What's the difference between `undefined` and `null`?

Before understanding the differences between undefined and null we must understand the similarities between them.

• They belong to JavaScript's 7 primitive types.

let primitiveTypes = ['string', 'number', 'null', 'undefined', 'boolean', 'symbol', 'bigint'];

• They are falsy values. Values that evaluated to false when converting it to boolean using Boolean(value) or !!value.

console.log(!!null); //logs false
console.log(!!undefined); //logs false

console.log(Boolean(null)); //logs false
console.log(Boolean(undefined)); //logs false

Ok, let's talk about the differences.

• undefined is the default value of a variable that has not been assigned a specific value. Or a function that has no explicit return value ex. console.log(1). Or a property that does not exist in an object. The JavaScript engine does this for us the assigning of undefined value.

let _thisIsUndefined;
const doNothing = () => {};
const someObj = {
  a: 'ay',
  b: 'bee',
  c: 'si',
};

console.log(_thisIsUndefined); //logs undefined
console.log(doNothing()); //logs undefined
console.log(someObj['d']); //logs undefined

• null is "a value that represents no value". null is value that has been explicitly defined to a variable. In this example we get a value of null when the fs.readFile method does not throw an error.

fs.readFile('path/to/file', (e, data) => {
  console.log(e); //it logs null when no error occurred
  if (e) {
    console.log(e);
  }
  console.log(data);
});

When comparing null and undefined we get true when using == and false when using ===.

console.log(null == undefined); // logs true
console.log(null === undefined); // logs false