Every codebase accumulates legacy code—patterns that made sense once but now slow down development. Refactoring this code is essential for long-term health but risky without proper testing. One wrong change can break functionality that's worked for years.
AI assistance changes the refactoring equation. By generating characterization tests, suggesting safe transformations, and explaining unfamiliar code, AI makes refactoring faster and safer than ever before.
This guide covers AI-assisted refactoring techniques that modernize code while preserving behavior.
The Refactoring Challenge#
Refactoring legacy code is difficult because:
Missing Tests: Legacy code often lacks tests, making changes risky.
Unclear Intent: Why was this code written this way? Without context, "improvements" may break things.
Hidden Dependencies: Legacy code has unexpected connections that aren't immediately visible.
Fear of Change: Past experiences with breaking changes make developers hesitant to refactor.
AI addresses each of these challenges by providing context, generating safety nets, and identifying dependencies.
Step 1: Understanding Before Changing#
Before refactoring, understand what the code does. AI excels at explaining legacy code.
Code Archaeology#
1"Explain this legacy function. It's been in production for years
2and works, but nobody knows exactly what it does:
3
4```javascript
5function processOrder(o, f, x) {
6 if (!o || !o.i) return null;
7 var r = {t: 0, d: 0};
8 for (var i = 0; i < o.i.length; i++) {
9 var it = o.i[i];
10 if (it.a && it.q > 0) {
11 var p = it.p * it.q;
12 if (x && x.d) p = p * (1 - x.d/100);
13 r.t += p;
14 if (it.s) r.d += it.p * it.q * 0.1;
15 }
16 }
17 if (f) r.t += f;
18 return r;
19}Explain:
- What this function does
- What each parameter means
- The business logic it implements
- Edge cases it handles"
AI explains:
```markdown
"**Function Analysis: processOrder**
**Purpose:** Calculate order total with discounts and shipping
**Parameters:**
- `o` (order): Order object containing items array (`i`)
- `f` (fees): Additional fees (likely shipping)
- `x` (context): Discount context with `d` property (discount percent)
**Business Logic:**
1. Validates order exists and has items
2. For each item:
- Skips inactive items (`it.a` = active flag)
- Skips zero-quantity items
- Calculates line total: price × quantity
- Applies percentage discount if provided
- If item is special (`it.s`), adds 10% to `d` (likely duty/tax)
3. Adds additional fees (shipping)
4. Returns object with `t` (total) and `d` (duty/tax)
**Edge Cases Handled:**
- Null/undefined order: returns null
- Missing items array: returns null
- Inactive items: skipped
- Zero quantity: skipped
- No discount context: no discount applied
**Variable Name Mapping:**
- `o.i` → order.items
- `it.a` → item.active
- `it.q` → item.quantity
- `it.p` → item.price
- `it.s` → item.special (duty applicable)
- `x.d` → context.discountPercent
- `r.t` → result.total
- `r.d` → result.duty"
Dependency Mapping#
Understand what depends on this code:
1"Find all usages of processOrder in the codebase.
2For each usage, identify:
31. What calls it
42. What parameters are passed
53. How the result is used
6
7This helps me understand what might break if I change it."Step 2: Creating Safety Nets#
Never refactor without tests. AI generates characterization tests that capture current behavior.
