How to use for in in js

for ... in ... in JavaScript is used to iterate over the enumerable properties of an object, it iterates over the keys (property names) instead of the values. Steps: Declare a variable key to store the current key. Specify the object to be traversed after the in keyword. The loop will traverse each enumerable property of the object and execute the code within the curly braces on each key.

How to use for ... in JavaScript

for ... in is a loop statement , used to iterate over the enumerable properties of an object. It iterates over the keys (property names) of the object, not the values.

Syntax

<code>for (let key in object) {
  // 对每个键执行代码
}</code>

Steps

1. Declare a variable:Declare a variablekey, used to store the current key.
2. Specify the object: Specify the object to be traversed after the in keyword.
3. Loop: The loop will continue to iterate through each enumerable property of the object.
4. Execution code: For each key, the code within the curly brackets will be executed.

Example

<code>const person = {
  name: "John Doe",
  age: 30,
  city: "New York"
};

for (let key in person) {
  console.log(key); // 输出：name, age, city
}</code>

Notes

• for ... in The loop traverses the keys of the object, not the value.
• It will traverse enumerable properties. Non-enumerable properties (such as Symbol values) are skipped.
• The loop sequence is undefined. It may not traverse the properties in the order they were added.
• For arrays, the for ... in loop will iterate over the array's indices, not the element values.
• For inherited properties, the for ... in loop will iterate over the properties from the parent object.

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