Testing JavaScript Functions

Tests make behaviour explicit and help detect regressions. A unit test checks a focused unit, often a pure function. Integration tests check that parts work together; end-to-end tests exercise user-visible flows. Use the test runner adopted by your project rather than adding multiple testing tools without a need.

Consider this function:

export function calculateTotal(price, quantity) {
  if (!Number.isFinite(price) || price < 0) throw new TypeError("Invalid price");
  if (!Number.isInteger(quantity) || quantity < 0) throw new TypeError("Invalid quantity");
  return price * quantity;
}

A good test set includes a normal example, boundary values, and invalid input:

console.assert(calculateTotal(10, 3) === 30);
console.assert(calculateTotal(0, 4) === 0);

console.assert is only a quick demonstration, not a substitute for a real test runner in a larger project. Tools such as Vitest, Jest, and Node's built-in test runner support structured assertions and automated reporting.

Test behaviour, not implementation details#

Tests should express the public contract. Avoid making them depend on private local variable names or exact internal steps unless those details are part of the requirement.

Practice#

Write tests for an array utility: empty array, one item, repeated values, and invalid input. Explain which test would catch a bug you might otherwise miss.