Home  >  Article  >  Web Front-end  >  Why Does jQuery Use `typeof variable === \"undefined\"` for Global Variables and `variable === undefined` for Local Variables?

Why Does jQuery Use `typeof variable === \"undefined\"` for Global Variables and `variable === undefined` for Local Variables?

Linda Hamilton
Linda HamiltonOriginal
2024-10-31 22:58:29541browse

Why Does jQuery Use `typeof variable ===

Understanding the Distinction: variable === undefined vs. typeof variable === "undefined"

In the jQuery Core Style Guidelines, two methods are proposed to verify if a variable is defined:

  • Global Variables: typeof variable === "undefined"
  • Local Variables and Properties: variable === undefined

Why this difference?

The explanation lies in the behavior of these operators when dealing with undeclared variables. For undeclared variables, typeof foo will return "undefined" as a string. However, the identity check foo === undefined will raise the error "foo is not defined."

Contrast this with local variables. Since they are explicitly declared somewhere, attempting the identity check variable === undefined will not trigger an error.

Therefore, jQuery uses the typeof operator for global variables, which may or may not be declared, to avoid potential errors. For local variables and properties, where declaration is ensured, the identity check is preferred due to its simplicity.

The above is the detailed content of Why Does jQuery Use `typeof variable === \"undefined\"` for Global Variables and `variable === undefined` for Local Variables?. For more information, please follow other related articles on the PHP Chinese website!

Statement:
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn