I am writing a script the utilizes a switch statement. When I declare the variables, they have a default boolean value of true, correct? Well, not so much when being utilized in a switch statement.

Here is the JavaScript I have: /

Basically, what I am trying to do is ask the user what type of list-style they prefer based upon that data that is entered into a window.prompt() method. If they enter in a 1, 2, or 3, they will get a list based upon the directions in the prompt. But, if they do not enter in a valid integer, the variable validInput is set with a boolean value of false.

From here, an if statement is ran to check whether the validInput variable has a boolean value of true, if it does, then it outputs the values of the many variables to the screen, if not, it outputs text saying "Invalid Choice".

Why is it that the code will recognize validInput as only having a value of false in the if statement? When it is only assigned the value of false if a different value is entered into the prompt window? To get this program to run properly I have to explicitly define the validInput value as true in each switch case.

Why is this? Can someone explain, please?

Thank you! Aaron

I am writing a script the utilizes a switch statement. When I declare the variables, they have a default boolean value of true, correct? Well, not so much when being utilized in a switch statement.

Here is the JavaScript I have: http://codepen.io/anon/pen/IDLqd/

Basically, what I am trying to do is ask the user what type of list-style they prefer based upon that data that is entered into a window.prompt() method. If they enter in a 1, 2, or 3, they will get a list based upon the directions in the prompt. But, if they do not enter in a valid integer, the variable validInput is set with a boolean value of false.

From here, an if statement is ran to check whether the validInput variable has a boolean value of true, if it does, then it outputs the values of the many variables to the screen, if not, it outputs text saying "Invalid Choice".

Why is it that the code will recognize validInput as only having a value of false in the if statement? When it is only assigned the value of false if a different value is entered into the prompt window? To get this program to run properly I have to explicitly define the validInput value as true in each switch case.

Why is this? Can someone explain, please?

Thank you! Aaron

• javascript
• switch-statement

Share Improve this question Follow asked Mar 29, 2014 at 17:57 Aaron BrewerAaron Brewer 3,6771919 gold badges5151 silver badges7979 bronze badges 8

• 4 A declared but unset variable defaults to undefined, which is "falsey", i.e. undefined == false, undefined != true, but is not actually false, i.e. undefined !== false, undefined !== true. – Dave Commented Mar 29, 2014 at 17:59
• Isnt that because in your case the execution will go to the default and set validinput as false as it is not initialized beforehand? – pj013 Commented Mar 29, 2014 at 18:01
• 2 Incidentally, you are using mas and multiple vars in your declarations; pick one style or the other, you can't use both simultaneously. – Dave Commented Mar 29, 2014 at 18:03
• 1 undefined == null but undefined !== null. Undefined means literally that; it is not defined to be anything. Whereas null is a sort of blank object (which makes more sense if you have a C or Java background). In JavaScript the distinction isn't really necessary, but can be useful for example when checking which parameters were sent to a function. Just remember that undefined, null, false, 0 and '' are all "falsey" (as well as a bunch of strings like '0'), but they are all different. – Dave Commented Mar 29, 2014 at 18:07
• 1 And no, you don't need to explicitly set all variables to something. If you're happy with the default undefined (which is sometimes OK), just the var a; statement is enough. Just remember that it's really more like var a = undefined; – Dave Commented Mar 29, 2014 at 18:08

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2 Answers 2

Sorted by: Reset to default 4

Javascript is a dynamic language and there is nothing like a default boolean value.

When you define a variable without a value it's default value is always undefined:

var variable; // variable is undefined

So you have to set the value:

var variable = true;
// or
var variable = false;

If you want to toggle this boolean value, you can do the following:

variable = !variable;

You are checking if the input is valid with

if (validInput == true) {
  // Your code
}

The more mon way of doing this would be

if (validInput) {
  // Your code
}

What's the difference between these two?

The first checks if validInput is equal to true - nothing else will do (well, pretty much nothing else - you're using == rather than ===, which can sometimes have surprising results because of javascript's type conversion algorithm, but that's another question altogether).

To understand the second, you need to understand javascript's concept of "truthiness". If you put a value into the condition of an if statement, then javascript decides is it's "truth-y" or "false-y", and acts accordingly.

true is truthy, as is any non-zero number, any non-empty string, and any object. Other things are falsey, including false, 0, "", null and undefined.

The last of these is probably the most relevant to you, as variables are undefined until you set them to something.