Typing composables in Vue - Step-by-Step Execution

Concept Flow - Typing composables

Define composable function

↓

Add TypeScript types to inputs/outputs

↓

Use ref/reactive with typed values

↓

Return typed state and methods

↓

Consume composable with type safety

This flow shows how to create a Vue composable with TypeScript types to ensure inputs and outputs are correctly typed and safe to use.

Execution Sample

Vue

import { ref } from 'vue'

function useCounter(initial: number) {
  const count = ref<number>(initial)
  function increment() { count.value++ }
  return { count, increment }
}

A simple typed composable that manages a counter with a typed initial value and typed state.

Execution Table

Step	Action	Input/State	Output/State Change
1	Call useCounter with initial=5	initial=5	count = ref(5) with type number
2	Define increment function	count=ref(5)	increment() ready to increase count.value
3	Return { count, increment }	count=ref(5)	Composable returns typed count and increment
4	Call increment()	count.value=5	count.value becomes 6
5	Access count.value	count.value=6	Output is 6

💡 Composable returns typed state and methods; usage updates count.value correctly.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
count.value	undefined	5	5	5	6	6

Key Moments - 3 Insights

Why do we write ref<number>(initial) instead of just ref(initial)?

Typing ref<number> explicitly tells TypeScript the value inside is a number, ensuring type safety as shown in execution_table step 1.

What happens if increment() is called before count is initialized?

In this composable, count is initialized immediately, so increment() safely updates count.value as seen in step 4.

How does TypeScript help when using the returned composable?

TypeScript knows count is a Ref<number> and increment is a function, preventing mistakes when accessing or modifying them, as shown in step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of count.value after step 4?

A undefined

B 6

C 5

D 7

💡 Hint

Check the 'Output/State Change' column at step 4 in execution_table.

At which step is the increment function defined and ready to use?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column describing function definition in execution_table.

If we change initial to a string, what will happen to the typing?

A increment function will convert string to number automatically

B count.value will accept string without error

C TypeScript will show an error because count expects a number

D No change, code runs fine

💡 Hint

Refer to the typing in the composable definition and variable_tracker for count.value type.

Concept Snapshot

Typing composables in Vue:
- Define composable function with typed parameters
- Use ref<T> or reactive with explicit types
- Return typed state and methods
- TypeScript ensures safe usage and catches errors
- Helps maintain clear contracts for composables

Full Transcript

Typing composables in Vue means adding TypeScript types to the inputs and outputs of your composable functions. You start by defining a composable function that takes typed parameters. Inside, you use Vue's ref or reactive with explicit type annotations to hold state. You then return the typed state and any methods. This approach helps TypeScript understand what types your composable expects and returns, so it can catch mistakes early. For example, a useCounter composable takes a number initial value, creates a ref<number> count, and returns count and an increment function. When increment is called, count.value increases safely. If you try to pass a wrong type, TypeScript will warn you. This makes your composables safer and easier to use.