What is a flag variable?

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 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. It is a variable you can use to 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 variable
var errors = 0;

for(var i = 0; i < 10; i++) {
  if(i == 6) {  // Your error condition
    errors++;
  }
}

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