Add Testing Guide

Author: cjyuan

Sprint-3/testing-guide.md

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+# A Beginner's Guide to Testing Functions
+
+## 1. What Is a Function?
+
+```
+Input  ──▶  Function  ──▶  Output
+```
+
+A ***pure function***
+- Takes **input** (via **arguments**)
+- Does some work
+- Yields **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?
+  
+#### 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()`
+
+```javascript
+  // 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
+
+```javascript
+  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.