Report an unassigned variable — practice exercise

What you'll be asked to do

1. 1.Report an unassigned variableexercise · easy · 10 min

   Report the value of a variable that was declared but not set.

   Passes when: #out should show undefined.

2. 2.Report a missing propertyexercise · easy · 10 min · practice plan

   Read a property that does not exist and report it.

   Passes when: #out should show undefined.

3. 3.Empty a value on purposeexercise · easy · 10 min · practice plan

   Set a variable to null and report it.

   Passes when: #out should show null.

4. 4.Check for eitherexercise · easy · 10 min · practice plan

   Use one check that catches both null and undefined.

   Passes when: #out should show nothing there.

5. 5.See what JSON keepsexercise · medium · 10 min · practice plan

   Stringify an object with a null and an undefined property.

   Passes when: #out should show {"a":null}.

6. 6.Fix the check that misses undefinedbug fix · easy · 8 min

   Strict equality against null does not catch undefined.

   Passes when: #out should show nothing there.

7. 7.Fix the zero rejected as missingbug fix · easy · 8 min · practice plan

   A truthy check treats a valid zero as absent.

   Passes when: #out should show got 0.

8. 8.Fix typeof used to detect nullbug fix · easy · 8 min · practice plan

   typeof null is object, so it cannot identify null.

   Passes when: #out should show empty on purpose.

9. 9.Fix the property set to undefinedbug fix · easy · 8 min · practice plan

   Setting a property to undefined leaves it in the object but drops it from JSON.

   Passes when: #out should show the JSON with a set to null.

10. 10.Fix the wrong emptiness assignedbug fix · easy · 8 min · practice plan

    A deliberate emptying should use null, not the text undefined.

    Passes when: #out should show really null.

Between them these tasks cover null, undefined and Absence checks.

Looking for a different chapter? See every practice set, or switch to the interview theory questions.