Read-only arrays in Typescript - Step-by-Step Execution

Concept Flow - Read-only arrays

Declare array with readonly type

↓

Try to read elements

→Allowed

↓

Try to modify elements

→Compile error: modification not allowed

↓

Use array safely without changes

↓

End

You create an array that cannot be changed. Reading is allowed, but any change causes an error.

Execution Sample

Typescript

const nums: readonly number[] = [1, 2, 3];
console.log(nums[0]);
// nums[1] = 5; // Error: cannot assign to readonly array
const newNums = nums.map(x => x * 2);
console.log(newNums);

This code shows a readonly array, reading elements, and creating a new array by mapping.

Execution Table

Step	Action	Code Line	Result	Notes
1	Declare readonly array nums	const nums: readonly number[] = [1, 2, 3];	nums = [1, 2, 3]	Array created, cannot be changed
2	Read first element	console.log(nums[0]);	Output: 1	Reading allowed
3	Attempt to modify element	// nums[1] = 5;	Compile error	Modification not allowed on readonly array
4	Create new array by map	const newNums = nums.map(x => x * 2);	newNums = [2, 4, 6]	map returns new array, original unchanged
5	Print newNums	console.log(newNums);	Output: [2, 4, 6]	Shows doubled values

💡 Execution stops because modification line is commented out due to compile error

Variable Tracker

Variable	Start	After Step 1	After Step 4	Final
nums	undefined	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]
newNums	undefined	undefined	[2, 4, 6]	[2, 4, 6]

Key Moments - 3 Insights

Why can't we assign a new value to nums[1]?

Because nums is declared as a readonly array, any attempt to change its elements causes a compile error, as shown in step 3 of the execution table.

Can we read elements from a readonly array?

Yes, reading elements is allowed and safe, as shown in step 2 where nums[0] is read and printed.

How can we create a changed version of a readonly array?

By using methods like map that return a new array without modifying the original, as shown in step 4 where newNums is created.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of nums after step 4?

A [2, 4, 6]

B undefined

C [1, 2, 3]

D Compile error

💡 Hint

Check the variable_tracker row for nums after step 4

At which step does the code try to modify the readonly array?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column in the execution_table for modification attempts

If we remove the comment from the modification line, what happens?

A The array nums changes to [1, 5, 3]

B A compile error occurs

C The program prints 5

D The map function fails

💡 Hint

Refer to the 'Result' column in step 3 of the execution_table

Concept Snapshot

Readonly arrays in TypeScript:
- Declared with 'readonly' keyword
- Elements can be read but not changed
- Any modification causes compile error
- Methods like map return new arrays
- Useful for safe, immutable data

Full Transcript

This visual trace shows how readonly arrays work in TypeScript. First, we declare an array with the readonly keyword, which means we can read elements but cannot change them. Reading nums[0] prints 1 successfully. Trying to assign a new value to nums[1] causes a compile error, so that line is commented out. We then use the map method to create a new array with doubled values, which is allowed because it does not change the original array. The new array is printed as [2, 4, 6]. The variable tracker confirms nums stays unchanged throughout. Key moments clarify why modification is disallowed and how to work with readonly arrays safely. The quiz tests understanding of these steps.