What is a flag variable?

Recently I came across flag variables, but I have no idea what they do.

I am little unsure about when to use a flag variable and how to go about it.

I Googled it but there weren't any specific examples related to my context (of JavaScript).



Flag Variables Defined and Uses says:

A flag variable, in its simplest form, is a variable you define to have one value until some condition is true, in which case you change the variable's value. Using flag variable(user defined variable, not a predefined) you can control the flow of a function or statement, allowing you to check for certain conditions while your function progresses.

As an example:

// errors is the flag var
var errors = 0;

for(var i = 0; i < 10; i++)
  if(i == 6) // your error condition

if(errors)  // is the flag "up"? (i.e. > 0)
  alert("There was a problem!");


A flag is a variable used to have some value until some condition becomes true,then we change it to false and print the output (Initially flag considered as true)


Flag variables are the same for all languages, whether it's RUBY, Python, Javascript or C++.

A flag variable is usually given one value, 0 or 1, True or False. It's used as a Boolean variable where the result toggles between 0 (False) and 1 (True) or as used by the programmer. Some prefer flag = 0 and change it to flag = 1 in the program to perform an action.


No matter what language you are using, the concept of flag variables is always the same: a variable that stores a boolean value. A boolean value is one which is always either true or false or is either 0 or 1 (depending on the language and programmer's personal preference).

Flag variables (also called boolean variables, or often just flags) are used to indicate a condition which can only be either of the two boolean values. The value of the variable can toggle depending on events in the program.

A JavaScript example:

var isRaining = false; // It is either raining or not raining.

var statusCode = true; // Status codes may be boolean values.


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