JS pure function

Pure function

1. This function when the same input value , Always produce the same output . The output of the function is independent of the context state of the current running environment .

Pure functions are like functions in Mathematics , Function maps input parameters to return values , in other words , For each set of inputs , There is an output .

function test(a){
    
    console.log(a*a);
}
test(10);
test(10);
test(10);
test(10);

2. The running process of this function does not affect the running environment , That is, no side effects （ Such as trigger event 、 launch http request 、 Print /log etc. ）.

 A function execution process produces externally observable changes to , Then say , This function has side effects . and   Pure functions do not produce any observable side effects , That is, it cannot change any external state .

var num = 18;
function compare(x){
    
    return x > num;
}
console.log(compare(20))
//  Not a pure function 

//  Revise it 
var num = 18;
function compare(x){
    
    return x > 18;
}
console.log(compare(20))

//  Pass two parameters 
var num = 18;
function compare(x,num){
    
    return x > num;
}
console.log(compare(20,num))
//  Not using external num  But within its scope num

3. If it's a reference type ？

var arr = [];
function add(_arr){
    
    var obj = {
    name:'cts'};
    _arr.push(obj);
    return _arr;
}
add(arr);
console.log(arr);
//  Not a pure function   Reference type 


//  Revise it 
var arr = [{
    name:'bailibn'}];
function add(_arr){
    
    var obj = {
    name:'cts'};
    //  Create a new arr
    var newArr = [];
    //  take arr Clone the contents of 
    for(var i = 0;i<_arr.length;i++){
    
        //  Clone by cloning 
        newArr[i] = _arr[i];
    }
    newArr.push(obj);
    return newArr;
}
console.log(add(arr))

//  To simplify the 
function add(x,y){
    
    return x+y;
}
var num1 = 1;
var num2 = 2;
add(num1,num2);//  Pure function 

var GArr = [];
function add(arr){
    
    arr.push(1);
    return arr;
}
add(gArr);//  Not a pure function 


//  A lot of bullshit , What's the use of pure functions ？

Simply speaking , That is, when the output of a function is not affected by the external environment , At the same time, it does not affect the external environment , This function is a pure function , That is, it only focuses on logical operations and mathematical operations , The same input always gets the same output .

javascript Built in functions have many pure functions , There are also many non pure functions .

Pure function ：

Array.prototype.slice

Array.prototype.map

String.prototype.toUpperCase

Meromorphic function ：

Math.random

Date.now

Array.ptototype.splice

Pure function action

JavaScript It is easy to create global variables in , These variables can be accessed in all functions . This is also a cause bug Common causes of , Because any part of the program may modify global variables, resulting in abnormal bold style of function behavior Pure functions are very easy to unit test , Because you don't need to think about context , Just consider the inputs and outputs