when passed as parameters to

• <array-like> [when] passed as parameters to [constructor] <function> <assertion?>
• <array-like> [when] passed as parameters to [async] <function> <assertion?>

Apply a function to the subject array (or array-like object), then delegate the return value to another assertion.

function add(a, b) {
  return a + b;
}
 
expect([1, 2], 'when passed as parameters to', add, 'to equal', 3);

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

expect([1, 2], 'when passed as parameters to', add, 'to equal', 9);

expected [ 1, 2 ]
when passed as parameters to function add(a, b) { return a + b; } to equal 9
  
expected 3 to equal 9

To call an node-style async function, use the async flag to automatically add a callback to the parameter list and do further assertions on the value it passes to the callback.

function delayedAdd(a, b, cb) {
  setTimeout(function () {
    cb(null, a + b);
  }, 1);
}
 
return expect(
  [1, 2],
  'when passed as parameters to async',
  delayedAdd,
  'to equal',
  3
);

The assertion will fail if the async function passes an error to the callback.

You can also use the constructor flag to create an instance of a constructor function (using the new operator):

function Foo(value) {
  this.value = value;
}
 
expect([123], 'when passed as parameters to constructor', Foo, 'to be a', Foo);

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([1, 3], 'passed as parameters to', add).then(function (result) {
  expect(result, 'to equal', 4);
});