Generating Characterization Tests#
1"Generate characterization tests for processOrder that document
2its current behavior. Include:
3- Normal cases
4- Edge cases (null, empty, missing properties)
5- Various discount scenarios
6- Special items handling
7
8These tests should pass with the current implementation
9and will catch any behavior changes during refactoring."AI generates tests:
1describe('processOrder - characterization tests', () => {
2 describe('input validation', () => {
3 it('returns null for null order', () => {
4 expect(processOrder(null, 0, null)).toBeNull();
5 });
6
7 it('returns null for undefined order', () => {
8 expect(processOrder(undefined, 0, null)).toBeNull();
9 });
10
11 it('returns null for order without items', () => {
12 expect(processOrder({}, 0, null)).toBeNull();
13 });
14
15 it('returns null for order with undefined items', () => {
16 expect(processOrder({ i: undefined }, 0, null)).toBeNull();
17 });
18 });
19
20 describe('empty order', () => {
21 it('returns zero totals for empty items array', () => {
22 const result = processOrder({ i: [] }, 0, null);
23 expect(result).toEqual({ t: 0, d: 0 });
24 });
25 });
26
27 describe('basic calculation', () => {
28 it('calculates total for single active item', () => {
29 const order = {
30 i: [{ a: true, q: 2, p: 10 }]
31 };
32 const result = processOrder(order, 0, null);
33 expect(result.t).toBe(20);
34 expect(result.d).toBe(0);
35 });
36
37 it('calculates total for multiple items', () => {
38 const order = {
39 i: [
40 { a: true, q: 2, p: 10 },
41 { a: true, q: 1, p: 30 }
42 ]
43 };
44 const result = processOrder(order, 0, null);
45 expect(result.t).toBe(50);
46 });
47 });
48
49 describe('inactive and zero-quantity items', () => {
50 it('skips inactive items', () => {
51 const order = {
52 i: [
53 { a: false, q: 2, p: 100 },
54 { a: true, q: 1, p: 10 }
55 ]
56 };
57 const result = processOrder(order, 0, null);
58 expect(result.t).toBe(10);
59 });
60
61 it('skips zero-quantity items', () => {
62 const order = {
63 i: [
64 { a: true, q: 0, p: 100 },
65 { a: true, q: 1, p: 10 }
66 ]
67 };
68 const result = processOrder(order, 0, null);
69 expect(result.t).toBe(10);
70 });
71 });
72
73 describe('discounts', () => {
74 it('applies percentage discount when context provided', () => {
75 const order = { i: [{ a: true, q: 1, p: 100 }] };
76 const result = processOrder(order, 0, { d: 10 });
77 expect(result.t).toBe(90); // 10% off
78 });
79
80 it('no discount when context is null', () => {
81 const order = { i: [{ a: true, q: 1, p: 100 }] };
82 const result = processOrder(order, 0, null);
83 expect(result.t).toBe(100);
84 });
85 });
86
87 describe('special items (duty)', () => {
88 it('adds 10% duty for special items', () => {
89 const order = { i: [{ a: true, q: 1, p: 100, s: true }] };
90 const result = processOrder(order, 0, null);
91 expect(result.t).toBe(100);
92 expect(result.d).toBe(10);
93 });
94
95 it('no duty for non-special items', () => {
96 const order = { i: [{ a: true, q: 1, p: 100, s: false }] };
97 const result = processOrder(order, 0, null);
98 expect(result.d).toBe(0);
99 });
100 });
101
102 describe('fees', () => {
103 it('adds fees to total', () => {
104 const order = { i: [{ a: true, q: 1, p: 100 }] };
105 const result = processOrder(order, 15, null);
106 expect(result.t).toBe(115);
107 });
108 });
109});Run these tests to confirm they pass with the current implementation. They become your safety net during refactoring.
Step 3: Incremental Refactoring#
Refactor in small steps, running tests after each change.
Rename for Clarity#
First pass: meaningful names.
1"Refactor this function with clear names, keeping exact behavior:
2- Don't change logic
3- Just rename variables and parameters
4- Add TypeScript types
5
6Current function:
7[paste function]
8
9Characterization tests must still pass after changes."AI produces renamed version:
1interface OrderItem {
2 active: boolean;
3 quantity: number;
4 price: number;
5 isSpecial?: boolean;
6}
7
8interface Order {
9 items: OrderItem[];
10}
11
12interface DiscountContext {
13 discountPercent: number;
14}
15
16interface OrderTotal {
17 total: number;
18 duty: number;
19}
20
21function processOrder(
22 order: Order | null | undefined,
23 shippingFee: number,
24 discountContext: DiscountContext | null
25): OrderTotal | null {
26 if (!order || !order.items) return null;
27
28 let total = 0;
29 let duty = 0;
30
31 for (let i = 0; i < order.items.length; i++) {
32 const item = order.items[i];
33
34 if (item.active && item.quantity > 0) {
35 let lineTotal = item.price * item.quantity;
36
37 if (discountContext && discountContext.discountPercent) {
38 lineTotal = lineTotal * (1 - discountContext.discountPercent / 100);
39 }
40
41 total += lineTotal;
42
43 if (item.isSpecial) {
44 duty += item.price * item.quantity * 0.1;
45 }
46 }
47 }
48
49 if (shippingFee) {
50 total += shippingFee;
51 }
52
53 return { total, duty };
54}Run tests. They should still pass.
