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JavaScript null vs undefined

In JavaScript null and undefined are rather strange values, both serve a very similar purpose, which is to indicate the absence of a value.

Null

Null is used to assign a reference to an object that you will no longer need or, directly, you want to have the variable declared but initialize it with a value that you still do not know what it will be exactly. In all these cases the best thing to do is to assign a null value.

  var miVariable = null;
  console.log(miVariable);

//log null

undefined

For undefined means that the variable is declared but its value has not yet been defined.

  var miVariable
  console.log(miVariable);

//log null

Both values are values of type false, so if you do a non-strict comparison you will get true undefined, which means that the variable is declared but its value has not yet been defined.

if (null == undefined) {
  return true
 }
//log true

and if you do a strict comparison, because they are not really the same, it returns a false:

if (null === undefined) {
   return true
 }
return false
//log false

Learned by leyaim-jimenez on Sep 21, 2021

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