when to use var?

Question

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Javascript: is using ‘var’ to declare variables optional?

how come in this code example below when i put var before the variables it does not work, however if i remove the var it works? i thought you had to use var when you were creating new variables.

function myfunction () {
    if (document.getElementById('ramyes').checked) {
      var itischecked = "yes"
    } else if (document.getElementById('ramno').checked) {
      var itischecked = "no"
    }
}   



function display () {
  myfunction()

  if (itischecked == "yes") {
    alert ("it sure is");
  } else if (itischecked == "no") {
    alert ("it is not");
  }
}

Matti Virkkunen · Answer 1 · 2013-01-01T23:25:42.417

If you use var, the variable is only visible inside the current function (it's a local variable). If you don't use var when first setting the variable, it creates a global variable, which is visible to all functions.

Defining global variables by just setting them is usually frowned upon because most of the time you want local variables (in your case you should return your "itischecked" value from the function, instead of storing it in a global variable), and setting a variable without var looks like a mistake. It is also an error in strict mode (which everybody should use, always). To explicitly define a global variable in (browser) JS, use window.variableName, or var in the global scope (outside any functions).

But before you do that, think carefully whether that's a good idea. The more global variables you have, the higher the chance you will get a name collision when multiple scripts use the same variable name.

ah i see!, good explanation :) i was wondering why my script kept returning undefined on a website project of mine, its only a small one for college so i think global variables will be fine :) — Ryan Conway, Jan 01 '13 at 23:20
@Ryan: School is about learning. If you're gonna learn something, learn to do things the right way... — Matti Virkkunen, Jan 01 '13 at 23:21

score 1 · Answer 2 · answered Jan 01 '13 at 23:25

You should use var when you create a variable, but if you want to use the same variable in two functions, the variable has to be created in a scope outside both functions:

var itischecked;

function myfunction () {
  if(document.getElementById('ramyes').checked) {
    itischecked = "yes"
  } else if(document.getElementById('ramno').checked) {
    itischecked = "no"
  }
}   

function display () {
  myfunction();
  if (itischecked == "yes") {
    alert ("it sure is");
  } else if (itischecked == "no") {
    alert ("it is not");
  }
}

If you create a variable inside a function, that becomes a local variable. It is only visible inside that function, and goes away when you return from the function.

In your case you should rather use the return value of the function instead of putting a value in a variable. That way it is easier to follow how the data flows in the program, and you don't need to create a global variable:

function myfunction () {
  if(document.getElementById('ramyes').checked) {
    return "yes"
  } else if(document.getElementById('ramno').checked) {
    return "no"
  }
}   

function display () {
  var itischecked = myfunction();
  if (itischecked == "yes") {
    alert ("it sure is");
  } else if (itischecked == "no") {
    alert ("it is not");
  }
}

score 0 · Answer 3 · answered Jan 01 '13 at 23:23

Just to add to the previous answer that suggests you shouldn't create global variables: to avoid doing so, you need in some way to make a larger scope so that variables can be shared between your two functions. That's where the var comes. For instance:

function myProgramme() {
    var itischecked;

    function myfunction() {
        if (document.getElementById('ramyes').checked) {
            itischecked = "yes"
        }
        else if (document.getElementById('ramno').checked) {
            itischecked = "no"
        }
    }

    function display() {
        myfunction()
        if (itischecked == "yes") {
            alert("it sure is");
        }
        else if (itischecked == "no") {
            alert("it is not");
        }
    }
}

(Although in your specific example you could more simply have myfunction pass itischecked to display as a return value.)

when to use var?

3 Answers3