undefined is a property of the global object. That is, it is a variable in global scope. The initial value of undefined is the primitive value undefined.
In modern browsers (JavaScript 1.8.5 / Firefox 4+), undefined is a non-configurable, non-writable property, per the ECMAScript 5 specification. (Even when this is not the case, avoid overriding it.)
A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned.
null表示当前变量指向一个空对象
The value null is written with a literal: null. null is not an identifier for a property of the global object, like undefined can be. Instead, null expresses a lack of identification, indicating that a variable points to no object. In APIs, null is often retrieved in a place where an object can be expected but no object is relevant.
undefined 指变量未初始化
undefinedis a property of the global object. That is, it is a variable in global scope. The initial value ofundefinedis the primitive valueundefined.In modern browsers (JavaScript 1.8.5 / Firefox 4+),
undefinedis a non-configurable, non-writable property, per the ECMAScript 5 specification. (Even when this is not the case, avoid overriding it.)A variable that has not been assigned a value is of type
undefined. A method or statement also returnsundefinedif the variable that is being evaluated does not have an assigned value. A function returnsundefinedif a value was notreturned.null表示当前变量指向一个空对象
The value
nullis written with a literal:null.nullis not an identifier for a property of the global object, likeundefinedcan be. Instead,nullexpresses a lack of identification, indicating that a variable points to no object. In APIs,nullis often retrieved in a place where an object can be expected but no object is relevant.