You can use the typeof operator to check if a variable is undefined in JavaScript. For example:

if (typeof variableName === 'undefined') {
  // variable is undefined

To check if a variable is null, you can use a simple comparison:

if (variableName === null) {
  // variable is null

You can also check both in a single line using logical OR operator :

if(variableName === undefined || variableName === null) {
  // variable is undefined or null

Note that null is a value and undefined is a value as well as type.

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