Guowei Lv

9 minute read

null and undefined null is a language keyword and undefined is a predefined global object. undefined represents a system level, unexpected or error-like absence of value. nullrepresents program-level, normal or expected absence of value. When writing programs yourself always use null. global object When the JavaScript interpreter starts, it creates a new global object and gives it an initial set of properties. In client-side JavaScript, the window object serves as the global object.