JavaScript isset() Equivalent

If you are familiar with PHP’s isset() function, you know that it is used to determine if a variable has been set and is not NULL. If you do not use the isset function, you risk throwing a PHP error saying that the variable is undefined.

The isset() equivalent in JavaScript is the following code:

if (typeof variable_name !== 'undefined') {
  // variable_name has been set, so run this code

If you want to check if a JavaScript object’s property exists, run the following code:

if (object_name.hasOwnProperty('action')) {
  // the object_name object has the action property, so run this code

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