// adder
const sum = (...nums) => {
 return nums.reduce((sum, num) => sum + num);
};
console.log(sum(1, 2, 3, 4, 10)); // 20
// accumulator
const accumulator = (...nums) => {
 return nums.reduce((acc, num) => acc * num);
};
console.log(accumulator(1, 2, 3)); // 6
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.

const array = [-1, 10, 6, 5];
const max = Math.max(...array); // 10
const min = Math.min(...array); // -1
1.
2.
3.

const array = [-1, 10, 6, 5];
const max = array.reduce((max, num) => (max > num ? max : num));
const min = array.reduce((min, num) => (min < num ? min : num));
1.
2.
3.

// url https://qianlongo.github.io/vue-demos/dist/index.html?name=fatfish&age=100#/home
// format the search parameters
{
 "name": "fatfish",
 "age": "100"
}
1.
2.
3.
4.
5.
6.

const parseQuery = () => {
 const search = window.location.search;
 let query = {};
 search
   .slice(1)
   .split("&")
   .forEach((it) => {
     const [key, value] = it.split("=");
     query[key] = decodeURIComponent(value);
   });
 return query;
};
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.

const parseQuery = () => {
 const search = window.location.search;
 return search
   .replace(/(^\?)|(&$)/g, "")
   .split("&")
   .reduce((query, it) => {
     const [key, value] = it.split("=");
     query[key] = decodeURIComponent(value);
     return query;
   }, {});
};
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.

/ url https://qianlongo.github.io/vue-demos/dist/index.html?name=fatfish&age=100#/home
// 1. First get the search parameter
const search = window.location.search; // ?name=fatfish&age=100
// 2. Remove the beginning "?" or ending "&".
search.replace(/(^\?)|(&$)/g, "");
// ?name=fatfish&age=100 => name=fatfish&age=100
// 3. Use reduce to collect parameters
// ...
1.
2.
3.
4.
5.
6.
7.
8.

const searchObj = {
 name: "fatfish",
 age: 100,
 // ...
};
const link = `https://medium.com/?name=${searchObj.name}&age=${searchObj.age}`;
// https://medium.com/?name=fatfish&age=100
1.
2.
3.
4.
5.
6.
7.

const stringifySearch = (search = {}) => {
 return Object.entries(search)
   .reduce(
     (t, v) => `${t}${v[0]}=${encodeURIComponent(v[1])}&`,
     Object.keys(search).length ? "?" : ""
   )
   .replace(/&$/, "");
};
const search = stringifySearch({
 name: "fatfish",
 age: 100,
});
const link = `https://medium.com/${search}`;
console.log(link); // https://medium.com/?name=fatfish&age=100
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.

const array = [1, [2, [3, [4, [5]]]]];
// expected output [ 1, 2, 3, 4, 5 ]
const flatArray = array.flat(Infinity); // [1, 2, 3, 4, 5]
1.
2.
3.

const flat = (array) => {
 return array.reduce(
   (acc, it) => acc.concat(Array.isArray(it) ? flat(it) : it),
   []
 );
};
const array = [1, [2, [3, [4, [5]]]]];
const flatArray = flat(array); // [1, 2, 3, 4, 5]
1.
2.
3.
4.
5.
6.
7.
8.

// Expand one layer by default
Array.prototype.flat2 = function (n = 1) {
 const len = this.length
 let count = 0
 let current = this
 if (!len || n === 0) {
   return current
 }
 // Confirm whether there are array items in current
 const hasArray = () => current.some((it) => Array.isArray(it))
 // Expand one layer after each cycle
 while (count++ < n && hasArray()) {
   current = current.reduce((result, it) => {
     result = result.concat(it)
     return result
   }, [])
 }
 return current
}
const array = [ 1, [ 2, [ 3, [ 4, [ 5 ] ] ] ] ]
// Expand one layer
console.log(array.flat()) // [ 1, 2, [ 3, [ 4, [ 5 ] ] ] ]  
console.log(array.flat2()) // [ 1, 2, [ 3, [ 4, [ 5 ] ] ] ]  
// Expand all
console.log(array.flat(Infinity))
console.log(array.flat2(Infinity))
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.

const array = [ 1, 2, 1, 2, -1, 10, 11 ]
const uniqueArray1 = [ ...new Set(array) ]
const uniqueArray2 = array.reduce((acc, it) => acc.includes(it) ? acc : [ ...acc, it ], [])
1.
2.
3.

const count = (array) => {
 return array.reduce((acc, it) => (acc.set(it, (acc.get(it) || 0) + 1), acc), new Map())
}
const array = [ 1, 2, 1, 2, -1, 0, '0', 10, '10' ]
console.log(count(array))
1.
2.
3.
4.
5.

// There is an object with many properties
const obj = {
 a: 1,
 b: 2,
 c: 3,
 d: 4,
 e: 5
 // ...
}
// We just want to get some properties above it to create a new object
const newObj = {
 a: obj.a,
 b: obj.b,
 c: obj.c,
 d: obj.d
 // ...
}
// Do you think this is too inefficient?
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.

const getObjectKeys = (obj = {}, keys = []) => {
 return Object.keys(obj).reduce((acc, key) => (keys.includes(key) && (acc[key] = obj[key]), acc), {});
}
const obj = {
 a: 1,
 b: 2,
 c: 3,
 d: 4,
 e: 5
 // ...
}
const newObj = getObjectKeys(obj, [ 'a', 'b', 'c', 'd' ])
console.log(newObj)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.

const reverseString = (string) => {
 return string.split("").reduceRight((acc, s) => acc + s)
}
const string = 'fatfish'
console.log(reverseString(string)) // hsiftaf
1.
2.
3.
4.
5.