Jest expect.any() not working as expected : <p>So when testing one of my <strong>reducers</strong> in a Preact(not much different to React while testing with JEST) based project, I got bumped into this issue:</p>

<p>Following output comes up when running jest test - </p>

<pre><code>● should setup 

    expect(received).toEqual(expected)

    Expected value to equal:
      {"ID": Any&lt;String&gt;, "active": true, "data": Any&lt;Array&gt;}
    Received:
      {"ID": "BysnEuMlm", "active": true, "data": [{"ID": "Hy7wMAz1lm", "code": "dat.fle", "label": "CRES/datum14.cdata", "name": "File", "status": "READY", "value": {"format": "cdata", "name": "datum14.cdata", "path": "CRES"}}, {"ID": "rkB7RMkeX", "code": "prp.kcv", "label": "3 folds", "name": "k-Fold Cross-Validation", "status": "READY", "value": "3"}, {"ID": "ByCmRfygQ", "code": "ats", "label": undefined, "name": " Best First +  Cfs Subset Eval", "status": "READY", "value": {"evaluator": {"name": "CfsSubsetEval"}, "search": {"name": "BestFirst", "options": ["-D", "1", "-N", "5"]}, "use": true}}, {"ID": "HkmVAM1l7", "code": "lrn", "label": undefined, "name": "Naive Bayes", "status": "READY", "value": {"label": "Naive Bayes", "name": "bayes.NaiveBayes", "use": true}}], "output": {"format": "pipeline", "name": "jestReact.cpipe", "path": "/home/rupav/opensource/candis/CRES"}}

    Difference:

    - Expected
    + Received

      Object {
    -   "ID": Any&lt;String&gt;,
    +   "ID": "BysnEuMlm",
        "active": true,
    -   "data": Any&lt;Array&gt;,
    +   "data": Array [
    +     Object {
    +       "ID": "Hy7wMAz1lm",
    +       "code": "dat.fle",
    +       "label": "CRES/datum14.cdata",
    +       "name": "File",
    +       "status": "READY",
    +       "value": Object {
    +         "format": "cdata",
    +         "name": "datum14.cdata",
    +         "path": "CRES",
    +       },
    +     },
    +     Object {
    +       "ID": "rkB7RMkeX",
    +       "code": "prp.kcv",
    +       "label": "3 folds",
    +       "name": "k-Fold Cross-Validation",
    +       "status": "READY",
    +       "value": "3",
    +     },
    +     Object {
    +       "ID": "ByCmRfygQ",
    +       "code": "ats",
    +       "label": undefined,
    +       "name": " Best First +  Cfs Subset Eval",
    +       "status": "READY",
    +       "value": Object {
    +         "evaluator": Object {
    +           "name": "CfsSubsetEval",
    +         },
    +         "search": Object {
    +           "name": "BestFirst",
    +           "options": Array [
    +             "-D",
    +             "1",
    +             "-N",
    +             "5",
    +           ],
    +         },
    +         "use": true,
    +       },
    +     },
    +     Object {
    +       "ID": "HkmVAM1l7",
    +       "code": "lrn",
    +       "label": undefined,
    +       "name": "Naive Bayes",
    +       "status": "READY",
    +       "value": Object {
    +         "label": "Naive Bayes",
    +         "name": "bayes.NaiveBayes",
    +         "use": true,
    +       },
    +     },
    +   ],
    +   "output": Object {
    +     "format": "pipeline",
    +     "name": "jestReact.cpipe",
    +     "path": "/home/rupav/opensource/candis/CRES",
    +   },
      }
</code></pre>

<p>Following is the test case:</p>

<pre><code>test('should setup ', () =&gt; {
    const state = documentProcessor(
        undefined,
        {
            type: ActionType.Asynchronous.READ_SUCCESS,
            payload: dokuments.active
    })
    // expect(state.active.ID).toEqual(expect.any(String)) - Test case passes iff I run this test with this command only.
    expect(state.active).toEqual({
        data: expect.any(Array),
        active: true,
        ID: expect.any(String),
    })

})
</code></pre>

<p>Since state gets changed while calling that reducer, I needed to use <code>expect.any</code> function, but as per the output, although types are same, test is not getting passed.
Rather in expected its showing up <code>Any&lt;String&gt;</code>.</p>
