Prototype chain in Javascript - Step-by-Step Execution

Concept Flow - Prototype chain

Create object A

↓

Set prototype of A to B

↓

Access property on A

↓

Check if property in A

Yes No↓

Return

↓

Check if property in B

↓

Return

When accessing a property, JavaScript looks first on the object, then up its prototype chain until it finds the property or reaches null.

Execution Sample

Javascript

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

Create an object 'child' with 'parent' as prototype, then access properties from both.

Execution Table

Step	Action	Object Checked	Property Searched	Found?	Result
1	Create object 'parent'	N/A	N/A	N/A	{a:1} created
2	Create object 'child' with prototype 'parent'	N/A	N/A	N/A	child created with prototype parent
3	Assign property 'b=2' to child	child	b	N/A	child.b = 2
4	Access child.a	child	a	No	Look in prototype
5	Access parent.a	parent	a	Yes	Value 1 found
6	Access child.b	child	b	Yes	Value 2 found
7	End	N/A	N/A	N/A	All properties accessed

💡 Property found or prototype chain ends at null

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 6	Final
parent	undefined	{a:1}	{a:1}	{a:1}	{a:1}	{a:1}	{a:1}
child	undefined	undefined	Object with prototype parent	Object with b=2	Object with b=2	Object with b=2	Object with b=2

Key Moments - 3 Insights

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

Because child’s prototype is parent, and JavaScript looks up the prototype chain to find 'a' in parent (see execution_table rows 4 and 5).

What happens if a property is not found on the object or any prototype?

JavaScript returns undefined after reaching the end of the prototype chain (null). This is shown in the exit_note.

Does assigning child.b = 2 affect the parent object?

No, child.b is its own property and does not change parent (see execution_table row 3).

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the property 'a' found when accessing child.a?

A Step 4

B Step 5

C Step 3

D Step 6

💡 Hint

Check the 'Found?' and 'Result' columns for child.a access in steps 4 and 5.

According to variable_tracker, what is the value of 'child' after step 3?

A Undefined

B Object with prototype parent

C Object with b=2

D Object with a=1

💡 Hint

Look at the 'After Step 3' column for 'child' in variable_tracker.

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

A Return value from parent.c

B Return undefined after checking all prototypes

C Throw an error

D Return null

💡 Hint

Refer to the exit_note about property not found in prototype chain.

Concept Snapshot

Prototype chain in JavaScript:
- Objects have a prototype link to another object.
- Property access checks own properties first.
- If not found, looks up the prototype chain.
- Stops when property found or prototype is null.
- Enables inheritance and shared properties.

Full Transcript

This visual trace shows how JavaScript uses the prototype chain to find properties. When accessing a property on an object, JavaScript first checks if the property exists directly on that object. If not found, it looks at the object's prototype, then the prototype's prototype, and so on until it finds the property or reaches the end of the chain (null). In the example, 'child' is created with 'parent' as its prototype. Accessing 'child.a' looks up to 'parent' where 'a' is found. Accessing 'child.b' finds 'b' directly on 'child'. This chain allows objects to share properties and methods efficiently.