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Overload the in operator

Written byPhuoc Nguyen
06 Jan, 2024
In JavaScript, you can use the `in` operator to check if an object has a specific property. When you use the `in` operator, it returns `true` if the property exists in the object, and `false` otherwise.
Here's an example of how to use the `in` operator to check if a property exists in an object:
const person = {
name: 'John Smith',
age: 42,
occupation: 'developer',

console.log('name' in person); // true
console.log('email' in person); // false
In this example, we have an object called `person` that includes properties for `name`, `age`, and `occupation`. Using the `in` operator, we can check if the properties `name` and `email` exist in the `person` object. The first console log returns `true`, indicating that `name` is a property of the object. The second console log, on the other hand, returns `false`, indicating that `email` is not a property of the object.
The `in` operator is not just limited to checking objects. It can also be used to verify if an element exists in an array. For instance, we can use the following code to determine if the number 3 is an element of an array called `arr`:
const arr = [1, 2, 3, 4, 5];
console.log(3 in arr); // true
console.log(8 in arr); // false
In this post, we'll learn how to overload the in operator with JavaScript Proxy.

Overloading the in operator in JavaScript

In programming, overloading is when an operator or function can have multiple meanings and implementations depending on the context. In JavaScript, we can overload the `in` operator using a `Proxy` object.
A `Proxy` object acts as an intermediary between the code and an object, allowing us to intercept and customize various operations such as property lookup, assignment, enumeration, and more. By using this feature, we can overload the `in` operator to create custom behavior.
Here's an example of how we can overload the `in` operator in JavaScript:
const handler = {
has(target, key) {
console.log(`Checking for ${key} property...`);
return key in target;
In this example, we're creating a `handler` object with a `has()` method that intercepts calls to the `in` operator. It does two things: logs a message to the console and delegates the operation to the original object using the `target` parameter of the `has()` method.
Let's give this Proxy trap handler a spin:
const person = {
name: 'John Smith',
age: 42,
occupation: 'developer',

const proxy = new Proxy(person, handler);
// Logs "Checking for name property..." and returns `true`
console.log('name' in proxy);

// Logs "Checking for email property..." and returns `false`
console.log('email' in proxy);
We've created a `Proxy` object called `proxy` that wraps around our original object, `person`, and uses our custom handler. Now, when we call `'name' in proxy`, it logs `"Checking for name property..."` to the console and returns `true`. On the other hand, when we call `'email' in proxy`, it logs `"Checking for email property..."` to the console and returns `false`.
This example is pretty simple, but in the next section, we'll explore more advanced usage of Proxy.

Checking if a number belongs to a given range

Many programming languages have built-in support for ranges, making it easy for developers to work with sequences of numbers or other types in a concise and readable way. For example, Python has a `range()` function that generates a sequence of numbers within a specified range, while Ruby has a `Range` class that represents an interval of values.
How do we check if a number belongs to a range? In Python, we can simply use the membership operator (`in`) with a range object created using the `range()` function.
3 in range(1, 6) # True
10 in range(1, 6) # False
Similarly, Ruby provides a variety of methods, such as `include?` and `cover?`, to check if a value falls within a given range.
(1..5).include?(3) # true
(1..5).include?(10) # false
By using these built-in functions and methods, developers can easily check if a number belongs to a given range and streamline their code.
While JavaScript doesn't have a similar API natively, we can still achieve similar functionality by using Proxy. Check out how we can implement the `range` function using JavaScript:
const range = (min, max) => new Proxy(
has: (target, key) => (+key >= min && +key <= max),
The `range` function in the code creates a new `Proxy` object with an empty target. Then, the `has` trap checks if a given key falls between the minimum and maximum values passed as arguments to the function. If it does, the `has` trap returns `true`, meaning the key belongs to the range. If not, it returns `false`.
Using this proxy object makes it easy to check if a given number is in a particular range without having to use conditional statements.
2 in range(1, 5); // true
10 in range(1, 5); // false


To sum up, in JavaScript, we can use the `in` operator to check if a property exists in an object or if an element exists in an array. Additionally, we can customize the `in` operator with Proxy to create unique behavior for checking properties.
Furthermore, with the help of Proxy, we can create a `range` function that allows us to check if a given number is within a specific range, without any need for conditional statements.
Overloading the `in` operator is just one example of how Proxy objects can enhance our JavaScript code. Proxies provide us with the ability to personalize and intercept various operations on objects and functions, giving us greater flexibility and control over our code.
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