Detect when your site is visible to users

document.onvisibilitychange = function () {
          document.visibilityState === "visible"
             ? (document.title = "👋 Hello")
             : (document.title = "😟 Bye!");
       };

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Spread Operator for Objects

Using the spread operator during an Object declaration will assign the properties of the referenced Object to the new Object.

Detect when your site is visible to users

Rest Operator for Objects

Now we can use rest on the properties of an Object. It allows us to explicitly extract certain named variables, and assign any uncalled variables into a catchall Object.

Extend Object javascript

Đếm có bao nhiêu items giống nhau trong một array

reduce javascript. Đếm có bao nhiêu items giống nhau trong một array