Template literal with unions in Typescript - Step-by-Step Execution

Concept Flow - Template literal with unions

Define union types

↓

Create template literal type using unions

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Use template literal type in variable

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Assign valid string matching template

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TypeScript checks correctness

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Accept or error based on match

This flow shows how TypeScript builds a template literal type from unions and checks if assigned strings match the pattern.

Execution Sample

Typescript

type Color = "red" | "blue";
type Size = "small" | "large";
type Shirt = `${Color}-${Size}`;

let myShirt: Shirt = "red-small";

Defines unions for colors and sizes, combines them into a template literal type, and assigns a valid string.

Execution Table

Step	Action	Evaluation	Result
1	Define union type Color	"red" | "blue"	Color = "red" or "blue"
2	Define union type Size	"small" | "large"	Size = "small" or "large"
3	Create template literal type Shirt	`${Color}-${Size}`	Shirt = "red-small" | "red-large" | "blue-small" | "blue-large"
4	Declare variable myShirt of type Shirt	myShirt: Shirt	myShirt must be one of Shirt strings
5	Assign "red-small" to myShirt	Check if "red-small" in Shirt	Valid assignment, no error
6	Assign "green-small" to myShirt	Check if "green-small" in Shirt	Error: "green-small" not assignable to Shirt

💡 Stops after assignment check; errors if string does not match template literal type.

Variable Tracker

Variable	Start	After Step 5	After Step 6
myShirt	undefined	"red-small"	Error (invalid assignment)

Key Moments - 2 Insights

Why does assigning "green-small" to myShirt cause an error?

Because "green" is not part of the Color union (see execution_table step 6), so "green-small" does not match the Shirt template literal type.

How does TypeScript know which strings are valid for Shirt?

It combines all Color and Size union values into all possible strings using the template literal (execution_table step 3). Only those strings are valid.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of myShirt after step 5?

A "blue-large"

B undefined

C "red-small"

D Error

💡 Hint

Check the variable_tracker row for myShirt after step 5.

At which step does TypeScript detect an invalid assignment?

A Step 6

B Step 5

C Step 4

D Step 3

💡 Hint

See execution_table row describing assignment of "green-small".

If we add "green" to Color union, what happens to the Shirt type?

A It causes a syntax error

B It includes "green-small" and "green-large" strings

C It stays the same

D It removes "red-small" from Shirt

💡 Hint

Refer to execution_table step 3 where Shirt is created from all Color and Size combinations.

Concept Snapshot

Template literal with unions:
- Define unions (e.g., Color = "red" | "blue")
- Create template literal type: type Shirt = `${Color}-${Size}`
- Shirt includes all combinations ("red-small", "blue-large", etc.)
- Assigning strings outside these causes errors
- TypeScript checks assignments at compile time

Full Transcript

This visual trace shows how TypeScript uses union types combined with template literals to create a new string type. First, union types for Color and Size are defined. Then, a template literal type Shirt is created by combining these unions with a dash. Variables of type Shirt can only hold strings matching these combinations. Assigning a string like "red-small" is valid, but "green-small" causes an error because "green" is not in the Color union. The execution table tracks each step, showing how TypeScript builds the type and checks assignments. The variable tracker shows the value of myShirt changing from undefined to "red-small" and then an error on invalid assignment. Key moments clarify why certain assignments fail and how TypeScript generates all valid strings. The quiz tests understanding of these steps and the effect of changing unions.

Practice

1.

Which of the following best describes a template literal with unions in TypeScript?

easy

A. A method to concatenate numbers and strings without type checking.

B. A way to create specific string patterns by combining fixed text and union types.

C. A function that returns a template string without any type restrictions.

D. A way to declare variables without specifying their types.

Solution

1. Step 1: Understand template literals and unions

   Template literals allow embedding expressions in strings. Union types restrict values to a set of options.

2. Step 2: Combine both concepts

   Template literal with unions creates strings with fixed parts and limited variable parts, improving type safety.

3. Final Answer:

   A way to create specific string patterns by combining fixed text and union types. -> Option B

4. Quick Check:

   Template literal + union = specific string patterns [OK]

Hint: Think of fixed text plus limited choices in strings [OK]

Common Mistakes:

• Confusing unions with simple string concatenation
• Ignoring type safety benefits
• Thinking it's only about functions

2.

