Numeric enums in Typescript - Time & Space Complexity

Time Complexity: Numeric enums

O(1)

Understanding Time Complexity

We want to understand how the time it takes to work with numeric enums changes as the number of enum members grows.

Specifically, how does accessing or searching enum values behave when the enum size increases?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

    enum Colors {
      Red,    // 0
      Green,  // 1
      Blue    // 2
    }

    function getColorName(value: number): string | undefined {
      return Colors[value];
    }
    

This code defines a numeric enum and a function that looks up the name of a color by its numeric value.

Identify Repeating Operations

• Primary operation: Direct lookup in an object (enum reverse mapping)
• How many times: Single operation per lookup, no loops or recursion

How Execution Grows With Input

Looking up a value in the enum is like checking a label on a box directly, no matter how many boxes there are.

Input Size (n)	Approx. Operations
10	1
100	1
1000	1

Pattern observation: The time to find a name does not grow with the number of enum members; it stays constant.

Final Time Complexity

Time Complexity: O(1)

This means looking up a numeric enum value takes the same amount of time no matter how many items are in the enum.

Common Mistake

[X] Wrong: "Looking up a numeric enum value takes longer as the enum gets bigger because it has to check each item."

[OK] Correct: Numeric enums in TypeScript create an object with direct keys, so the lookup is a quick direct access, not a search through all items.

Interview Connect

Understanding how enum lookups work helps you explain efficient data access in TypeScript, showing you know how language features impact performance.

Self-Check

"What if we changed the enum to a string enum? How would the time complexity of looking up a name by value change?"

Practice

1. What is a numeric enum in TypeScript used for?

easy

A. To declare variables without types

B. To assign names to numeric values for better code clarity

C. To define functions with numeric parameters

D. To create string-only constants

Solution

1. Step 1: Understand enum purpose

   Numeric enums give meaningful names to numbers, making code easier to read.

2. Step 2: Compare options

   Only To assign names to numeric values for better code clarity correctly describes numeric enums as naming numbers for clarity.

3. Final Answer:

   To assign names to numeric values for better code clarity -> Option B

4. Quick Check:

   Numeric enums = named numbers [OK]

Hint: Enums name numbers to avoid magic numbers [OK]

Common Mistakes:

• Confusing enums with string constants
• Thinking enums declare variables
• Assuming enums define functions

2. Which of the following is the correct syntax to declare a numeric enum in TypeScript?

easy

A. enum Colors { Red = 1, Green, Blue }

B. enum Colors = { Red: 1, Green: 2, Blue: 3 }

C. enum Colors (Red = 1, Green, Blue)

D. enum Colors { Red: 1; Green; Blue }

Solution

1. Step 1: Recall enum syntax

   TypeScript enums use curly braces with members separated by commas, and assignment uses '='.

2. Step 2: Check each option

   enum Colors { Red = 1, Green, Blue } uses correct syntax: enum name, braces, members with '=' and commas.

3. Final Answer:

   enum Colors { Red = 1, Green, Blue } -> Option A

4. Quick Check:

   Correct enum syntax uses braces and '=' [OK]

Hint: Use braces and '=' for enum members [OK]

Common Mistakes:

• Using '=' after enum name
• Using colons instead of '=' for values
• Separating members with semicolons

3. What will be the output of this code?

enum Status {
  Ready = 3,
  Waiting,
  Done
}
console.log(Status.Waiting);

medium

A. 3

B. 5

C. 4

D. undefined

Solution

1. Step 1: Understand enum auto-increment

   Ready is set to 3, so Waiting becomes 4, and Done becomes 5 automatically.

2. Step 2: Identify logged value

   console.log(Status.Waiting) prints 4, the auto-incremented value after 3.

3. Final Answer:

   4 -> Option C

4. Quick Check:

   Auto-increment after 3 = 4 [OK]

Hint: Next enum value auto-increments by 1 [OK]

Common Mistakes:

• Assuming Waiting equals 3
• Thinking enum values reset automatically
• Expecting string output

4. Identify the error in this enum declaration:

enum Direction {
  Up = 1,
  Down = 2
  Left = 3,
  Right = 4
}

medium

A. Cannot mix assigned and unassigned members

B. Enum members cannot have assigned values

C. Enum name must be lowercase

D. Missing comma after Down = 2

Solution

1. Step 1: Check enum member separators

   Enum members must be separated by commas. After 'Down = 2' there is no comma.

2. Step 2: Validate other rules

   Assigned values are allowed, enum names can be uppercase, and mixing assigned/unassigned is allowed.

3. Final Answer:

   Missing comma after Down = 2 -> Option D

4. Quick Check:

   Enum members need commas [OK]

Hint: Check commas between enum members [OK]

Common Mistakes:

• Forgetting commas between members
• Thinking enum names must be lowercase
• Believing all members must be unassigned

5. Given this enum:

enum Levels {
  Low = 1,
  Medium = Low * 2,
  High = Medium + 2
}

What is the value of Levels.High?

hard

A. 4

B. 6

C. 5

D. 8

Solution

1. Step 1: Calculate Medium value

   Medium = Low * 2 = 1 * 2 = 2.

2. Step 2: Calculate High value

   High = Medium + 2 = 2 + 2 = 4.

3. Step 3: Re-check calculations

   Medium is 2, High is 4. This is correct.

4. Step 4: Confirm enum constant expressions

   TypeScript allows constant expressions. Low=1, Medium=2, High=4.

5. Final Answer:

   4 -> Option A

6. Quick Check:

   Levels.High = 4 [OK]

Hint: Calculate enum expressions step-by-step [OK]

Common Mistakes:

• Ignoring expression evaluation order
• Assuming High equals Medium + Low
• Confusing enum values with indexes