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Manage multiple boolean flags

Written byPhuoc Nguyen
25 Feb, 2021
Sometimes a function has to deal with different conditions. It's not ideal to pass all of them as separated parameters.
To demonstrate the problem, let's say that we need to validate a password in a registration form. It's up to you to define differents rules for a strong password, but this example covers rules listed as following:
  • Should have at least 8 characters
  • Should contain at least one upper case character
  • Should contain at least one lower case character
  • Should contain at least one digit
As we imagine, each rule could be represented by a `boolean` parameter. The validator function looks like:
const validatePassword = (password, checkLength, checkUpperCase, checkLowerCase, checkDigit) => {
// returns `true` or `false`
// ...
The number of parameters can be increased quickly depending on the number of rules. Also, the implementation is a mess if we combine the rules together. To fix the problem, we introduce the rule flag by different numbers. Each number is a power of 2:
const RULE_LENGTH = 1 << 0; // 1
const RULE_UPPER_CASE = 1 << 1; // 2
const RULE_LOWER_CASE = 1 << 2; // 4
const RULE_DIGIT = 1 << 3; // 8
We can create a combination of rules via the bitwise (`|`) operator. Using `|` with different rule(s) will generate an unique number, because a number can be written as the sum of different power of 2.
The validator function can be shortent as following, where `rule` is a number:
// The default rule should cover all the cases

const validatePassword = (password, rule = RULE_DEFAULT) => {};
You just need to pass the required rules instead of indicating all of them:
// Only check the length rule
validatePassword(password, RULE_LENGTH);

// Check length and upper case rules
validatePassword(password, RULE_LENGTH | RULE_UPPER_CASE);
From the `rule` number, we can know whether a given rule is enabled by using the `&` operator:
const validatePassword = (password, rule = RULE_DEFAULT) => {
// `RULE_LENGTH` is enabled
if (rule & RULE_LENGTH && password.length < 8) {
return false;

if (rule & RULE_UPPER_CASE && password === value.toUpperCase()) {
return false;

if (rule & RULE_LOWER_CASE && password === value.toLowerCase()) {
return false;

if (rule & RULE_DIGIT && password.search(/[0-9]/) < 0) {
return false;

return true;

See also

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