get and set Traps
Customize property reads and writes.
get and set Traps is a free JavaScript Academy lesson on CoddyKit. This is lesson 2 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the JavaScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The JavaScript Academy course includes 4 lessons in total.
The get and set Traps
The get and set traps are the most-used proxy hooks. They intercept reading and writing properties, letting you transform values, validate input, or log access.
get Trap Signature
get(target, property, receiver) runs on every property read. Return whatever you want the caller to see.
const proxy = new Proxy({ price: 10 }, {
get(target, prop) {
return target[prop];
}
});
console.log(proxy.price);Default Values via get
A get trap can supply a fallback for missing properties instead of returning undefined.
const proxy = new Proxy({ a: 1 }, {
get(target, prop) {
return prop in target ? target[prop] : 0;
}
});
console.log(proxy.a);
console.log(proxy.missing);Transforming Read Values
The get trap can reshape values on the way out, for example uppercasing strings.
const proxy = new Proxy({ name: 'ada' }, {
get(target, prop) {
const value = target[prop];
return typeof value === 'string' ? value.toUpperCase() : value;
}
});
console.log(proxy.name);set Trap Signature
set(target, property, value, receiver) runs on every write. It must return true for success, otherwise strict mode throws.
const proxy = new Proxy({}, {
set(target, prop, value) {
target[prop] = value;
return true;
}
});
proxy.x = 5;
console.log(proxy.x);Validation in set
The classic use of a set trap is validation: reject invalid values before they reach the target.
const person = new Proxy({}, {
set(target, prop, value) {
if (prop === 'age' && (typeof value !== 'number' || value < 0)) {
throw new TypeError('age must be a non-negative number');
}
target[prop] = value;
return true;
}
});
person.age = 30;
console.log(person.age);
try { person.age = -5; } catch (e) { console.log(e.message); }Coercing Values in set
Beyond rejecting, a set trap can normalize input, for example trimming strings before storing.
const proxy = new Proxy({}, {
set(target, prop, value) {
target[prop] = typeof value === 'string' ? value.trim() : value;
return true;
}
});
proxy.name = ' Ada ';
console.log('[' + proxy.name + ']');Logging Both Operations
Combine get and set to trace every access. This is invaluable for debugging how an object is used.
const proxy = new Proxy({ v: 1 }, {
get(target, prop) {
console.log('GET ' + prop);
return target[prop];
},
set(target, prop, value) {
console.log('SET ' + prop + ' = ' + value);
target[prop] = value;
return true;
}
});
proxy.v = 42;
console.log(proxy.v);Enforcing Read-Only
A set trap that always rejects writes turns a proxy into a read-only view of its target.
const readonly = new Proxy({ pi: 3.14 }, {
set() {
console.log('writes are blocked');
return false;
}
});
try {
'use strict';
readonly.pi = 0;
} catch (e) {
console.log(e.name);
}
console.log(readonly.pi);Symbol Keys and Internals
Traps fire for symbol keys too. Be careful: logging every property including internal symbols can be noisy, so filter when needed.
const proxy = new Proxy({ a: 1 }, {
get(target, prop) {
if (typeof prop === 'string') console.log('reading ' + prop);
return target[prop];
}
});
console.log(proxy.a);Why get and set Matter
Together these traps enable validation, computed properties, defaults, logging, and reactivity, all without modifying the target. But manually writing target[prop] = value inside traps has edge cases. The Reflect API, covered next, handles them correctly.
const tracked = new Proxy({ total: 0 }, {
set(target, prop, value) {
console.log(prop + ' changed to ' + value);
target[prop] = value;
return true;
}
});
tracked.total = 100;
console.log(tracked.total);Quick Check
What must a set trap return to indicate a successful assignment?
Recap: get and set Traps
You learned the core proxy traps:
get(target, prop, receiver)intercepts reads; return any value.set(target, prop, value, receiver)intercepts writes; returntrueon success.- Use them for defaults, transforms, validation, and logging.
- For correct internal handling inside traps, use
Reflect, covered next.
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Frequently Asked Questions
Is the “get and set Traps” lesson free?
Yes — any 3 lessons of the JavaScript Academy learning path, including “get and set Traps”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The JavaScript Academy course includes 4 lessons in total.
What will I learn in “get and set Traps”?
Customize property reads and writes. You practise JavaScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start JavaScript Academy?
No prior experience is required. JavaScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.
How long does the “get and set Traps” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this JavaScript Academy lesson?
Yes. Every JavaScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Creating a Proxy
- get and set Traps
- The Reflect API
- Practical Proxy Use Cases