Javascript isset() equivalent

Comprehending existence checks

Existential dilemma for a Variable?: typeof operator

The typeof operator is your friend when dealing with uncertain existence of a variable.

// This is safe - just like asking: "Who are you?" instead of "Are you John Doe?"
if (typeof whoKnows !== 'undefined') {
  // We found someone who's not undefined!
}


Can't trust the inheritance: hasOwnProperty method

If you prefer direct ownership over inherited properties, hasOwnProperty will serve you well:

// More picky than 'in', only trusts direct ownership
if (myObj.hasOwnProperty('prop')) {
  // 'prop' is literally myObj's legit kid 🧒👌
}


Generous heritage inspector: in operator

To check for a property existent anywhere, whether in direct ownership or inheritance, go for the in operator:

// Doesn't care about direct ownership, loves all properties alike
if ('prop' in myObj) {
  // 'prop' spotted in myObj's vicinity 🚀
}


Coding like hipsters: Optional Chaining (?.) & Nullish Coalescing (??)

Embrace the modern JavaScript syntax of Optional Chaining and Nullish Coalescing to handle uncertainties:

// Oh please dear existence, give me some value or undefined, no error am I seeking!
let value = myObj?.prop;

// Dear null or undefined, should you dare to exist, become 'default' you will!
let secureValue = myObj?.prop ?? 'default';
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Need a sturdy isset(): Libraries & NPM packages

For deeply nested property checks, consider using utilities from libraries such as Lodash or Underscore:

// Going deep with Lodash
_.has(myObj, 'deeply.nested.prop');
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The isset-php package is a robust PHP-style isset() function port for JavaScript, handling checks on multiple arguments altogether:

// This is like buying PHP's isset() a JavaScript ticket 
const isset = require('isset-php');
if (isset(() => myObj.prop1, () => myObj.prop2)) {
  // All properties exist and we're good to go!
}


The global search: Checking global variables

Global variables are accessible via the window object, hence their existence is verifiable through it:

// Caught the global variable yet? Sherlock 'window' Holmes to the rescue!
if ('myGlobalVar' in window) {
  // Global variables can't hide from me 🕵️
}


Custom built isset(): Arrow function

No one said you couldn't craft your own isset() function using an arrow function:

// All you need: a portable isset() because JavaScript won't complain
const isset = (prop) => typeof prop !== 'undefined';
if (isset(myObj?.prop)) {
  // Live long and prosper with myObj.prop
}


Daredevil check: Negation of the in operator

Remember, the in operator can be negated, much like a double-edged sword:

// 'prop': Not in myObj? Let's find out!
if (!('prop' in myObj)) {
  // 'prop' isn't partying in this myObj club 🕺🚫
}


Do remember it doesn't confirm prop is null or undefined. It merely signifies the absence of recognition as a property.

Decoding these methods

Checking associative arrays

Ensure you're looking in the right place when it comes to associative arrays:

// For hunting a specific key in myArray
if (myArray.hasOwnProperty('key')) { 
  // Found the 'key' to our problem! 🔑💡
}


Distrust in falsy friends

Watch your back when dealing with falsy values. They might make you fall in the trap of non-existent existence:

if (myObj.prop) { 
  // Falsy values lurk around, making this unreliable 😨❌
}


Consistency: The ace in the hole

Maintaining a consistent way of checking property presence fends off errors and pain in general:

if (typeof someArray['key'] !== 'undefined') {
  // Because 'key' might like playing the game of hide and seek!
}


In-depth knowledge is power

The more, the merrier! Deep dive into provided references for an oceanic understanding of typeof, hasOwnProperty, in, and the nuances of JavaScript existence checks.