Tan's Notes

Closures is (a fancy term for) a function that remembers outside things that are used inside const createPrinter() {   const name = 'Alex'   const printName = () => {     console.log(name)   }   return printName } const myPrinter = createPrinter() myPrinter() // Alex This function that was return from from createPrinter function is called a closure . // Here is printName Closure is a combination of a function and the environment where it was declared .

setState is asynchronous action - If you need to use the updated state to do something, you should use a callback function in setState , so it will call after the state is updated: this.setState({data: newData}, () => console.log(this.state.data)) - If you wanna update the state with anything else, for example, set the '"title" to be "Hello", you can use the object in setState function, like this: this.setState({title: 'Hello'}) - But if you have to take the current state and use it to evaluate what to update, you should pass a function to setState function, so you will have access to the previous state and previous props. It will guarantees that you always get the latest update of the state or props: this.setState((prevState, prevProps) => {     return {count: prevState.count + 1}   } )

You can not bind arrow function They automatically get what called lexical scoping - which mean, they bind " this " context to the place where they were defined in the first place It's easier to write class method by using arrow function, so you can stop worry about bind " this "

Caveat: If you .fill() an Array with an object, all elements refer to the same instance (i.e., the object isn’t cloned) So if you do something like: let a = Array(3).fill([]) // create an array named a and fill it with 3 empty array a[2].push('poop') // push new value to the element array at index 2 expect [ [], [], [2]] But because all element refer to the same instance, so the result is: [[2], [2], [2]] More details here:  https://2ality.com/2018/12/creating-arrays.html