This is the 20th day of my participation in the August More Text Challenge

Two, interception splicing

2.1 slice

The slice() method returns a new array object that is a shallow copy of the original array determined by begin and end. The original array will not be changed.

arr.slice([begin[, end]])

Note: The copied array contains the start subscript element, not the end subscript element

  • @paramsBy default, end is copied to the end of the array. You can use negative values to select elements from the end of the array.
  • @return: returns the copied subarray
  • Whether to change the original array: don’t change
var arr = [1.2.3.6.7.8];
var res = arr.slice(1.4);    
console.log(res);  / /,3,6 [2]
console.log(arr);  / /,2,3,6,7,8 [1]
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2.2 the concat

The concat() method is used to merge two or more arrays. This method does not change an existing array, but returns a new array.

var new_array = old_array.concat(value1[, value2[, ...[, valueN]]])

  • @params: multiple arbitrary items, which can be arrays or single items
  • @return: Returns the new array after the merge
  • Whether to change the original array: don’t change
var arr = [1.2.3];
var res = arr.concat(1[0.0]);    
console.log(res);  / /,2,3,1,0,0 [1]
console.log(arr);  / / [1, 2, 3]
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Third, find judgment

3.1 includes

The includes() method is used to determine whether an array contains a specified value, returning true if it does and false otherwise.

arr.includes(valueToFind[, fromIndex])

  • @params: Parameter 1 is mandatory, indicating the element to be queried. Parameter 2 is optional, indicating that the element to be queried starts from the specified position. (If the value is negative, the value is set to 0 if the negative value exceeds the length of the array.)
  • @return: Returns a Boolean value
  • Whether to change the original array: don’t change
var arr = [1.2.3];
var res = arr.includes(2);  

console.log(res);  // true
console.log(arr);  / / [1, 2, 3]
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3.2 the find

The find() method returns the value of the first element in the array that satisfies the provided test function. Otherwise return undefined.

arr.find(function(item, index, array){}, thisArg)

  • @paramsThisAr represents the object to which this points when the callback is executed
  • @return: The first element or undefined that satisfies the condition
  • Whether to change the original array: don’t change
const array = [5.12.8.130.44];
const found = array.find(element= > element > 10);

console.log(found);  / / 12
console.log(array);  // [5, 12, 8, 130, 44]
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3.3 findIndex

The findIndex() method returns the index of the first element in the array that satisfies the provided test function. Returns -1 if no corresponding element is found.

arr.findIndex(callback[, thisArg])

  • @paramsThisAr represents the object to which this points when the callback is executed
  • @return: the first index that meets the condition, or -1
  • Whether to change the original array: don’t change
const array = [5.12.8.130.44];
const found = array.findIndex(element= > element > 10);

console.log(found);  / / 1
console.log(array);  // [5, 12, 8, 130, 44]
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3.4 indexOf

The indexOf() method returns the first index in the array where a given element can be found, or -1 if none exists.

arr.indexOf(searchElement[, fromIndex])

  • @params: Mandatory: indicates the elements to be queried. Optional: indicates that the elements to be queried start from the specified location
  • @return: Returns the index of the search item if it exists, or -1 if it does not
  • Whether to change the original array: don’t change
var arr = [1.2.3];
console.log(arr.indexOf(2));  / / 1
console.log(arr.indexOf(8));  // -1
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3.5 lastIndexOf

The lastIndexOf() method returns the index of the last element in the array specified (that is, a valid JavaScript value or variable), or -1 if none exists. Look forward from the back of the array, starting at fromIndex.

arr.lastIndexOf(searchElement[, fromIndex])

  • @params: Mandatory: indicates the elements to be queried. Optional: indicates that the elements to be queried start from the specified location
  • @return: Returns the index of the search item if it exists, or -1 if it does not
  • Whether to change the original array: don’t change
var arr = [1.2.3.2.2.3.4];
console.log(arr.lastIndexOf(3));  / / 5
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