Which of the following TypeScript declarations correctly uses a template literal with unions?

type Size = "small" | "medium" | "large";
type Message = `Size is ${Size}`;

easy

A. type Message = `Size is ${string}`;

B. type Message = `Size is ${boolean}`;

C. type Message = `Size is ${number}`;

D. type Message = `Size is ${Size}`;

Solution

1. Step 1: Check the union type used inside template literal

   The union type Size is "small" | "medium" | "large" and should be used inside the template literal.

2. Step 2: Match the template literal syntax

   The correct syntax is `Size is ${Size}` to restrict the string to those sizes.

3. Final Answer:

   type Message = `Size is ${Size}`; -> Option D

4. Quick Check:

   Use union type inside template literal exactly [OK]

Hint: Use the union type name inside ${} exactly [OK]

Common Mistakes:

• Replacing union with general types like string or number
• Using incorrect syntax for template literals
• Forgetting backticks around the template

3.

What is the type of result in the following code?

type Color = "red" | "green" | "blue";
type Alert = `Alert: ${Color}`;

const result: Alert = "Alert: green";

medium

A. "Alert: red" | "Alert: green" | "Alert: blue"

B. "Alert: red" | "Alert: green"

C. "Alert: green"

D. string

Solution

1. Step 1: Understand the template literal with union

   The type Alert creates strings by combining "Alert: " with each value in Color.

2. Step 2: Expand the union to all possible strings

   So Alert is the union of "Alert: red", "Alert: green", and "Alert: blue".

3. Final Answer:

   "Alert: red" | "Alert: green" | "Alert: blue" -> Option A

4. Quick Check:

   Template literal expands union to all combined strings [OK]

Hint: Expand union inside template literal to all string combos [OK]

Common Mistakes:

• Assuming only one string value is allowed
• Confusing union with single string
• Using general string type instead of union

4.

Identify the error in this TypeScript code using template literals with unions:

type Status = "success" | "error";
type Message = `Status: ${string}`;

const msg: Message = "Status: success";

medium

A. Using string inside template literal instead of union type Status.

B. Missing backticks around the string value assigned to msg.

C. Incorrect union type declaration for Status.

D. Assigning a string literal to a non-string type.

Solution

1. Step 1: Check the template literal type definition

   The template literal uses ${string} which allows any string, not restricted to Status.

2. Step 2: Identify the correct usage

   It should use ${Status} to restrict values to "success" or "error" only.

3. Final Answer:

   Using string inside template literal instead of union type Status. -> Option A

4. Quick Check:

   Use union type inside template literal for safety [OK]

Hint: Use union type inside ${}, not general string [OK]

Common Mistakes:

• Using string instead of union type inside template
• Ignoring type safety benefits
• Confusing string literals with template literals

5.

Given the following types, which of these assignments is valid without TypeScript errors?

type Direction = "up" | "down";
type Action = "move" | "stop";
type Command = `${Action}-${Direction}`;

const cmd1: Command = "move-up";
const cmd2: Command = "stop-left";
const cmd3: Command = "move-down";
const cmd4: Command = "jump-up";

hard

A. Only cmd2 is valid.

B. All commands are valid.

C. Only cmd1 and cmd3 are valid.

D. Only cmd4 is valid.

Solution

1. Step 1: Understand the Command type

   Command is a template literal combining Action and Direction with a dash, so valid strings are "move-up", "move-down", "stop-up", "stop-down".

2. Step 2: Check each assignment

   cmd1 = "move-up" (valid), cmd2 = "stop-left" (invalid, "left" not in Direction), cmd3 = "move-down" (valid), cmd4 = "jump-up" (invalid, "jump" not in Action).

3. Final Answer:

   Only cmd1 and cmd3 are valid. -> Option C

4. Quick Check:

   Match both parts to union values exactly [OK]

Hint: Check both parts of template literal match union values [OK]

Common Mistakes:

• Assuming any string with dash is valid
• Ignoring union type restrictions
• Confusing Action and Direction values