Typeof and null

Problem

What becomes confusing, fragile, or impossible without understanding typeof and null? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.

JavaScript began as a small browser scripting language, then accumulated object, functional, metaprogramming, module, and runtime capabilities while preserving early coercion rules.

Concept explanation

Typeof and null belongs to “Values, types, and coercion”. JavaScript values have precise types, coercions, and comparison algorithms. This lesson turns typeof and null into a working part of weekly fixture.

For typeof and null, trace concrete input, state transition, output, and failure through a JavaScript semantic rule observable through values, property operations, scope, calls, or protocol steps.

Execution evaluates expressions against lexical environments. Objects are property tables linked through prototypes; functions are callable objects with closures. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Example bank

Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.

Worked example

Start from supplied lab.mjs. Focus: Build typeof and null around this exact fixture: Evaluate a table of typeof, Boolean, Number, String, ==, ===, and Object.is cases.

1. Run: node lab.mjs
2. Save baseline evidence. Use property descriptors, scope panels, call stacks, console probes, and deliberately tiny cases.
3. Boundary case: Add typeof and null support for weekly boundary case: Include null, undefined, NaN, -0, BigInt, empty string, and empty array.
4. Failure case: Make typeof and null surface—not hide—this failure: Mix BigInt and Number arithmetic and capture thrown error.

RESULT
typeof and null handles baseline and boundary while preserving this observable result: Every output is predicted and explained by named conversion/comparison algorithm. Starter-level baseline: Two labeled records print in stable order; each exposes value, runtime type, and string representation.

Starter material

Create lab.mjs, paste this exact content, then run the command below.

const cases = [
  { label: "baseline", input: 0 },
  { label: "boundary", input: 1 },
];

function observe(value) {
  // Change only this function while investigating typeof and null.
  return { value, type: typeof value, text: String(value) };
}

for (const test of cases) console.log(test.label, observe(test.input));

STOP / CLEANUPScripts exit by themselves; no cleanup command required.

Independent exercise

Add one real case: typeof and null

1. Create second case from blank file: Add typeof and null support for weekly boundary case: Include null, undefined, NaN, -0, BigInt, empty string, and empty array.
2. Then create controlled failure: Make typeof and null surface—not hide—this failure: Mix BigInt and Number arithmetic and capture thrown error.
3. Use Chromium Sources · console · hx · Node REPL to prove behavior, then repair controlled failure.
4. Compare result against supplied acceptance checks and reference approach before marking complete.

Expected result

• typeof and null handles baseline and boundary while preserving this observable result: Every output is predicted and explained by named conversion/comparison algorithm.
• Two labeled records print in stable order; each exposes value, runtime type, and string representation.
• Controlled typeof and null failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.