index.md

null

null is a primitive type that represents the absence of value. It is common in DOM functions that perform a search to return null when they are not successful.

const button = document.querySelector('button');
const config = localStorage.getItem('config');

// Check if button is not null before calling click() method
if (button !== null) {
  button.click();
}

// Using truthy check to call click() method
if (button) {
  button.click();
}

// Shortened syntax for truthy check
if (button) button.click();

// Optional chaining operator to safely call click() method
button?.click();

// Log the type of null, which is 'object'
console.log(typeof null);

• Here, button is a reference to a button element in the HTML document, and config is a variable storing settings retrieved from local storage.

• In the conditional examples, we're checking if button is not null before calling the click() method. This is necessary to avoid an error if button is null. In TypeScript, the ! operator is used to assert that button is not null before calling click().

• The last console.log() statement demonstrates that typeof null returns 'object', which is a quirk of JavaScript.

undefined

undefined means that a variable or property has been declared but doesn't have a value yet.

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

const data = {};
console.log(data.name);

• In the code above, we declare a variable total without assigning it a value. When we log total to the console, it outputs undefined, which means that total has been declared but does not yet have a value assigned to it.

• Similarly, we declare an object data with no properties initialized. When we try to access the property name of the data object and log it to the console, it also outputs undefined. This is because the property nome has not been defined within the data object.