NaN and undefined in Javascript

Great question! NaN (Not-a-Number) and undefined are different in JavaScript, and understanding their differences is crucial. Here's a breakdown:

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🔹 Difference Between NaN and undefined

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🔥 Key Cases You Should Remember

1. When does NaN appear?

   • Trying to convert a non-numeric string into a number:

       parseFloat("hello"); // NaN
       Number("abc"); // NaN
     

   • Mathematical operations that don't make sense:

       Math.sqrt(-1); // NaN (in JavaScript, not complex numbers)
       0 / 0; // NaN
     

   • Performing operations with NaN results in NaN:

       let x = NaN;
       console.log(x + 5); // NaN
     

2. When does undefined appear?

   • Accessing an undeclared variable:

       console.log(myVar); // ReferenceError: myVar is not defined
     

   • Accessing an uninitialized variable:

       let x;
       console.log(x); // undefined
     

   • Accessing an object property that doesn't exist:

       let obj = {};
       console.log(obj.prop); // undefined
     

   • Function without return:

       function doSomething() {}
       console.log(doSomething()); // undefined
     

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✅ Important Things to Remember

• NaN is actually a number, but it means "invalid number".

• undefined is its own type and usually means "missing value".

• NaN is NOT equal to itself (NaN !== NaN), so you should check it using:

    Number.isNaN(value); // ✅ Best way to check for NaN
  

• Avoid using isNaN(value) directly, because it tries to convert values to numbers:

    isNaN("hello"); // true (confusing)
    Number.isNaN("hello"); // false (correct)
  

• Use typeof value === "undefined" to check for undefined:

    let a;
    console.log(typeof a === "undefined"); // true
  

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TL;DR

• Use Number.isNaN(value) to check for NaN.

• Use typeof value === "undefined" to check for undefined.

• NaN usually means an invalid number operation.

• undefined usually means a missing or uninitialized value.