no-nil.md

Forbid the use of null and undefined

Accessing properties or calling methods of a variable with value null or undefined is source of a lot of errors. The use of those values is hard to ignore in JavaScript, but avoiding their explicit use in your code may help restrain the number of occurrences of this kind of error.

This rule:

• Prohibits the use of null and undefined in any way, except in a comparison that directly compares with either value
• Enforces that all variables are initialized, as they would otherwise evaluate to undefined
• Enforces that every function returns a value, that is not either value

Fail

let a; // Equivalent to `let a = undefined;`
let a = null;

// No return statement, so it will return `undefined`
function foo() {}
function foo() {
  a + b;
}

function foo() {
  return null;
}

function foo(a=null) {
  return a;
}

function foo() {
  if (bar) {
    return;
  }
  return 1;
}

Pass

const a = 1;

if (value === null || value === undefined) {}

function foo(a=1) {
  return a;
}

function foo() {
  if (bar) {
    return 2;
  }
  return 1;
}

Option: Switch Default

You may want switch statements with a default clause that returns a default, non-nil value.

Fail: Switch

The following function will trigger the fp/no-nil as having no return value.

const someFunction = (state, action) => {
    switch (action.type) {
      case 'SOME_ACTION': {
        return assoc('anything', true, state);
      }
      default: {
        return state;
      }
    }
  }

Pass: Switch

You can turn this off via a configuration in your .eslintrc file by setting allowSwitchDefault to true:

"rules": {
  "fp-jxl/no-nil": ["error", {
      "allowSwitchDefault": true
  }],

Now it'll allow it to pass as long as your switch returns a non-nil value.

Option: Class Constructor

For class constructors, they typically do not return values.

Fail: constructor

class Sup {
    constructor() {
    }
}

Pass: constructor

To allow a constructor that doesn't return a value, add this option to the rule in your .eslintrc:

"rules": {
  "fp-jxl/no-nil": ["error", {
      "allowConstructors": true
  }],