Custom refs with customRef in Vue

Introduction

Custom refs let you control how Vue tracks and updates reactive data. They help you create special reactive behaviors beyond the usual ones.

When you want to create a reactive value that updates only under certain conditions.

When you need to customize how Vue tracks dependencies and triggers updates.

When you want to wrap a non-reactive object and make it reactive in a custom way.

When you want to optimize performance by controlling when updates happen.

When you want to add side effects or custom logic during get or set of a reactive value.

Syntax

Vue

import { customRef } from 'vue'

const myRef = customRef((track, trigger) => {
  let value
  return {
    get() {
      track()
      return value
    },
    set(newValue) {
      value = newValue
      trigger()
    }
  }
})

track() tells Vue to track dependencies when the value is read.

trigger() tells Vue to update dependents when the value changes.

Examples

This custom ref only updates if the new value is zero or positive.

Vue

const count = customRef((track, trigger) => {
  let value = 0
  return {
    get() {
      track()
      return value
    },
    set(newValue) {
      if (newValue >= 0) {
        value = newValue
        trigger()
      }
    }
  }
})

This custom ref delays updates by 300ms, useful for debouncing input.

Vue

const debouncedRef = customRef((track, trigger) => {
  let value
  let timeout
  return {
    get() {
      track()
      return value
    },
    set(newValue) {
      clearTimeout(timeout)
      timeout = setTimeout(() => {
        value = newValue
        trigger()
      }, 300)
    }
  }
})

Sample Program

This Vue component uses a custom ref to only update the count if the input is zero or positive. Negative numbers typed in the input will not change the displayed count.

Vue

<template>
  <div>
    <input v-model.number="count" type="number" aria-label="Number input" />
    <p>Count (only positive numbers update): {{ count }}</p>
  </div>
</template>

<script setup>
import { customRef } from 'vue'

const count = customRef((track, trigger) => {
  let value = 0
  return {
    get() {
      track()
      return value
    },
    set(newValue) {
      if (newValue >= 0) {
        value = newValue
        trigger()
      }
    }
  }
})
</script>

Important Notes

Use customRef when you need fine control over reactivity.

Always call track() in get() and trigger() in set() to keep Vue's reactivity system working.

Custom refs can help improve performance by reducing unnecessary updates.

Summary

Custom refs let you create reactive values with custom get/set behavior.

They use track() and trigger() to connect with Vue's reactivity.

Useful for debouncing, validation, or conditional updates.

Practice

1. What is the main purpose of using customRef in Vue?

easy

A. To create reactive references with custom get and set behavior

B. To define a component's template

C. To register global components

D. To handle lifecycle hooks automatically

Solution

1. Step 1: Understand what customRef does

   customRef allows you to create reactive references where you control how the value is read and written.

2. Step 2: Compare with other options

   Options A, C, and D relate to other Vue features, not customRef.

3. Final Answer:

   To create reactive references with custom get and set behavior -> Option A

4. Quick Check:

   customRef = custom reactive behavior [OK]

Hint: Remember: customRef customizes reactive get/set [OK]

Common Mistakes:

• Confusing customRef with component registration
• Thinking customRef manages lifecycle hooks
• Assuming customRef defines templates

2. Which of the following is the correct way to import customRef in a Vue 3 component?

easy

A. import { CustomRef } from 'vue';

B. import customRef from 'vue';

C. import { customRef } from 'vue';

D. import * as customRef from 'vue';

Solution

1. Step 1: Recall Vue 3 import syntax

   Vue exports customRef as a named export, so it must be imported with curly braces.

2. Step 2: Check each option

   import { customRef } from 'vue'; uses correct named import syntax. import customRef from 'vue'; tries default import which is incorrect. import { CustomRef } from 'vue'; uses wrong capitalization. import * as customRef from 'vue'; imports everything as an object, which is not typical for this usage.

