How to use for in in js

The for...in statement is used to traverse the enumeration properties of the object. Usage: 1. Traverse attribute names: for (key in object). 2. Get the attribute value: for (key in object) {console.log(object[key])}. 3. Set the attribute value: for (key in object) {object[key] = "Updated"}. Note: Only enumerable properties are traversed, the order is undefined.

Usage of for...in in JavaScript

for...in is a statement in JavaScript , used to iterate over the enumerable properties of an object. It returns an array of property names that can be used to get or set the property's value.

Syntax:

<code>for (key in object) {
  // 操作
}</code>

Variables:

• key: Attribute name.
• object: The object to be traversed.

Usage:

1. Traverse object properties:

<code>const person = { name: "John", age: 30 };
for (let key in person) {
  console.log(key); // 输出："name", "age"
}</code>

2. Get attribute value:

<code>for (let key in person) {
  console.log(person[key]); // 输出："John", "30"
}</code>

3. Set attribute value:

<code>for (let key in person) {
  person[key] = "Updated";
}
// person 对象的属性值现在都被更新为 "Updated"</code>

Notes:

• for...in traverses the enumerable properties of the object. Non-enumerable properties (such as those on the prototype chain) are not traversed.
• for...in may traverse properties in an undetermined order. Therefore, if you need to traverse in a specific order, you can use the Object.keys() or Object.getOwnPropertyNames() method.

Example:

<code>const array = [1, 2, 3];
for (let key in array) {
  console.log(key); // 输出：0, 1, 2
}
// 注意：数组的属性名是它的索引，而不是实际的数字值。</code>

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