Oppiskele Enumerability and Iteration | Understanding and Working with Objects

Understanding enumerable properties is crucial for controlling how your object's properties are accessed and manipulated in JavaScript. Properties in JavaScript objects have an enumerable attribute, which determines whether a property shows up during property enumeration, such as when using a for...in loop or Object.keys. By default, properties you add directly to an object are enumerable, meaning they are visible during iteration and methods that list object keys. However, sometimes you may want to have properties that are not visible during enumeration, such as internal values or helper functions. This is where controlling enumerability becomes valuable: it allows you to hide certain properties from loops and methods that operate on object keys, making your objects more robust and predictable.


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const user = { name: "Alice" };

// Add an enumerable property
Object.defineProperty(user, "age", {
  value: 30,
  enumerable: true
});

// Add a non-enumerable property
Object.defineProperty(user, "password", {
  value: "secret123",
  enumerable: false
});

console.log(JSON.stringify(user)); // { name: 'Alice', age: 30 }
console.log(user.password); // 'secret123'

With these properties in place, you can see how enumeration works in practice. When you use the for...in loop or Object.keys, only enumerable properties are included. In the user object above, "name" and "age" are enumerable, but "password" is not. This means "password" will not appear when you iterate through the object's properties. This behavior is important for protecting sensitive information or internal implementation details from being exposed unintentionally.


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const user = { name: "Alice" };

// Add an enumerable property
Object.defineProperty(user, "age", {
  value: 30,
  enumerable: true
});

// Add a non-enumerable property
Object.defineProperty(user, "password", {
  value: "secret123",
  enumerable: false
});

// Using for...in to iterate over properties
for (let key in user) {
  console.log(key); // Outputs: name, age
}

// Using Object.keys to get enumerable property names
console.log(Object.keys(user)); // ['name', 'age']

// Non-enumerable property "password" is not included
console.log(user.password); // 'secret123'

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Can you explain how to make a property non-enumerable after it has already been added to an object?

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              12345678910111213141516
            
const user = { name: "Alice" };

// Add an enumerable property
Object.defineProperty(user, "age", {
  value: 30,
  enumerable: true
});

// Add a non-enumerable property
Object.defineProperty(user, "password", {
  value: "secret123",
  enumerable: false
});

console.log(JSON.stringify(user)); // { name: 'Alice', age: 30 }
console.log(user.password); // 'secret123'


              123456789101112131415161718192021222324
            
const user = { name: "Alice" };

// Add an enumerable property
Object.defineProperty(user, "age", {
  value: 30,
  enumerable: true
});

// Add a non-enumerable property
Object.defineProperty(user, "password", {
  value: "secret123",
  enumerable: false
});

// Using for...in to iterate over properties
for (let key in user) {
  console.log(key); // Outputs: name, age
}

// Using Object.keys to get enumerable property names
console.log(Object.keys(user)); // ['name', 'age']

// Non-enumerable property "password" is not included
console.log(user.password); // 'secret123'

Oliko kaikki selvää?

Kiitos palautteestasi!

Osio 1. Luku 5