JavaScript check if variable exists (is defined/initialized)

Question

Which method of checking if a variable has been initialized is better/correct? (Assuming the variable could hold anything (string, int, object, function, etc.))

Answer 1

You could use a try...catch block like the following:

var status = 'Variable exists'

try {
  myVar
} catch (ReferenceError) {
  status = 'Variable does not exist'
}

console.log(status)

A disadvantage is you cannot put it in a function as it would throw a ReferenceError

function variableExists(x) {
  var status = true
  try {
    x
  } catch (ReferenceError) {
    status = false
  }
  
  return status
}

console.log(variableExists(x))

Edit:

If you were working in front-end Javascript and you needed to check if a variable was not initialized (var x = undefined would count as not initialized), you could use:

function globalVariableExists(variable) {
  if (window[variable] != undefined) {
    return true
  }
  
  return false
}

var x = undefined

console.log(globalVariableExists("x"))

console.log(globalVariableExists("y"))

var z = 123

console.log(globalVariableExists("z"))

Edit 2:

If you needed to check if a variable existed in the current scope, you could simply pass this to the function, along with the name of the variable contained in a string:

function variableExists(variable, thisObj) {
  if (thisObj[variable] !== undefined) {
    return true
  }
  
  return false
}

class someClass {
  constructor(name) { 
    this.x = 99
    this.y = 99
    this.z = 99
    this.v = 99
    
    console.log(variableExists(name, this))
  }
}

new someClass('x')
new someClass('y')
new someClass('z')
new someClass('v')
new someClass('doesNotExist')