How to Use Proxy for Validation in JavaScript

Use a Proxy in JavaScript to intercept property assignments and validate values before they are set. Define a set trap in the proxy handler to check values and throw errors or reject invalid data.

Syntax

A Proxy wraps a target object and intercepts operations like property setting. The set trap is a function that runs when a property is assigned a value.

It has this form:

• target: the original object to wrap
• handler: an object with traps like set
• set(target, property, value, receiver): trap called on property assignment

javascript

const proxy = new Proxy(target, {
  set(target, property, value, receiver) {
    // validation logic here
    target[property] = value;
    return true; // indicate success
  }
});

Example

This example shows how to use a Proxy to validate that the age property is always a positive number before setting it.

javascript

const person = {};

const validator = {
  set(target, property, value) {
    if (property === 'age') {
      if (typeof value !== 'number' || value <= 0) {
        throw new Error('Age must be a positive number');
      }
    }
    target[property] = value;
    return true;
  }
};

const proxyPerson = new Proxy(person, validator);

proxyPerson.age = 25;
console.log(proxyPerson.age); // 25

try {
  proxyPerson.age = -5; // Throws error
} catch (e) {
  console.log(e.message);
}

Output

25
Age must be a positive number

Common Pitfalls

Common mistakes when using Proxy for validation include:

• Not returning true from the set trap, which can cause silent failures.
• Forgetting to actually set the property on the target object.
• Throwing errors without try-catch, which can crash the program unexpectedly.
• Not handling all properties if validation is needed for multiple keys.

javascript

const obj = {};

const badValidator = {
  set(target, prop, value) {
    if (prop === 'score' && value < 0) {
      throw new Error('Score cannot be negative');
    }
    // Missing target[prop] = value; and return true
  }
};

const proxyObj = new Proxy(obj, badValidator);

try {
  proxyObj.score = 10; // Does not set value
  console.log(proxyObj.score); // undefined
} catch (e) {
  console.log(e.message);
}

// Correct way:
const goodValidator = {
  set(target, prop, value) {
    if (prop === 'score' && value < 0) {
      throw new Error('Score cannot be negative');
    }
    target[prop] = value;
    return true;
  }
};

Output

undefined

Quick Reference

Tips for using Proxy for validation:

• Always return true from the set trap if the assignment succeeds.
• Throw errors to reject invalid values clearly.
• Use typeof and other checks to validate data types.
• Remember to set the property on the target object inside the set trap.
• Wrap assignments in try-catch when using proxies that throw errors.

Key Takeaways

Use a Proxy's set trap to intercept and validate property assignments.

Always set the property on the target and return true to confirm success.

Throw errors inside the set trap to reject invalid values clearly.

Wrap proxy property assignments in try-catch to handle validation errors safely.

Validate data types and values explicitly inside the set trap for robust checks.