reduce usage introduction
1. Introduction to reduce grammar
array.reduce((prev, item, index, arr)=> {
/***/
}, initialValue)
Parameters:
parameter one callback function, which contains four parameters:
| prev | If initialValue is provided, it is; if initialValue is provided, it is the first item of the array |
|---|---|
| item | The array element currently being processed by the array loop |
| index | the index of the current currentValue |
| array | The array on which to perform the reduce operation |
The second parameter initialValue is the initial value, the initial value can be provided or not provided. Provide an initial value, item starts from the first item of the array, if no initial value is provided, item starts to execute from the second item, and the corresponding first prev is the value of the first item in the array Return value
:
Return the last execution result
Two, example analysis
let arr = ['h','i','n','a']
let str = arr.reduce((prev, item, index, arr)=> {
console.log(prev, item, index, arr)
return prev + item
}, 'C')
console.log(str) // China
var arr = [1, 2, 3, 4];
var sum = arr.reduce(function (prev, cur, index, arr) {
console.log(prev, cur, index);
return prev + cur;
});
console.log(arr, sum);
print result
1 2 1
3 3 2
6 4 3
[ 1, 2, 3, 4 ] 10
3. Matters needing attention
-
reduce is a method for accumulating operations on arrays, and return must be added to return the data of the accumulation operation. In this way, prev can get the result of the last execution, otherwise it is undefined
-
When the reduce operation is performed on an empty array and the initial value is not provided, the reduce will report an error. The error message is as follows:
4. Specific use
let arr = [1, 2, 2, 2, 3, 3, 4, undefined, undefined, null]
arr.reduce((prev, item) => {
return prev.includes(item) ? prev : prev.concat(item);
}, [])
// [1, 2, 3, 4, undefined, null]
let arr = [1,2,3,4,4,1]
let newArr = arr.reduce((pre,cur)=>{
if(!pre.includes(cur)){
return pre.concat(cur)
}else{
return pre
}
},[])
console.log(newArr);
2. Array flattening multi-dimensional to one-dimensional
array dimensionality reduction
let arr = [0,[1],[2, 3],[4, [5, 6, 7]]];
let reduceArray = function (arr) {
return arr.reduce((prev, item) => {
return prev.concat(Array.isArray(item) ? reduceArray(item) : item)
}, [])
}
reduceArray(arr)
//[0, 1, 2, 3, 4, 5, 6, 7]
let arr = [[0, 1], [2, 3], [4,[5,6,7]]]
const newArr = function(arr){
return arr.reduce((pre,cur)=>pre.concat(Array.isArray(cur)?newArr(cur):cur),[])
}
console.log(newArr(arr));
2D down to 1D
let arr = [[0, 1], [2, 3], [4, 5]]
let newArr = arr.reduce((pre,cur)=>{
return pre.concat(cur)
},[])
console.log(newArr);
打印结果[0, 1, 2, 3, 4, 5]
3. Array accumulation
let arr = [1, 2, 3, 4]
arr.reduce((prev, item) => {
return prev + item;
})
// 10
4. Count the number of occurrences of each element in the array
var arr = ["one", "two", "three", "six", "four", "five", "six"];
var newArr = arr.reduce((pre, cur) => {
console.log(cur, pre);
if (cur in pre) {
pre[cur]++;
} else {
pre[cur] = 1;
}
return pre;
}, {
});//这里注意初始值要默认赋予空对象，不然会报错
console.log(newArr);
print result
打印结果
one {
}
two {
one: 1 }
three {
one: 1, two: 1 }
six {
one: 1, two: 1, three: 1 }
four {
one: 1, two: 1, three: 1, six: 1 }
five {
one: 1, two: 1, three: 1, six: 1, four: 1 }
six {
one: 1, two: 1, three: 1, six: 1, four: 1, five: 1 }
{
one: 1, two: 1, three: 1, six: 2, four: 1, five: 1 }
5. Object attribute summation
var result = [
{
subject: 'math',
score: 10
},
{
subject: 'chinese',
score: 20
},
{
subject: 'english',
score: 30
}
];
var sum = result.reduce(function(prev, cur) {
return cur.score + prev;
}, 0);
console.log(sum)
打印结果 60