Prototype inheritance in Javascript - Step-by-Step Execution

Concept Flow - Prototype inheritance

Create object A

↓

Assign prototype of A to object B

↓

Access property on B

↓

Check if property exists on B

↓

Return property

↓

Return property from A if found

↓

If not found, return undefined

When accessing a property on an object, JavaScript looks first on the object itself. If not found, it looks up the prototype chain until it finds the property or returns undefined.

Execution Sample

Javascript

const parent = {a: 1};
const child = Object.create(parent);
child.b = 2;
console.log(child.a);
console.log(child.b);

This code creates a child object inheriting from parent and accesses properties from both child and parent.

Execution Table

Step	Action	Object Accessed	Property Looked Up	Found On	Value Returned
1	Create parent object	parent	-	-	-
2	Create child with prototype parent	child	-	-	-
3	Assign property b=2 on child	child	b	child	2
4	Access child.a	child	a	parent	1
5	Access child.b	child	b	child	2

💡 Property lookup stops when property is found or prototype chain ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
parent	{a:1}	{a:1}	{a:1}	{a:1}
child	undefined	prototype -> parent	prototype -> parent, b=2	prototype -> parent, b=2

Key Moments - 3 Insights

Why does child.a return 1 even though 'a' is not directly on child?

Because child inherits from parent via prototype. The lookup finds 'a' on parent as shown in execution_table step 4.

What happens if a property is not found on child or parent?

The lookup continues up the prototype chain until no prototype exists, then returns undefined. This is implied in the exit_note.

Does assigning child.b = 2 affect parent?

No, child.b is a direct property on child only, parent remains unchanged as shown in variable_tracker after step 3.

Visual Quiz - 3 Questions

Look at the execution_table step 4, where is property 'a' found when accessed on child?

A On parent

B On child

C Undefined

D On global object

💡 Hint

Check the 'Found On' column in step 4 of execution_table.

At which step is property 'b' assigned directly to child?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column in execution_table for property assignments.

If we tried to access child.c which does not exist, what would happen?

A Return value from parent if exists

B Throw an error

C Return undefined

D Return null

💡 Hint

Refer to the exit_note about property lookup stopping when property is not found.

Concept Snapshot

Prototype inheritance in JavaScript:
- Objects can inherit properties via prototype chain.
- Accessing a property checks own properties first.
- If not found, lookup continues up prototype chain.
- If property not found anywhere, returns undefined.
- Use Object.create(proto) to set prototype.

Full Transcript

Prototype inheritance means when you ask for a property on an object, JavaScript first looks on that object. If it doesn't find it, it looks on the object's prototype. This continues up the chain until the property is found or there are no more prototypes. In the example, child inherits from parent. So child.b is found directly on child, but child.a is found on parent. If a property is missing on both, the result is undefined.