What is a flag variable?

问题

Recently I just came across this term and I had no idea as to what purpose it serves?

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

I Googled it but there were not any specific examples related to it (in the context of JavaScript).

回答1:

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 var
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!");

回答2:

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)

回答3:

Flag variables are the same for all languages, whether it's RUBY or Python or JavaScript or C++ or even Microsoft Small Basic.

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

来源：https://stackoverflow.com/questions/17402125/what-is-a-flag-variable