Javascript Check Null or Undefined §

In JavaScript, a value could be nullish or not nullish, and a value could be falsy or truthy.
Nullish values are a proper subset of falsy values:

 ╭─ nullish ──────╮ ╭─ not nullish ─────────────────────────────────╮
┌───────────┬──────┬───────┬───┬────┬─────┬──────┬───┬─────────┬─────┐
│ undefined │ null │ false │ 0 │ "" │ ... │ true │ 1 │ "hello" │ ... │
└───────────┴──────┴───────┴───┴────┴─────┴──────┴───┴─────────┴─────┘
 ╰─ falsy ───────────────────────────────╯ ╰─ truthy ───────────────╯

Check if value is nullish (undefined or null) §

Use one of the following depending on your coding style:

if (value == null)                         { /* value is nullish */ }
if (value === undefined || value === null) { /* value is nullish */ }
if (value == undefined)                    { /* value is nullish */ }
if ((value ?? null) === null)              { /* value is nullish */ }

Notes:

• The == operator works because it has a special case for null vs undefined comparison
• The === operator is more readable (opinion based), eqeqeq friendly and allows checking for undefined and null separately
• The first and third examples work identically, however the third one is rarely seen in production code
• The fourth example uses nullish coalescing operator to change nullish values to null for straight forward comparison

Check if value is not nullish §

if (value != null)                         { /* value is not nullish, although it could be falsy */ }
if (value !== undefined && value !== null) { /* value is not nullish, although it could be falsy */ }
if (value != undefined)                    { /* value is not nullish, although it could be falsy */ }
if ((value ?? null) !== null)              { /* value is not nullish, although it could be falsy */ }

Check if value is falsy §

Use the ! operator:

if (!value) { /* value is falsy */ }

Check if value is truthy §

if (value) { /* value is truthy */ }

Reference §

Is there a standard function to check for null, undefined, or blank variables in JavaScript? - Stack Overflow