JavaScript arrays using collections

This time I will bring you JavaScriptarray usagecollection,JavaScript arraywhat are the precautionsfor using collections, the following is a practical case, let’s take a look take a look.

1.join() Convert all elements in the array into strings and join them together

var a=[1,2,3,4];
a.join(); //"1,2,3,4"

2.reverser() Reverse the order of the elements in the array and return the array in reverse order.

var a[1,2,3,4];
a.reverse(); //[4,3,2,1]

3.sort() Sorts the elements in the array and returns the sorted array.
When sort() is called without parameters, the array elements are sorted in alphabetical order.

var a=['ant','Bug','cat','Dog'];
a.sort(); //["Bug", "Dog", "ant", "cat"]
a.sort(function(s,t){
   var s1=s.toLowerCase();
   var t1=t.toLowerCase();   if(s1<t1) return -1;   if(s1>t1) return 1;   return 0});//["ant", "BUg", "cat", "Dog"]

4.concat() creates and returns a new array whose elements include the elements of the original array calling concat() and each parameter of concat(). If any of these arguments is itself an array, the array elements are concatenated, not the arrays themselves.

var a=[1,2,3];a.concat(4,5);// [1, 2, 3, 4, 5]a.concat([4,5]);// [1, 2, 3, 4, 5]a.concat([4,5],[6,7]);// [1, 2, 3, 4, 5, 6, 7]a.concat([4,5],[6,[8,7]]);// [1, 2, 3, 4, 5, 6,[8,7]]

5.slice() returns a slice or subarray of the specified array. Its two parameters specify the start and end positions of the fragment respectively. The returned array contains all data elements between the position specified by the first argument and all up to but not including the position specified by the second argument.
If only one parameter is specified, the returned array contains all elements from the beginning to the end of the array.
If a negative number appears in the parameter, it represents the position relative to the last element in the array. For example: parameter -1 specifies the last element, and -3 specifies the third to last element.
Note that slice() will not modify the called array.

var a=[1,2,3,4,5];a.slice(0,2);//[1, 2]a.slice(3);//[4, 5]a.slice(1,-1);//[2, 3, 4]a.slice(-3,-2);//[3]

6.splice() A general method to insert or delete elements in an array. Unlike slice() and concat(), splice() modifies the calling array. Note: splice() and slice() have very similar names, but their functions are essentially different.
splice() can delete elements from an array, insert elements into an array, or complete both operations at the same time. Array elements after the insertion or deletion point have their index values ​​increased or decreased as necessary, so the rest of the array remains contiguous. The first parameter of splice() specifies the starting position of insertion and/or deletion. The second parameter specifies the number of elements that should be deleted from the array. If the second parameter is omitted, all elements from the starting point to the end of the array will be deleted. splice() returns an array of deleted elements, or an empty array if no elements were deleted.

var a=[1,2,3,4,5,6,7,8];a.splice(4);//返回[[5, 6, 7, 8]],a是[1, 2, 3, 4]a.splice(1,2)//返回[2, 3]，a是[1, 4, 5, 6, 7, 8]a.splice(1,1);//返回[2],a是 [1, 3, 4, 5, 6, 7, 8]

7.push() and pop()
push() adds one or more elements to the end of the array.
pop() deletes the last element of the array.

8.unshift() and shift()
unshift() adds one or more elements to the head of the array.
shift() deletes the first element of the array.

9.toString() and toLocaleString()

[1,2,3].toString();//"1,2,3"[1,[2,'c']].toString();//"1,2,c"

toLocaleString() is the localized version of the toString() method. It calls the element's toLocaleString() method to convert each array element to a string, and concatenates these strings using localized delimiters to generate the final string.

The function passed is the first parameter of forEach(), and then forEach() calls the function with three parameters: the array element, the index of the element and the array itself.

var data=[1,2,3,4,5];//计算数组元素的和值var sum=0;
data.forEach(function(value){
sum+=value
});  
sum //15//每个数组元素的值加1data.forEach(function(value,index,arr){
arr[index]=value+1;
});
data  //[2, 3, 4, 5, 6]

of the function. Note: The function passed to mao() should have a return value. map() returns a new array and does not modify the original array. If the original array is a sparse array, the sparse array returned is the same way, with the same length and the same missing elements.

var a=[1,2,3];var b=a.map(function(value){return value*value;
});
b// [1, 4, 9]

var a=[1,2,3,5];var b=a.filter(function(value){return value>2;
});
b  // [3, 5]

The logical judgment of the array, they apply the specified function to the array elements Determine, return true or false.
every() means that all elements in the array meet the filtering conditions, then return true.
some() means that there are elements in the array that meet the filtering conditions, then return true;

var a =[1,2,3,4,5];
a.every(function(value){return value<10;
})  //true a中所有元素都小于10a.every(function(value){return value%2===0;
});//false a中不是所有元素都是偶数a.some(function(value){return value%2===0;
})//true a中存在偶数

Use the specified function to combine array elements to generate a single value.
reduce() requires two parameters.
The first one is the function that performs the simplification operation. The task of the simplify function is to combine or simplify two values ​​into one value in some way and return the simplified value. The second (optional) parameter is an initial value passed to the function.
reduceRight() is used in the same way as reduce(), except that it processes the array from high to low (right to left) according to the array index.

var a=[1,2,3,4,5];var sum=a.reduce(function(x,y){return x+y;},0);
sum //15  数组求和var max=a.reduce(function(x,y){return x>y?x:y});
max // 5求最大值

indexOf()The index of the first qualifying value, if not, returns -1
lastIndexOf()The index of the last qualifying value , if not, return -1

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