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A Beginner's Guide to Testing Functions

1. What Is a Function?

Input  ──▶  Function  ──▶  Output

A function

  • Takes input (via arguments)
  • Does some work
  • Produces one output (via a return value)

Example:

sum(2, 3) → 5

Important idea: the same input should produce the same output.

2. Testing Means Predicting

Testing means:

If I give this input, what output should I get?

3. Choosing Good Test Values

Step 1: Determining the space of possible inputs

Ask:

  • What type of value is expected?
  • What values make sense?
    • If they are numbers:
      • Are they integers or floating-point numbers?
      • What is their range?
    • If they are strings:
      • What are their length and patterns?
  • What values would not make sense?

Step 2: Choosing Good Test Values

Normal Cases

These confirm that the function works in normal use.

  • What does a typical, ordinary input look like?
  • Are there multiple ordinary groups of inputs? e.g. for an age checking function, maybe there are "adults" and "children" as expected ordinary groups of inputs.

Boundary Cases

Test values exactly at, just inside, and just outside defined ranges. These values are where logic breaks most often.

Consider All Outcomes

Every outcome must be reached by at least one test.

  • How many different results can this function produce?
  • Have I tested a value that leads to each one?

Crossing the Edges and Invalid Values

This tests how the function behaves when assumptions are violated.

  • What happens when input is outside of the expected range?
  • What happens when input is not of the expected type?
  • What happens when input is not in the expected format?

4. How to Test

1. Using console.assert()

  // Report a failure only when the first argument is false
  console.assert( sum(4, 6) === 10, "Expected 4 + 6 to equal 10" );

It is simpler than using if-else and requires no setup.

2. Jest Testing Framework

  test("Should correctly return the sum of two positive numbers", () => {
    expect( sum(4, 6) ).toEqual(10);
    ... // Can test multiple samples 
  });

Jest supports many useful functions for testing but requires additional setup.