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 parameters 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.