Type alias for unions in Typescript - Step-by-Step Execution

Concept Flow - Type alias for unions

Define union types

↓

Create type alias

↓

Use alias in variables

↓

Assign values matching union

↓

Type checks pass or error

↓

End

This flow shows how to define a union type, create a type alias for it, use the alias for variables, assign values, and check types.

Execution Sample

Typescript

type Status = "success" | "error" | "loading";
let currentStatus: Status;
currentStatus = "loading";
currentStatus = "success";
// currentStatus = "done"; // Error

Defines a type alias 'Status' as a union of string literals and assigns valid values to a variable.

Execution Table

Step	Action	Evaluation	Result
1	Define type alias 'Status'	Status = "success" | "error" | "loading"	Alias created
2	Declare variable 'currentStatus' of type Status	currentStatus: Status	Variable declared
3	Assign 'loading' to currentStatus	currentStatus = "loading"	Valid assignment
4	Assign 'success' to currentStatus	currentStatus = "success"	Valid assignment
5	Assign 'done' to currentStatus	currentStatus = "done"	Type error: 'done' not in Status union

💡 Execution stops because 'done' is not part of the union type 'Status'

Variable Tracker

Variable	Start	After 3	After 4	After 5 (error)
currentStatus	undefined	"loading"	"success"	Error - invalid assignment

Key Moments - 2 Insights

Why does assigning "done" to currentStatus cause an error?

Because "done" is not one of the allowed string literals in the union type 'Status', as shown in execution_table step 5.

What does the type alias 'Status' represent?

It represents a union of specific string values "success", "error", and "loading", allowing variables of this type to hold only these values (execution_table step 1).

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of currentStatus?

A "success"

B "loading"

C "error"

D undefined

💡 Hint

Check the 'Result' column for step 3 in execution_table.

At which step does the type error occur when assigning a value to currentStatus?

A Step 5

B Step 3

C Step 2

D Step 4

💡 Hint

Look for the 'Type error' mention in the execution_table rows.

If we add "done" to the union type alias, what changes in the execution_table?

A Step 3 assignment causes error

B Variable declaration fails

C Step 5 assignment becomes valid

D No changes

💡 Hint

Consider how the union type controls valid assignments shown in execution_table.

Concept Snapshot

Type alias for unions syntax:
type AliasName = Type1 | Type2 | ...;
Allows variables to hold any value from the union.
Assigning values outside union causes errors.
Use for clear, reusable type definitions.

Full Transcript

This example shows how to create a type alias named 'Status' that is a union of string literals "success", "error", and "loading". We declare a variable 'currentStatus' of this type. Assigning "loading" and "success" works fine because they are in the union. Assigning "done" causes a type error because it is not part of the union. This helps catch mistakes early by restricting values to a set of allowed options.

Practice

1.

What does a type alias for unions in TypeScript do?

easy

A. Defines a function that returns different types

B. Creates a new variable with multiple values

C. Gives a single name to multiple possible types

D. Declares a constant with union values

Solution

1. Step 1: Understand the purpose of type alias

   A type alias lets you give a name to a type or group of types for easier use.

2. Step 2: Understand union types

   A union type means a value can be one of several types, like string or number.

3. Final Answer:

   Gives a single name to multiple possible types -> Option C

4. Quick Check:

   Type alias for unions = single name for multiple types [OK]

Hint: Type alias names multiple types combined with | [OK]

Common Mistakes:

• Thinking it creates variables instead of naming types
• Confusing union with function return types
• Assuming it declares constants

2.

Which of the following is the correct syntax to create a type alias MyType that can be either string or number?

easy

A. type MyType = string | number;

B. alias MyType = string & number;

C. type MyType = string & number;

D. alias MyType = string | number;

Solution

1. Step 1: Recall correct keyword for type alias

   Use the keyword type to create a type alias, not alias.

2. Step 2: Use union operator for multiple types

   The union operator is |, not &. So string | number is correct.

3. Final Answer:

   type MyType = string | number; -> Option A

4. Quick Check:

   type + union (|) = correct syntax [OK]

Hint: Use 'type' keyword and '|' for union types [OK]

Common Mistakes:

• Using 'alias' instead of 'type'
• Using '&' instead of '|' for unions
• Mixing syntax from other languages

3.

What will be the output of the following TypeScript code?

type Status = "success" | "error";

function printStatus(s: Status) {
  console.log(s);
}

printStatus("success");
printStatus("error");
// printStatus("pending"); // Uncommenting this line causes error

medium

A. No output

B. success\nerror\npending

C. Error at runtime

D. success\nerror

Solution

1. Step 1: Understand the type alias Status

   Status can only be "success" or "error" strings.

2. Step 2: Check function calls

   Calling printStatus with "success" and "error" prints those values. "pending" is invalid and commented out.

3. Final Answer:

   success\nerror -> Option D

4. Quick Check:

   Valid union values print correctly [OK]

Hint: Only allowed union values print without error [OK]

Common Mistakes:

• Assuming invalid union values print without error
• Thinking it causes runtime error
• Ignoring commented invalid call

4.

Identify the error in this TypeScript code using a type alias for unions:

type ID = number | string;

let userId: ID = true;

medium

A. Type 'boolean' is not assignable to type 'number | string'

B. Missing semicolon after type alias

C. Type alias cannot use union types

D. Variable 'userId' must be declared with 'const'

Solution

1. Step 1: Check type alias definition

   ID allows only number or string values.

2. Step 2: Check assigned value type

   Assigning true is boolean, which is not allowed by the union.

3. Final Answer:

   Type 'boolean' is not assignable to type 'number | string' -> Option A

4. Quick Check:

   Assigning wrong type causes error [OK]

Hint: Assigned value must match union types exactly [OK]

Common Mistakes:

• Thinking union allows any type
• Ignoring type mismatch errors
• Confusing syntax errors with type errors

5.

Given the type alias type Response = { status: "ok"; data: string } | { status: "error"; error: string };, which of the following objects matches the Response type?

hard

A. { status: "ok", error: "File missing" }

B. { status: "ok", data: "File loaded" }

C. { status: "error", data: "File loaded" }

D. { status: "error", error: 404 }

Solution

1. Step 1: Analyze the first union type

   When status is "ok", object must have a string property named data.

2. Step 2: Analyze the second union type

   When status is "error", object must have a string property named error.

3. Step 3: Check each option

   { status: "ok", data: "File loaded" } matches first type correctly. Options B and C mix properties incorrectly. { status: "error", error: 404 } has error property as number, not string.

4. Final Answer:

   { status: "ok", data: "File loaded" } -> Option B

5. Quick Check:

   Match status with correct property name and type [OK]

Hint: Match status value with correct property name and type [OK]

Common Mistakes:

• Mixing properties from different union types
• Using wrong property types
• Ignoring the status-property relationship