JavaScript In

property in object
#privateIdentifier in object

const obj = { customProp: "value" };

"customProp" in obj; // true (own property)
"toString" in obj;   // true (inherited from Object.prototype)

"length" in "string"; // TypeError: Cannot use 'in' operator to search for 'length' in string
"length" in new String("string"); // true (String object wrapper)

const arr = ["apple", , "cherry"]; // Sparse array with an empty slot at index 1

0 in arr;       // true (index 0 exists)
1 in arr;       // false (empty slot)
2 in arr;       // true (index 2 exists)
"length" in arr;// true (arrays have a 'length' property)

// Explicitly setting undefined creates the property key
arr[1] = undefined;
1 in arr;       // true

const config = { timeout: undefined, retries: 3 };

"timeout" in config; // true (key exists, value is undefined)

delete config.retries;
"retries" in config; // false (key was removed from the object)

class DataNode {
  #internalState = true;

  static isNode(obj) {
    return #internalState in obj;
  }
}

const node = new DataNode();
DataNode.isNode(node); // true
DataNode.isNode({});   // false

const uniqueKey = Symbol("meta");
const node = { [uniqueKey]: true };

uniqueKey in node; // true