(JS) pure function in array

1. Do not change the source array （ No side effects ）
2. Returns an array

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1. Arrat.concat(value)
   Merge arrays and return a new array , Do not change the original array （ And push be similar , Add... At the end of the array value）

	let array = [1,2,3,4,5];
	let x = array.concat([10,20])
		console.log(x) // [1,2,3,4,5,10,20]

2. Array.map(function)
   Traversal array , If return Output array value , Will become a new array , If judgment is used , The unqualified return false

let array = [1,2,3,4,5];
let x = array.map(result=>{
    
	return result*10;
});
console.log(x) //[10, 20, 30, 40, 50]

3. Array.filter(function)
   Filter array , Returns a value that meets the criteria

	let array = [1,2,3,4,5];
	let x = array.filter(num=>{
    
		return num>2
	})
	console.log(x)//[3, 4, 5]

4. Array.slice(start ,end)
   Copy array from start To end( barring end) Value , Returns a new array , No changes will be made to the source array

	let array = [1,2,3,4,5];
	let x = array.slice(2,3)
	console.log(array) //[1,2,3,4,5]
	console.log(x) //[3]