Setup function basics in Vue - Step-by-Step Execution

Concept Flow - Setup function basics

Component starts

↓

Call setup()

↓

Declare reactive state

↓

Return state and methods

↓

Template uses returned values

↓

Component renders with reactive data

The setup function runs first in a Vue component, sets up reactive state and methods, then returns them for the template to use.

Execution Sample

Vue

import { ref } from 'vue'
export default {
  setup() {
    const count = ref(0)
    function increment() { count.value++ }
    return { count, increment }
  }
}

This code creates a reactive count variable and an increment function inside setup, then returns them for the template.

Execution Table

Step	Action	Evaluation	Result
1	Call setup()	Runs setup function	Setup function starts
2	Declare count = ref(0)	Creates reactive count with value 0	count.value = 0
3	Define increment()	Function increments count.value	increment() ready
4	Return { count, increment }	Expose reactive state and method	Returned object with count and increment
5	Template uses count	Reads count.value	Displays 0 initially
6	Call increment()	count.value++	count.value becomes 1
7	Template re-renders	Reflects updated count.value	Displays 1

💡 Setup function completes after returning reactive state and methods

Variable Tracker

Variable	Start	After Step 2	After Step 6	Final
count.value	undefined	0	1	1

Key Moments - 3 Insights

Why do we use ref() for count inside setup?

ref() makes count reactive so Vue tracks changes and updates the template automatically, as shown in execution_table step 2 and 7.

What happens if we don't return count and increment from setup?

The template cannot access them, so no reactive updates happen. This is why returning them in step 4 is essential.

Why do we access count.value instead of count directly?

ref() wraps the value inside an object; .value accesses the actual data, as seen in steps 2 and 6.

Visual Quiz - 3 Questions

Look at the execution table, what is count.value after step 2?

A undefined

B 0

C 1

D null

💡 Hint

Check the 'Result' column at step 2 in the execution_table.

At which step does the template first display the updated count value?

A Step 7

B Step 6

C Step 5

D Step 4

💡 Hint

Look for when the template re-renders with the new count in execution_table step 7.

If we forget to return increment from setup, what happens?

A Template can still call increment

B count stops being reactive

C increment is not accessible in template

D Setup function errors out

💡 Hint

Refer to key_moments about returning values from setup and execution_table step 4.

Concept Snapshot

Setup function runs first in Vue component.
Declare reactive state with ref() or reactive().
Define methods inside setup.
Return state and methods for template use.
Template accesses returned values reactively.
Use .value to access ref() data.

Full Transcript

In Vue, the setup function is the first code that runs when a component starts. Inside setup, you create reactive variables using ref() or reactive(). You can also define functions to change these variables. After setting up, you return the reactive variables and functions so the template can use them. The template then shows the current values and updates automatically when the reactive data changes. For example, a count variable created with ref(0) starts at zero. When you call an increment function that adds one to count.value, the template updates to show the new count. Remember, ref() wraps the value, so you use .value to get or set the actual data. If you forget to return a variable or function from setup, the template cannot access it and won't update. This flow helps Vue components stay reactive and easy to manage.