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.
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.
undefined:
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 }
null:
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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