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MediumPro challengePythonJavaScriptTypeScript

Implement expect.extend()

TestingUtilities

Jest lets you register custom matchers with expect.extend({ ... }), then use them like built-ins — including through .not.

The starter code provides the custom matchers in MATCHER_LIBRARY. Each takes (received, ...args) and returns { pass, message }, where message describes the failure.

solve(ops) runs a list of operations in order, starting from an empty registry, and returns their results:

OpResult
['extend', names]null — registers the named matchers from MATCHER_LIBRARY
['match', name, received, ...args]{ pass, message } — runs a registered matcher
['list']registered matcher names, in registration order, without duplicates

Rules for match:

  • message is '' whenever the result passes.
  • A name prefixed with not. inverts the matcher. If the inverted check fails, the message is 'toBeEven unexpectedly passed for 4'.
  • A name that was never registered → { pass: false, message: 'Unknown matcher: toBeOdd' }.
solve([
  ['extend', ['toBeEven']],
  ['match', 'toBeEven', 3],
  ['match', 'not.toBeEven', 3],
])
// → [null, { pass: false, message: 'expected 3 to be even' }, { pass: true, message: '' }]

Sample tests

Test #1a registered matcher passes
Input: [[["extend",["toBeEven"]],["match","toBeEven",4]]]
Output: [null,{"pass":true,"message":""}]
Test #2a registered matcher fails with its message
Input: [[["extend",["toBeEven"]],["match","toBeEven",3]]]
Output: [null,{"pass":false,"message":"expected 3 to be even"}]
Test #3list returns registered names in order
Input: [[["extend",["toBeEven","toBePositive"]],["list"]]]
Output: [null,["toBeEven","toBePositive"]]
Test #4unknown matcher
Input: [[["match","toBeOdd",3]]]
Output: [{"pass":false,"message":"Unknown matcher: toBeOdd"}]
Test #5.not inverts the matcher
Input: [[["extend",["toBeEven"]],["match","not.toBeEven",3],["match","not.toBeEven",4]]]
Output: [null,{"pass":true,"message":""},{"pass":false,"message":"toBeEven unexpectedly passed for 4"}]
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