Basic usage of three JS high-order functions --filter---map---reduce

1. fillter

Array of filter Method , You need to pass a callback function as a parameter , The parameters of the callback function n, Namely filter The array elements you get every time you cycle through the array , The callback function will return a Boolean, If true, Represents that the element will be placed in the final filter In the returned array .
filter It doesn't change the original array , It returns the filtered new array .

let newarr = arr.filter(function(n){
    
					return n<0
				}
			)
			// Arrow function writing 
let newarr = arr.filter(n => n<0)

2. map

Array of map Method , You need to pass a callback function as a parameter , The parameters of the callback function n, Namely map The array elements you get every time you cycle through the array , The return value of the callback function will be put into the final map In the returned array .map The return is an array .

let newarr = arr.map(function(n){
    
				return n*2
			})
// Arrow function writing 
let newarr = arr.map(n => n*2)

3. reduce

reduce Returns a value

let arr = [1,2,3,4]
	let sum = arr.reduce(function(initvalue,n){
    
		//reduce The second parameter 0 Will act as initvalue Pass to callback function 
		// If reduce The second parameter is not passed , Then the first element of the array will be taken as the first initvalue Pass to callback function ,
		// also reduce From the index 1 Where we started  callback  Method 
		//n It's every traversal sum Get element value 
		// Of the callback function return The value of will be as initvalue Pass to the next callback function execution 
		return initvalue+n
	})
	console.log(sum);