How to find whether a specified array element exists in javascript

4 methods: 1. Use includes() to detect, the syntax is "array.includes(element value)"; 2. Use some() to detect, the syntax is "function f(v) {if (v== =value) {return true;}}var b= array.some(f);”; 3. Use indexOf() to detect, the syntax is “array.indexOf(value)”; 4. Use lastIndexOf(), the syntax is “array. lastIndexOf(value)".

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

Method 1: Using the includes() method of the array

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

arr.includes(searchElement [, fromIndex])

• searchElement Required. The element value to be found.

• fromIndex Optional. Start looking for searchElement at this index. If negative, the search starts at the index of array.length fromIndex in ascending order. Default is 0.

Example: Determine whether the fruits array contains the given element "Durian"

var fruits = ['苹果',"香蕉", '榴莲', '橘子', '菠萝蜜',"梨子"];
if(fruits.includes('榴莲')){
	console.log("给定元素是存在的");
}else{
	console.log("给定元素是不存在的");
}

Method 2: Use the some() method of the array

The some() method can detect whether there are elements that meet the conditions in the array. Thinking about it from another angle, it can also be used to detect whether all elements in the array do not meet the specified conditions. If they do not meet the specified conditions, it will return false. If one or more elements match, it will return true.

array.every(function callbackfn(Value,index,array),thisValue)

Parameter	Description
function(currentValue, index,arr)	must. Function, each element in the array will execute this function Function parameters: Parameter description currentValue must be. The value index of the current element is optional. The index value arr of the current element is optional. The array object to which the current element belongs
thisValue	Optional. The object is used as the execution callback, passed to the function, and used as the value of "this". If thisValue is omitted, the value of "this" is "undefined"

Return value: Boolean value. Returns true if any element in the array satisfies the condition, otherwise returns false.

Example:

function f(value, index, ar) {
    if (value === '橘子') {
        return true;
    }
}
var fruits = ['苹果',"香蕉", '榴莲', '橘子', '菠萝蜜',"梨子"];
var b = fruits.some(f);
if (b) {
   console.log("给定元素是存在的");
} else {
   console.log("给定元素是不存在的");
}

The output result is:

Method 3: Utilization The indexOf() method of array

indexOf() method can return the first occurrence position of a specified element in the array. If the element to be retrieved does not appear, the method returns -1.

array.indexOf(item,start)

Parameters	Description
item	must. The element to find.
start	Optional integer parameter. Specifies the position in the array to start searching. Its legal values ​​are 0 to stringObject.length - 1. If this parameter is omitted, the search will start from the first character of the string.

Implementation idea: Use this method to check the first occurrence position of the specified value in the array. If the position exists, the given element is included. If -1 is returned, the given element is not contained.

The implementation code is given below:

var fruits = ['苹果',"香蕉", '榴莲', '橘子', '菠萝蜜',"梨子"];
var b = fruits.indexOf("桃子");
if (b>0) {
   console.log("给定元素是存在的");
} else {
   console.log("给定元素是不存在的");
}

The output result is:

Method 4: Use the lastIndexOf() method of the array

lastIndexOf() method to search for elements in the array and return the position where it last appeared. If the element to be retrieved does not appear, the method returns -1.

array.lastIndexOf(item,start)

Parameters	Description
item	Required. Specifies the string value to be retrieved.
start	Optional integer parameter. Specifies the position in the string to begin searching. Its legal values ​​are 0 to stringObject.length - 1. If this parameter is omitted, the search will start from the last character of the string.

实现思想：利用该方法检查指定值在数组的最后出现位置，如果位置是存在的，则包含该给定元素；如果返回-1，则不包含该给定元素。

下面给出实现代码：：

var fruits = ['苹果',"香蕉", '榴莲', '橘子', '菠萝蜜',"梨子"];
var b = fruits.lastIndexOf("葡萄");
if (b>0) {
   console.log("给定元素是存在的");
} else {
   console.log("给定元素是不存在的");
}

输出结果为：

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