In JavaScript, you can use the typeof operator to check if a variable is Exists Or Is Defined. The typeof operator returns the type of a variable or an expression. If a variable is not defined, it will return “undefined”.

For example, you can use the following code to check if a variable myVariable is defined:

if (typeof myVariable !== 'undefined') {
  console.log('myVariable is defined');
} else {
  console.log('myVariable is not defined');
}

Another way to check if a variable is defined is to use the in operator, which returns a boolean value indicating whether an object has a given property.

if ('myVariable' in window) {
  console.log('myVariable is defined');
} else {
  console.log('myVariable is not defined');
}

Additionally, you can use the !== undefined or !== void 0 to check if a variable is defined.

if (myVariable !== undefined) {
  console.log('myVariable is defined');
} else {
  console.log('myVariable is not defined');
}
if (myVariable !== void 0) {
  console.log('myVariable is defined');
} else {
  console.log('myVariable is not defined');
}

Note:

  • ‘window’ is the object representing the global scope, you can use it to check if a variable is defined in the global scope.
  • ‘void 0’ is undefined , it’s a way to get the undefined value.

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