:

expect(1, 'when passed as parameter to', increment, 'to equal', 3);

expected 1
when passed as parameter to function increment(n) { return n + 1; } to equal 3
  
expected 2 to equal 3

This assertion delegates to the when passed as parameters to assertion and also supports the async and constructor flags.

If you don't provide an assertion to delegate to, the return value will be provided as the fulfillment value of the promise:

return expect(10, 'passed as parameter to', increment).then(function (result) {
  expect(result, 'to equal', 11);
});