3. Final Answer:

   import { customRef } from 'vue'; -> Option C

4. Quick Check:

   Named import uses curly braces [OK]

Hint: Use curly braces for named imports like customRef [OK]

Common Mistakes:

• Using default import instead of named import
• Wrong capitalization of customRef
• Importing everything instead of named import

3. Given the following code, what will be logged when myRef.value = 10 is called?

import { customRef } from 'vue';

const myRef = customRef((track, trigger) => {
  let value = 0;
  return {
    get() {
      track();
      return value;
    },
    set(newValue) {
      if (newValue !== value) {
        value = newValue;
        trigger();
        console.log('Value updated');
      }
    }
  };
});

myRef.value = 10;

medium

A. Value updated

B. No output

C. Error: track is not defined

D. Value updated twice

Solution

1. Step 1: Understand the set function behavior

   The set function checks if the new value differs from the current one. Since 10 !== 0, it updates value, triggers reactivity, and logs 'Value updated'.

2. Step 2: Confirm no repeated logs or errors

   There is no repeated assignment or error in the code, so only one log occurs.

3. Final Answer:

   Value updated -> Option A

4. Quick Check:

   Set logs once on value change [OK]

Hint: Set logs only when value changes [OK]

Common Mistakes:

• Expecting no output because of track confusion
• Thinking trigger causes multiple logs
• Assuming error due to track usage

4. Identify the error in this customRef implementation:

import { customRef } from 'vue';

const myRef = customRef((track, trigger) => {
  let value = 0;
  return {
    get() {
      return value;
    },
    set(newValue) {
      value = newValue;
      trigger();
    }
  };
});

medium

A. customRef cannot be used with arrow functions

B. Missing call to trigger() inside get()

C. Value should be declared with ref()

D. Missing call to track() inside get()

Solution

1. Step 1: Check get() method for reactivity tracking

   The get() method must call track() to register dependencies. Here, it returns value without calling track(), so reactivity won't work properly.

2. Step 2: Verify other parts

   Trigger is correctly called in set(). Value does not need to be a ref inside customRef. Arrow functions are allowed.

3. Final Answer:

   Missing call to track() inside get() -> Option D

4. Quick Check:

   track() needed in get() for reactivity [OK]

Hint: Always call track() inside get() [OK]

Common Mistakes:

• Forgetting to call track() in get()
• Thinking trigger() is needed in get()
• Assuming value must be a ref inside customRef

5. You want to create a customRef that debounces updates by 300ms before triggering reactivity. Which code snippet correctly implements this behavior?

hard

A. const debouncedRef = customRef((track, trigger) => {
let value;
return {
get() {
track();
return value;
},
set(newValue) {
value = newValue;
trigger();
}
};
});

B. const debouncedRef = customRef((track, trigger) => {
let value;
let timeout;
return {
get() {
track();
return value;
},
set(newValue) {
clearTimeout(timeout);
timeout = setTimeout(() => {
value = newValue;
trigger();
}, 300);
}
};
});

C. const debouncedRef = customRef((track, trigger) => {
let value;
let timeout;
return {
get() {
return value;
},
set(newValue) {
clearTimeout(timeout);
timeout = setTimeout(() => {
value = newValue;
trigger();
}, 300);
}
};
});

D. const debouncedRef = customRef((track, trigger) => {
let value;
let timeout;
return {
get() {
track();
return value;
},
set(newValue) {
value = newValue;
clearTimeout(timeout);
timeout = setTimeout(() => {
trigger();
}, 300);
}
};
});

Solution

1. Step 1: Understand debounce logic in set()

   Debounce means delaying the update and trigger until 300ms after the last set call. The correct code clears previous timeout, sets new timeout to update value and trigger after 300ms.

2. Step 2: Check get() calls track()

   The correct code calls track() in get() to register dependencies. Missing track() in get() breaks reactivity. The remaining options either lack track(), do not debounce properly, or have incorrect debouncing logic.

3. Final Answer:

   Correctly implements debounced updates with track() in get() -> Option B

4. Quick Check:

   Debounce needs clearTimeout + delayed trigger [OK]

Hint: Debounce by delaying trigger with setTimeout and clearTimeout [OK]

Common Mistakes:

• Not calling track() inside get()
• Triggering immediately without debounce
• Setting value outside timeout causing immediate update