Question 287971 in Javascript Question posted in
A very good W3school tutorial can be found here.

What are some best practices for JavaScript variables?

I am starting out with JavaScript and am having problems with the confusing operation of variables. I read that var was to be discouraged because it allows unexpected changes so let and const should replace var. I have been struggling with a program that produces the wrong results and I have narrowed the problem down to my use of a variable. I have determined that if I use let, the code fails but it works fine with var. I get that the problems is due to the scope of the let command but I would like to have some guidance on the best practice of how to make this simple sample work with let rather than var.

Here is the code that gets an undefined error:

    const yearBorn = 1970;
    console.log(yearBorn);
    if (yearBorn <= 2020) {
        console.log("Less than");  
        let baseYearCalc = 2021;
        console.log(baseYearCalc);
    } 
        console.log(baseYearCalc);

Output is:
1970
Less than
2021
baseYearCalc is not defined

Please let me know how someone who knows what they were doing would solve this problem.
Thanks, Larry

Tags:

2

Answers


  1. 0

    so when you use var that variable is hoisted. Essentially it is available outside the scope of your if block.

       if (yearBorn <= 2020) {
        console.log("Less than");  
        let baseYearCalc = 2021;     // making a variable here in this scope
        console.log(baseYearCalc);
    }                                // It goes out of scope here
        console.log(baseYearCalc);   // and when you try to access it here it errors.

    Solution is to define it outside the if block

       let baseYearCalc = 2021; 
   if (yearBorn <= 2020) {
        console.log("Less than");  
        console.log(baseYearCalc);
    }                                
        console.log(baseYearCalc);   // Now it is available here as well
    Login or Signup to reply.
  2. 0

    Question: What should baseYearCalc be if yearBorn is not less than or equal to 2020? That’s the fundamental problem here, you only declare the variable at all when the condition is true.

    The trick here is to remember that, generally, declarations using let and const after an open brace, {, end with the next closing brace, }. If you want the variable to exist beyond that, you need to declare them in the outer scope that needs them, even if you give them a value later.

    For example, in your case, you could do:

    const yearBorn = 1970;
let baseYearCalc;               // Declared here
console.log(yearBorn);
if (yearBorn <= 2020) {
    console.log("Less than");  
    baseYearCalc = 2021;        // Assigned here
    console.log(baseYearCalc);
} 
console.log(baseYearCalc);      // Still exists here thanks to being declared outside the `if` block's scope

    This is still flawed: baseYearCalc has no value if the if statement isn’t true. To fix this, you can give it a default value in the original let, or assign a different value in an else attached to the if.

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search