Reduce method of array

Array of reduce Method

reduce() Method to execute one provided by you in order for each element in the array reducer function , Every time it runs Pass the calculation result of the previous element as a parameter , Finally, the result Summarize to a single return value .

When the callback function is executed for the first time , non-existent “ The result of the last calculation ”, So if you need to index the callback function from the array to 0 The element of starts execution , You need to pass Initial value Otherwise, the index number is 0 The element of will be As an initial value initialValue, The iterator will execute from the second element

grammar ：

//  Arrow function writing 
	Array.reduce((perviousValue,currentValue) => {
     *** })
	Array.reduce((perviousValue,currentValue,currentIndex) => {
     *** })
	Array.reduce((perviousValue,currentValue,currentIndex,array) => {
     *** })
	Array.reduce((perviousValue,currentValue,currentIndex,array) => {
     *** },initialValue)
//  Normal function writing 
	...
	Array.reduce(function(perviousValue,currentValue,currentIndex,array){
     *** },initialValue)

Parameters ：

Array reduce Method , There are four parameters ：

• perviousValue： The return value of the last calculation result
• currentValue： The element being processed in the array . If an initial value is specified , Then the value is array index 0 The elements of , Otherwise 1
• currentIndex： The index of the element being processed in the array . If an initial value is specified , The starting index is 0, Otherwise 1
• array： Array of traversed objects

---

• initialValue（ Optional ）： This parameter is used as the parameter when calling the function for the first time previousValue Value ,
• If an initial value is specified initialValue, be currentValue The first element of the array will be used ;
• otherwise perviousValue The first element of the array will be used , and currentValue The second element of the array will be used

Example ：

1. Calculates the sum of all elements of an array

    //  Defines an initial value （ You can also write directly after the parameter ）
	const initialValue = 0; 
	//  An array is defined 
	const sum = [0,1,2,3,4,5,6];  
	//  Start the loop calculation 
	//  Receive a return value 
	const sumTotal = sum.reduce(function(previousValue,currentValue){
      
		return perviousValue + currentValue
	},initialValue)
	//  Print sum 
	console.log('sumTotal：' + sumTotal)  // sumTotal：21 

2. Accumulate the values of the object array

Be careful ： If you want to accumulate the values contained in the object array , must Provide initialValue Initial value

	//  Create initial values 
	const initialValue = 0;
	//  Define an array 
	const arr = [{
    x:1},{
    x:2},{
    x:3},{
    x:4}];
	//  Start circularly accumulating under each object in the array  x
	const sumTotal = arr.reduce(function(previousValue,currentValue){
    
		return previousValue + currentValue.x  //  object .x  Get every one of them  x  attribute 
	},initialValue)
	//  Print sum 
	console.log('sumTotal:' + sumTotal);  // sumTotal：10