Description

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Advanced · 12 min

Test small functions

Check normal cases, boundaries, and rejected inputs with explicit assertions.

Make a failure visible

A small assertion can throw when the actual result differs from the expected one. A test that never checks a result can pass while the function is wrong.

function assertEqual(actual, expected) { if (actual !== expected) throw new Error("Mismatch"); }
function total(price, quantity) { return price * quantity; }
assertEqual(total(4, 3), 12);
assertEqual(total(4, 0), 0);
console.log("2 checks passed");

Output

2 checks passed

Test rejected input

Check that invalid input actually throws. Keep the test failure outside the try block so it is not accidentally caught as the expected error.

function quantity(value) { if (value < 0) throw new Error("Negative"); return value; }
let threw = false;
try { quantity(-1); } catch { threw = true; }
if (!threw) throw new Error("Expected rejection");
console.log("Invalid input rejected");

Output

Invalid input rejected

Put it into practice

  1. Predict each result before running the example.
  2. Complete the coding task, then explain why your solution works.

Try it yourself

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Fix the implementation

Make total pass both the regular case and the zero-quantity case. Keep the checks unchanged.

Practice