Difference between 'in' operator and property value checking in JavaScript objects

Other Questions JavaScript
1

I’m working on a function that validates if each element in an array has its cube stored as a key in an object, and if the object values match the frequency count from the array. I’m looping through the array and using two different approaches to check for key existence, but they give different results.

In the first approach, I use the in operator to check if element ** 3 exists as a key:

let data = {
  8: 1,
  27: 1,
  64: 1,
  125: 2,
};

let numbers = [2, 3, 4, 3, 5];

for (let element of numbers) {
  if (!(element ** 3 in data)) {
    console.log("not found");
  } else {
    data[element ** 3] -= 1;
    console.log(data);
    console.log("found");
  }
}

In the second approach, I directly check the property value with data[element ** 3]:

let data = {
  8: 1,
  27: 1,
  64: 1,
  125: 2,
};

let numbers = [2, 3, 4, 3, 5];

for (let element of numbers) {
  if (!(data[element ** 3])) {
    console.log("not found");
  } else {
    data[element ** 3] -= 1;
    console.log(data);
    console.log("found");
  }
}

With the in operator, the subtraction continues even when values reach 0. But with direct property checking, when a property becomes 0, the condition !(data[element ** 3]) evaluates to true and logs “not found”.

Why do these two methods behave differently? Is there a specific rule that explains this behavior? The first approach seems more logical to me.

2

This tripped me up when I first started with JavaScript too. These operators test completely different things. When you use in, you’re asking “does this property exist on this object?” The answer stays yes even after the value becomes 0 because the property wasn’t deleted from the object. But when you use !(data[element ** 3]), you’re asking “is this value falsy?” and 0 is one of JavaScript’s falsy values along with false, null, undefined, empty string, and NaN. I learned this the hard way building a frequency counter like yours. The in operator is definitely the right choice for your validation function since you want to verify the key exists before decrementing. Just remember - if you ever need to actually remove processed keys entirely, you’d use delete data[element ** 3] and then both approaches would behave the same.

3

yeah, in is about the key presence in the obj, not the value. so even if data[27] is 0, key 27 is still valid. but !(data[element ** 3]) checks for truthy values, and 0 is falsy in js. that’s y the first method continues even when values r 0.

4

The key difference is what each method checks. The in operator looks for whether a property exists in the object, regardless of its value. So even when data[27] becomes 0, the key 27 is still present in the object.

On the other hand, !(data[element ** 3]) evaluates the property’s value for truthiness. In JavaScript, 0 is considered falsy, so when your counter hits 0, this condition returns true and triggers the ‘not found’ branch.

This behavior is due to JavaScript’s type coercion, where values like 0, null, undefined, false, and empty strings are treated as falsy. For your use case, the in operator is appropriate since you want to check for the existence of the key, not whether its value is truthy. If you need to verify both existence and a non-zero value, you could combine checks using data.hasOwnProperty(element ** 3) && data[element ** 3] > 0.