Can you please tell me how to create a variable in JavaScript using a condition if it has not been previously declared?

For example:

if(!a){ a = 'test'; }

In this case, the error in the console Uncaught ReferenceError: a is not defined

  • 2
    if (typeof a == 'undefined') - Alexey Shimansky
  • @ Alexey Shimansky, why not the answer? :-) - Grundy
  • And you can simply declare a variable :-) - Grundy
  • one
    @Pavel, and in the end what problem are you trying to solve? - Grundy
  • one
    @pavel Why don't you just want to declare a variable in advance? If you have checkboxes of an unknown amount, then their state is better not to add separate variables at all, but to an array or an object. - tutankhamun

2 answers 2

 console.log(this.a); if(!('a' in this)) { this.a = 'test'; } console.log(this.a);

if it has not been previously announced

And if declared, but not assigned?

    Write like this: if(a==undefined){ var a = 'test'; } if(a==undefined){ var a = 'test'; }

    I incorrectly wrote in the comment and deleted that comment. The point is that before executing the code, the interpreter runs through the code previously in order to form an object-context for executing the code, to fill it. in this case, this context object is called Global , because the code is global. Regarding variables, first the variables declared through var are assigned the value undefined , and then during the processing of the code these values ​​change to what the code stands for ( "test" ). If you look at your code

     if(!a){ a = 'test';

    }

    , the interpreter did not see the var to the left of a = 'test' ; and the Global variable object ( VO ) with the variable a inside it was not entered into the context object Global . All variables are pre-entered into the variable object ( VO - variable object ) as undefined , if the interpreter "saw" before them var .

    Here is a good article about the object-context of the code execution http://corporate.tuenti.com/en/dev/blog/functions-and-execution-contexts-in-javascript

    I give you a plus, so that the minus block it)))

    • @Pavel - damn it. minus again put you. I can not block the second one). making fun of people. I look, people got angry because of myok jokes. minuses scribbling). I do not care for the minuses). - Dimon
    • when using a==undefined will be an Uncaught ReferenceError exception : a is not defined when a is not declared, And using var inside if is meaningless, it is simpler to declare a variable before use. - Grundy
    • @Grundy - I have this code worked in chrome without errors. if you write as I wrote, then first (PRELIMINARY) the variable "a" is entered into the variable object VO (which is in the Global context object) as undefined, the comparison (a == undefined) will give the variable and the string will be assigned the string "test ". if, however, put a condition, for example, var a = 10; , the "test" is not assigned. - Dimon
    • and if var a; then assigned - Grundy
    • right. You, too, offer any option. I don’t know how to get around that moment that a variable must necessarily be inserted into the variable object before executing the code. - Dimon