What do undefined and null mean?

In JavaScript, undefined and null both represent the concept of "none": 1. undefined represents an uninitialized variable or a non-existent property. When a variable is declared but no value is assigned to it, The value of this variable is undefined. When accessing a property that does not exist in the object, the returned value is also undefined; 2. null represents an empty object reference. In some cases, the object reference can be set to null to release it. memory occupied.

# Operating system for this tutorial: Window10 system, Dell G3 computer.

In JavaScript, undefined and null both represent the concept of "nothing", but they have some semantic differences.

1. undefined:

   • In JavaScript, undefined represents an uninitialized variable or a non-existent property.
   • When a variable is declared but no value is assigned to it, the value of the variable is undefined.
   • When accessing properties that do not exist in the object, the returned value is also undefined.
   • When the function does not have a clear return value, it returns undefined by default.

   For example:

   var x; // 声明变量但未赋值，x 的值为 undefined
   var obj = {};
   console.log(obj.property); // 不存在的属性返回 undefined
   function foo() {
     // 没有明确返回值，默认返回 undefined
   }

2. null:

   • null represents an empty object reference, usually Used to mean "nothing" or "empty".
   • If you need to explicitly indicate that a variable is empty, you can assign it to null.
   • In some cases, the reference to an object can be set to null in order to release the memory it occupies.

   For example:

   var y = null; // 将变量赋值为 null，表示为空
   var obj = null; // 将对象引用设置为 null，释放内存

To sum up, undefined means undefined or uninitialized, while null means empty or nothing. value. In actual development, you usually choose to use undefined or null based on specific semantics and requirements.

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