javascript: Pure Function

// Pure function.const multiplyNumbers = (x,y) => x * y;multiplyNumbers(2, 3);> 6

// Impure function.let a = 4;const multiplyNumbers = (b) => a *= b;multiplyNumbers(3);console.log(a); // first time: 12> 12multiplyNumbers(3);console.log(a); // second time: 36> 36// Mutates external variable so it isn't pure.

// Impure function.addNumberarr = (arr, num) => {arr.push(num);};const testArr = [1,2,3];addNumberarr(testArr, 4);console.log(testArr);>[1, 2, 3, 4]// Mutates input array so it isn't pure.

// pure version of above.addNumberarr = (arr, num) => {return [...arr, num];};const testArr = [1,2,3];addNumberarr(testArr, 4);> [1, 2, 3, 4]

arr.reduce()arr.map()arr.filter()arr.concat()arr.slice()arr.each()arr.every()... - spread syntax 

arr.splice()arr.push()arr.sort()Math.random()