Extract Methods#
Second pass: decompose into smaller functions.
1"Extract helper functions from processOrder:
2- calculateLineTotal(item, discountContext)
3- calculateDuty(item)
4- isItemValid(item)
5
6Keep public interface identical. Tests must still pass."Modernize Patterns#
Third pass: use modern JavaScript features.
1"Modernize this function using:
2- Array methods (reduce, filter) instead of for loops
3- Optional chaining where appropriate
4- Early returns for validation
5- Destructuring
6
7Maintain exact same behavior. Tests must pass."AI produces modern version:
1function processOrder(
2 order: Order | null | undefined,
3 shippingFee: number = 0,
4 discountContext: DiscountContext | null = null
5): OrderTotal | null {
6 if (!order?.items) return null;
7
8 const activeItems = order.items.filter(
9 item => item.active && item.quantity > 0
10 );
11
12 const { total, duty } = activeItems.reduce(
13 (acc, item) => {
14 const lineTotal = calculateLineTotal(item, discountContext);
15 const itemDuty = item.isSpecial ? calculateDuty(item) : 0;
16
17 return {
18 total: acc.total + lineTotal,
19 duty: acc.duty + itemDuty
20 };
21 },
22 { total: 0, duty: 0 }
23 );
24
25 return {
26 total: total + shippingFee,
27 duty
28 };
29}
30
31function calculateLineTotal(
32 item: OrderItem,
33 discountContext: DiscountContext | null
34): number {
35 const baseTotal = item.price * item.quantity;
36 const discountMultiplier = discountContext?.discountPercent
37 ? 1 - discountContext.discountPercent / 100
38 : 1;
39 return baseTotal * discountMultiplier;
40}
41
42function calculateDuty(item: OrderItem): number {
43 const DUTY_RATE = 0.1;
44 return item.price * item.quantity * DUTY_RATE;
45}Run tests after each transformation. Any failures indicate behavior changes.
Step 4: Handling Side Effects#
Legacy code often has hidden side effects. AI helps identify them.
1"Analyze this function for side effects:
2- Global variable modifications
3- DOM manipulation
4- Network calls
5- Console logging
6- Date/random dependencies
7
8[paste code]
9
10Identify side effects that could cause issues during refactoring."For side effects, isolate them before refactoring:
1"Help me extract the side effects from this function
2into injected dependencies:
3- Replace Date.now() with injectable clock
4- Replace console.log with injectable logger
5- Replace fetch() with injectable HTTP client
6
7This lets us test the logic independently."Step 5: Verifying Behavior Preservation#
After refactoring, verify nothing changed.
Snapshot Testing#
"Generate snapshot tests that capture the output of
processOrder for a comprehensive set of inputs.
These snapshots let us verify the refactored version
produces identical output."Property-Based Testing#
1"Generate property-based tests for processOrder that verify:
21. Output total is never negative
32. Duty is always <= total
43. With zero discount, result equals sum of line totals
54. Inactive items never contribute to total
6
7Use a property testing library like fast-check."Common Refactoring Patterns#
Pattern: Extract Class#
"This function has grown to 300 lines with multiple
responsibilities. Identify cohesive groups of functionality
and suggest how to extract them into classes:
[paste function]"Pattern: Replace Conditional with Polymorphism#
"This switch statement handles different order types.
Show how to refactor using polymorphism:
[paste switch statement]"Pattern: Introduce Parameter Object#
"This function has 8 parameters. Suggest how to group
them into meaningful parameter objects:
[paste function signature]"Measuring Refactoring Success#
Track these metrics:
Code Quality:
- Cyclomatic complexity reduction
- Test coverage increase
- Code duplication decrease
Developer Experience:
- Time to understand code
- Time to make changes
- Confidence in changes
Risk Reduction:
- Tests added
- Dependencies clarified
- Documentation generated
Conclusion#
AI-assisted refactoring transforms a risky, tedious process into a systematic, safe one. By generating characterization tests, explaining code intent, and suggesting transformations, AI provides the confidence to modernize even the gnarliest legacy code.
The key is incremental transformation: understand first, test thoroughly, change gradually, verify constantly. With AI assistance, you can modernize legacy codebases without the fear of breaking everything.
Ready to tackle your legacy code? Try Bootspring free and access intelligent code analysis, refactoring assistance, and testing support that makes legacy modernization safe and systematic.