javascript check if variable exists

In Javascript, the common way to check whether a variable exists is if (variable ){.... // variable exists} else {.... // variable does not exist}. When variable is undefined | null, that is, the variable does not exist. However, if variable is 0,

*1, Variable type:JavaScript is a weak type, a variable can store any type, and the run-time type is variable;-> variables can be converted to type;*/ /*2, Variable declaration:*/var i;var index;var i, index;var i = 0, index = 2;/*Variable

Directory1 History 2 clarifying misunderstandings 3 basic concepts 4 features 5 different from Java 6 development toolsHistoryProbably in 1992, a company called Nombas began developing an embedded scripting language called C-minus (C-minus-minus, or

JavaScript is weak type, variables can store any type, and the type is variable during running The Code is as follows: /*1. Variable type:JavaScript is weak, variables can store any type, and the type is variable during running;-> Variable type

Recently, due to work needs, I began to develop some Python things. because I have been using Javascript before, I will unconsciously use some Javascript Concepts, syntaxes, and so on. I think it is necessary to summarize the differences between

Compared with Javascript, Python has recently started to develop some Python things due to work needs. Since Javascript has been used for a long time, it will unconsciously use some Javascript Concepts, syntaxes, and so on, it often falls into a

In contrast to C + +, the use of JavaScript in memory has made us more focused on the writing of business logic in development. However, with the continuous complexity of the business, single page applications, mobile HTML5 applications and node.js

Compared with C/C ++, the processing of the JavaScript we use in memory has made us pay more attention to writing business logic in development. However, as the business continues to become more complex and the development of single-page

In contrast to C + +, the use of JavaScript in memory has allowed us to focus more on the writing of business logic in development. But as business continues to complicate the development of single-page applications, mobile HTML5 applications, and

JavaScript memory optimizationIn contrast to C + +, the use of JavaScript in memory has allowed us to focus more on the writing of business logic in development. But as business continues to complicate the development of single-page applications,