Array flattening
Concept: Used to “flatten” an array of nested layers into a one-dimensional array
Method 1: Convert a two-dimensional array to a one-dimensional array using concat
How it works: Returns a new array by extending the operator, expanding the inside of the array, and concatenating two strings
let a = [12.3.45[6.7.8]]
console.log(a) // [12, 3, 45, Array(3)]
console.log([].concat(... a))// [12, 3, 45, 6, 7, 8]
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Method 2: Use the array method join and string method split to flatten the array
Principle: Through the join method will be converted into a dot separated string, in the use of split converted string into string array, through. The map method converts the internal string to a numeric type
let a = [4.1.2.3.6[7.8[3.9.10[4.6.11]]]];
let b = a.join(', ').split(', ').map(Number)
console.log(b) // [4, 1, 2, 3, 6, 7, 8, 3, 9, 10, 4, 6, 11]
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Method three: through the regular method and json. stringify method and array method
Principle: First convert array to string use string matching regular rule to replace all ‘[“]’ and method 2 similar split is mainly to convert string into array, map string array into numbers
let a = [4.1.2.3.6[7.8[3.9.10[4.6.11]]]];
let c = JSON.stringify(a).replace(/\[|\]/g.' ').split(', ').map(Number);
console.log(c) // [4, 1, 2, 3, 6, 7, 8, 3, 9, 10, 4, 6, 11]
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Method 4: function recursion
Principle: Determine whether the current value is an array, if it is an array, recursive call
let d = [];
let fn = arr= > {
for (let i = 0; i < arr.length; i++) {
if (Array.isArray(arr[i])) {
fn(arr[i]);
} else {
d.push(arr[i]);
}
}
}
fn(a)
console.log(d) // [4, 1, 2, 3, 6, 7, 8, 3, 9, 10, 4, 6, 11]
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Method 5: Use reduce to flatten the array
Principle: It is mainly through the sequential execution of reduce to determine whether the current object is an array or not. If it is an array, it will perform a recursive function to flatten all internal arrays (similar to method 4).
let a = [4.1.2.3.6[7.8[3.9.10[4.6.11]]]];
function flatten(arr) {
return arr.reduce((result, item) = > {
console.log(result, item); // If you look at the result, you will see that each array is split and extracted
return result.concat(Array.isArray(item) ? flatten(item) : item); } []); };console.log(flatten(a)) // [4, 1, 2, 3, 6, 7, 8, 3, 9, 10, 4, 6, 11]
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Method 6: ES6 added method Flat ()
let a = [4.1.2.3.6[7.8[3.9.10[4.6.11]]]];
let e = a.flat() // The array is converted from a two-dimensional array to a one-dimensional array
console.log(e) // [4, 1, 2, 3, 6, 7, 8, Array(4)]
let f = a.flat(2) // Passing a 2 converts a two-layer nested array to a one-dimensional array
console.log(f) // [4, 1, 2, 3, 6, 7, 8, 3, 9, 10, Array(3)]
let g = a.flat(Infinity) // Infinity uses this keyword to convert a contained so-called array to a one-dimensional array
console.log(g) // [4, 1, 2, 3, 6, 7, 8, 3, 9, 10, 4, 6, 11]
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Array to heavy
Concept: Remove duplicate values from an array
Method 1: cyclic traversal interception
Principle: By comparing the current value in the array each time through the loop, the current value is deleted and the index is decrement by one, which changes the original array
let arr = [1.23.1.1.1.3.23.5.6.7.9.9.8.5];
function removeDuplicatedItem(arr) {
for (var i = 0; i < arr.length - 1; i++) {
for (var j = i + 1; j < arr.length; j++) {
if (arr[i] === arr[j]) {
arr.splice(j, 1);//console.log(arr[j]);j--; }}}return arr;
}
let arr2 = removeDuplicatedItem(arr);
console.log(arr); // [1, 23, 3, 5, 6, 7, 9, 8]
console.log(arr2); // [1, 23, 3, 5, 6, 7, 9, 8]
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Method 2: Use indexOf()
Principle: It is similar to method 1 to determine whether the index of the element’s first occurrence in the array is equal to the index of the loop
let arr = [1.23.1.1.1.3.23.5.6.7.9.9.8.5];
function rep2(arr) {
for (var i = 0; i < arr.length; i++) {
// console.log(arr.indexOf(arr[i])); You can print the index of the first occurrence of the current element
if(arr.indexOf(arr[i]) ! == i) { arr.splice(i,1);// The element after the array length is reduced by 1 moves forward
i--;// Array subscript rollback}}return arr;
}
let arr2 = rep2(arr);
console.log(arr); // [1, 23, 3, 5, 6, 7, 9, 8]
console.log(arr2); // [1, 23, 3, 5, 6, 7, 9, 8]
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Method 3: Use the new array and indexOf() methods
If the indexOf the current element in the array is equal to the indexOf the loop, add it to the new array
let arr = [1.23.1.1.1.3.23.5.6.7.9.9.8.5];
function rep(arr) {
let ret = [];
for (var i = 0; i < arr.length; i++) {
if(arr.indexOf(arr[i]) == i) { ret.push(arr[i]); }}return ret;
}
let arr2 = rep(arr);
console.log(arr); // [1, 23, 1, 1, 1, 3, 23, 5, 6, 7, 9, 9, 8, 5]
console.log(arr2); // [1, 23, 3, 5, 6, 7, 9, 8]
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Method 4: Use empty objects
Principle: Using objects to record the stored elements in the new array without changing the original array is similar to method 3
let arr = [1.23.1.1.1.3.23.5.6.7.9.9.8.5];
let o = {};
let arr2 = [];
for (var i = 0; i < arr.length; i++) {
var k = arr[i];
if(! o[k]) { o[k] =true; arr2.push(k); }}console.log(arr); // [1, 23, 1, 1, 1, 3, 23, 5, 6, 7, 9, 9, 8, 5]
console.log(arr2); // [1, 23, 3, 5, 6, 7, 9, 8]
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Method 5: Filter method
Principle: Search the current element index position is equal to the current element index value, is true returns, if the current element index is not equal to the current index, it has appeared, does not return. The original array is unchanged
let arr = [1.23.1.1.1.3.23.5.6.7.9.9.8.5];
let arr2 = arr.filter(function (element, index, array) {
return array.indexOf(element) === index;
});
console.log(arr); // [1, 23, 1, 1, 1, 3, 23, 5, 6, 7, 9, 9, 8, 5]
console.log(arr2); // [1, 23, 3, 5, 6, 7, 9, 8]
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Method 6: Use the include method
Similar to indexOf, this index checks whether the current element exists and does not alter the array if it does not exist
let arr = [1.23.1.1.1.3.23.5.6.7.9.9.8.5];
function rep() {
let res = [];
for (let i = 0; i < arr.length; i++) {
if(! res.includes(arr[i])) res.push(arr[i]); }return res;
}
console.log(arr); // [1, 23, 1, 1, 1, 3, 23, 5, 6, 7, 9, 9, 8, 5]
console.log(rep()); // [1, 23, 3, 5, 6, 7, 9, 8]
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Method 7: Es6 new data structure new Set() method
Principle: Members of a new Set() are unique and cannot be repeated
let arr = [1.23.1.1.1.3.23.5.6.7.9.9.8.5];
let arr2 = new Set(arr)
console.log(arr); // [1, 23, 1, 1, 1, 3, 23, 5, 6, 7, 9, 9, 8, 5]
console.log(arr2) // {1, 23, 3, 5, 6,... }
console.log(rep()); // [1, 23, 3, 5, 6, 7, 9, 8]
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