JavaScript
Proxy use cases
Learn real proxy uses like validation, logging and tracing.
A validation proxy
One of the most common uses of Proxy is validation. By placing a condition inside the set trap, you make sure only valid values are written to a property.
const handler = {
set(target, prop, value) {
if (typeof value !== 'number') {
throw new TypeError('number required');
}
return Reflect.set(target, prop, value);
}
};
Here, assigning a non-number to age throws an error. The validation logic lives in one place, inside the proxy, so the same rule applies at every assignment site.
Logging and tracing proxies
Proxy is also handy for logging and tracing. Add a console.log line inside the get trap to record every property read, or inside the set trap to watch every assignment.
const handler = {
get(target, prop) {
console.log('read', prop);
return Reflect.get(target, prop);
}
};
This pattern is powerful for debugging, since you can see which property is accessed and when. After the log line you still apply the default behavior with Reflect, so the object keeps working as expected.
Exercise Write a validation proxy
Define an empty object named user. Inside a handler, write a set trap: if prop === 'age' and typeof value !== 'number', throw new TypeError('age must be a number'); otherwise return Reflect.set(target, prop, value). Create a proxy variable with new Proxy(user, handler). Assign proxy.age = 28 and log it with console.log(proxy.age).
Show example solution
const user = {};
const handler = {
set(target, prop, value) {
if (prop === 'age' && typeof value !== 'number') {
throw new TypeError('age must be a number');
}
return Reflect.set(target, prop, value);
}
};
const proxy = new Proxy(user, handler);
proxy.age = 28;
console.log(proxy.age);
Test yourself
-
What does the set trap of a validation proxy do?
- writes via Reflect.set but checks first
- always returns true
- ignores the value
- only logs to the console
Show answer
writes via Reflect.set but checks first — Correct! A validation proxy checks the value first, then writes it with Reflect.set.
-
Throw a validation error
if (typeof value !== 'number') {
____;
}
Show answer
if (typeof value !== 'number') {
throw new TypeError('age must be a number');
}
-
Match the Proxy use cases
- validation
- blocks invalid value
- logging
- records access
- tracing
- notifies changes
- default
- returns via Reflect
Next lesson: Summary - Proxy and Reflect
