Manage multiple boolean flags

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;
};

Manage multiple boolean flags

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