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.Combination | Number |
---|---|

``RULE_LENGTH`` | 1 |

``RULE_UPPER_CASE`` | 2 |

``RULE_LENGTH | RULE_UPPER_CASE`` | 3 |

``RULE_LOWER_CASE`` | 4 |

``RULE_LENGTH | RULE_LOWER_CASE`` | 5 |

``RULE_UPPER_CASE | RULE_LOWER_CASE`` | 6 |

``RULE_LENGTH | RULE_UPPER_CASE | RULE_LOWER_CASE`` | 7 |

... | ... |

``RULE_LENGTH | RULE_UPPER_CASE | RULE_LOWER_CASE | RULE_DIGIT`` | 15 |

The validator function can be shortent as following, where

``rule``

is a number:`// The default rule should cover all the cases`

const RULE_DEFAULT = RULE_LENGTH | RULE_UPPER_CASE | RULE_LOWER_CASE | RULE_DIGIT; // 15

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;

};