Summary of various JS operations on Array

Array should be the most frequently used when we write js code. In daily projects, a lot of data can be stored and manipulated through Array. In addition to Object, the Array type should be the most commonly used type in js.

Today I will summarize some simple and basic operations of Array, and also to consolidate my basic knowledge.

1. How to create an Array (let’s talk about arrays directly below)

There are two main ways to create an array. The first is to use the array constructor, and the second is to use the array literal representation.

1. Use the array constructor

For example: var arr = new Array();

If you know the length of the array in advance, you can also pass the length directly to the constructor.

For example: var arr = new Array(20);

If you know the items that should be included in the array, just pass the items that should be included in the array directly during construction.

For example: var arr = new Array(1,2,3);

2. Use array literal representation

For example: var arr = [1,2,3,4];

var arr2 = [ ; Tail data item

push();----This method is to add data items at the end of the array. The number of parameters of this method can be customized;

pop();---This method is to obtain the last item of the array. A data item, this function does not need to pass any parameters;

For example:

var colors = new Array();//Create an array

console.log(count);

var color = ["red","black"];

color[3]="yellow "; //Add an item
console.log(color);
console.log(color.push("blue"));
console.log(color.pop()); //Get the last item




2) Queue operation method: first in, first out principle---simulated by inserting data from the head of the array and obtaining data items

push();--adding data items to the end of the array;

shift ();---Get the data information of the head item of the array;

unshift();--Completely opposite to shift, it inserts the data item information into the head of the array;

var colorArr = new Array() ;//Create an array

console.log(colorArr);

console.log (length);

var item = colorArr.shift();//Get the first item
console.log(item);
console.log(colorArr.length);



2. Detect and verify the array

In daily project development, we often encounter the problem of judging whether an object is an array (during function parameter transfer). So if we want to judge whether an object is an array, there are the following two ways

1) The first method

　　if(value instanseof Array){
   }

2) The second method

　　if(Array.isArray(value)){
   }//该方法只使用与高版本的浏览器：IE9+、Firefox4+/Chrome

3. Specific programming examples

function append(arr, item) { 
  var newArr = arr.slice(0); // slice(start, end)浅拷贝数组 
  newArr.push(item); 
  return newArr; 
};

function append(arr, item) { 
  var length = arr.length, 
    newArr = []; 
  
  for (var i = 0; i < length; i++) { 
    newArr.push(arr[i]); 
  } 
  
  newArr.push(item); 
  
  return newArr; 
};

Method 3: Use concat

function append(arr, item) { 
  return arr.concat(item); 
}

2) Add elements (Add an element at any position)

function insert(arr, item, index) { 
  var newArr=[]; 
  for(var i=0;i<arr.length;i++){ 
    newArr.push(arr[i]); 
  } 
  newArr.splice(index,0,item); 
  return newArr; 
}

function insert(arr, item, index) { 
  var newArr=arr.slice(0); 
  newArr.splice(index,0,item); 
  return newArr; 
}

function insert(arr, item, index) { 
  var newArr=arr.concat(); 
  newArr.splice(index,0,item); 
  return newArr; 
}

3. Delete elements (delete the last one in the array element)

function truncate(arr, item) { 
  var newArr=[]; 
  for(var i=0;i<arr.length-1;i++){ 
    newArr.push(arr[i]); 
  } 
  return newArr; 
}

function truncate(arr) { 
  var newArr = arr.concat(); 
  newArr.pop(); 
  return newArr; 
}

Method 1: Use ordinary iterative copy

function curtail(arr) { 
  var newArr=[]; 
  for(var i=1;i<arr.length;i++){ 
    newArr.push(arr[i]); 
  } 
  return newArr; 
}

Method 2: Combine concat() and shift()

function curtail(arr) { 
  var newArr = arr.concat(); 
  newArr.shift(); 
  return newArr; 
}

function curtail(arr) { 
  return arr.slice(1); 
}

5, merge arrays

function concat(arr1, arr2) { 
  var newArr=[]; 
  for(var i=0;i<arr1.length;i++){ 
    newArr.push(arr1[i]); 
  } 
  for(var j=0;j<arr2.length;j++){ 
    newArr.push(arr2[j]); 
  } 
  return newArr; 
}

function concat(arr1, arr2) { 
  return arr1.concat(arr2); 
}

function concat(arr1, arr2) { 
  var newArr=arr1.slice(0); 
  for(var i=0;i<arr2.length;i++){ 
    newArr.push(arr2[i]); 
  } 
  return newArr; 
}

5. Remove elements from the array

function remove(arr, item) { 
      var newArr = arr.slice(0); 
      for(var i=0; i<newArr.length; i++) { 
        if(newArr[i] == item) { 
          newArr.splice(i, 1); 
        } 
      } 
      return newArr; 
    } 
    var arr = [1,2,3,4,2]; 
    var item = 2; 
    console.log(remove(arr, item)); 
    console.log(arr);

function remove(arr,item){ 
  var newarr = []; 
  for(var i=0;i<arr.length;i++){ 
    if(arr[i] != item){ 
      newarr.push(arr[i]); 
    } 
  } 
  return newarr; 
}