Common APIS of Array objects

The Array object

Generate instance objects

var a = new Array(a)Copy the code

attribute

a.length  / / the length
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judge

Array.isArray(a)  // true / false
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Used to determine whether a value is an array

methods

value

Array.valueof()
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Return the array itself

Array.toString()
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Returns the string form of an arrayCopy the code

The enumeration

Keys (), values(), entries()Copy the code

conversion

Array.from(object, mapFunction, thisValue);

var setObj = new Set(["a"."b"."c"]);
var objArr = Array.from(setObj);
objArr[1] = ="b";  // true


var arr = Array.from([1.2.3].x= > x * 10);
// arr[0] == 10;
// arr[1] == 20;
// arr[2] == 30;
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The method is to convert array-like objects (that is, objects with the Length attribute) and traversable objects into true arrays.

Array.of();

let arr1 = Array.of(1.2.3); / / [1, 2, 3]
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The method is to convert a set of values into an array. If the number of arguments is 0, an empty array is returned

Add or delete

Array.push(1.2)
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Adds one or more elements to the end of the array and returns the new array length

Array.pop()
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Removes and returns the last element of the array

Array.unshift(1.2)
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Adds one or more elements to the beginning of an array and returns the new array length

Array.shift()
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Removes the first item of the array and returns the value of the first element

Splicing/interception

Array.concat(arr1,arr2...)
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Combine two or more arrays to generate a new array. The original array remains the same.

Array.join(",")
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Concatenates each item of an array with the specified character to form a string

Array.slice(start , end)
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Returns a new array containing elements in the ARR from start to end (excluding the element)

arr.splice(index , howmany , item1,..... ,itemX)Copy the code

Add/remove items to/from the array and return the array of deleted items

Index: required. Integer to specify where items are added/removed, and negative numbers to specify positions from the end of the array.

Howmany: Required. Number of items to delete. If set to 0, the project will not be deleted.

item1, … , itemX: Optional. The new item added to the array.

The sorting

Array.reverse()
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Invert the array. The original array changes.

Array.sort();

// From small to big
arr.sort((a,b) = >{
    return a-b
})
//② From large to small
arr.sort((a,b) = >{
    return b-a
})
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Sort array elements

The query

indexOf(element,start)
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Returns the position of the given element’s first occurrence in the array, or -1 if none occurs. You can accept a second argument that indicates the start of the search

lastIndexOf(element)
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Returns the last occurrence of the given element in the array, or -1 if none occurred.Copy the code

find();

let Arr = [1.2.5.7.5.9];
let result1 = Arr.find(function(currentValue,index,arr){			
    return currentValue>5;
});
console.log(result1); / / 7
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Method returns the value of the first element of the array that passes the test.

findIndex();
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FindIndex is similar to find, except that it returns the index by default, or -1 if there is no element that matches the criteria

array.fill(value,  start,  end);
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The fill() method fills all elements of an array from the start index to the end index with a fixed value. Does not include terminating indexes.

arr.includes(searchElement, fromIndex);

[1.2.3].includes(2);
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An iterative approach

//5 iterative methods:ForEach (), map(), filter(), some(), every()// These methods have the same syntax and do not change the original array.
currentValue: required. Current element index: optional. The index value of the current element.arr: optional. The array object to which the current element belongs.ForEach () : Iterates through the array. This method returns no value.
Arr.forEach(function(currentValue, index, arr){
    console.log(index+"--"+currentValue+"--"+(arr === Arr));		
})

//map() : refers to "mapping", the method returns a new array
var arr2 = arr.map(function(currentValue){
    return currentValue*currentValue;
});

//filter() : The "filter" function creates a new array
var result1 = arr.filter(function(currentValue){
    return currentValue>5;
});

//every() : checks whether every item in the array meets the condition. Only all items meet the condition returns true.
var arr = [1.4.6.8.10];
var result1 = arr.every(function(currentValue){
 return currentValue< 12;
});
console.log(result1);  // true

//some() : checks whether there are any items in the array that satisfy the condition, and returns true as long as one item satisfies the condition.
var arr = [1.4.6.8.10];
var result1 = arr.some(function(currentValue){
     return currentValue> 10;
});
console.log(result1);  // false
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Merge method

Reduce () and reduceRight ()Copy the code

Method starts with the first item in the array and iterates to the end. ReduceRight (), on the other hand, starts from the last item of the array and traverses forward to the first item.

Reduce () syntax:

arr.reduce(function(total , cur , index , arr){//do something}, initialValue)
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ReduceRight () syntax:

arr.reduceRight(function(total , cur , index , arr){//do something}, initialValue)
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Total: required. The initial value, or the return value at the end of the calculation. Cur: Necessary. Current element. Index: Optional. The index of the current element. Arr: Optional. The array object to which the current element belongs. InitialValue: Optional. The initial value passed to the function.

var result2 = arr.reduce(function(total,cur,index,arr){	
    console.log("total:"+total+",cur:"+cur+",index:"+index);
 return total+cur;
},10);
console.log("Result:"+result2);
/ / output
// total:10,cur:1,index:0
// total:11,cur:2,index:1
// total:13,cur:3,index:2
// total:16,cur:4,index:3
// total:20,cur:5,index:4
// Result: 25
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