Understand the two ways to deduplicate arrays in ES6

This article will introduce to you two ways to deduplicate arrays in ES6. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

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Two ways to deduplicate arrays in ES6

Method 1:

function unique(arr) {
    const res = new Map();
    return arr.filter((a) => !res.has(a) && res.set(a, 1))
}

It’s as short as that, let’s explain why.

Map object

Map is a new data structure provided by ES6.
Map objects store key-value pairs. Any value (object or primitive) can be used as a key or a value.

The following table lists the methods of the Map object.

##clearDelete all Key/value pairs, no return value. deleteDelete a key and return true. If deletion fails, return false. forEachPerforms the specified operation on each element. getReturns the value corresponding to the Map object key. hasReturns a Boolean value indicating whether a key is in the current Map object. setSet the key/value key/value pair for the Map object.

Method	Description

#The Map object also has a size attribute, which returns the number of key/value pairs of the Map object.

The filter() method of array

The filter() method creates a new array. The elements in the new array are determined by checking all elements in the specified array that meet the conditions. .

Syntax:

array.filter(function(currentValue,index,arr), thisValue)

Parameter description:

Arrow function

return arr.filter((a) => !res.has(a) && res.set(a, 1))
//上面的代码可以改成这样
return arr.filter(function(a){
    return !res.has(a) && res.set(a, 1);
});

1. The arrow function has a more concise syntax when writing code;

2. This will not be bound.

To learn more, click here

Method one analysis

function unique(arr) {
    //定义常量 res,值为一个Map对象实例
    const res = new Map();
    
    //返回arr数组过滤后的结果，结果为一个数组
    //过滤条件是，如果res中没有某个键，就设置这个键的值为1
    return arr.filter((a) => !res.has(a) && res.set(a, 1))
}

Method two:

function unique(arr) {
    return Array.from(new Set(arr))
}

This method requires less code, which is incredible.

The from method of array

The Array.from() method extracts data from an array-like or iterable object (including Array, Map, Set, String, TypedArray, arguments objects, etc. )

Syntax:

Array.from(arrayLike[, mapFn[, thisArg]])

ParametersDescriptionarrayLikeRequired, an array-like object or iterable object that you want to convert to a real array. mapFnOptional, if this parameter is specified, the finally generated array will be processed by this function and then returned. thisArgOptional, the value of this when executing the mapFn function.

Sample code:

const bar = ["a", "b", "c"];
Array.from(bar);
// ["a", "b", "c"]

Array.from('foo');
// ["f", "o", "o"]

Set object

Set object allows you to store A unique value of any type, whether a primitive value or an object reference.

A Set object is a collection of values, and you can iterate its elements in the order of insertion.
The elements in the Set will only appear once, that is, the elements in the Set are unique.

Syntax:

new Set([iterable]);

Parameters: iterable, if you pass an iterable object (including Array, Map, Set, String, TypedArray, arguments object, etc.), all its elements will be added to the new Set. If this parameter is not specified or its value is null, the new Set is empty.

The following table lists the methods of the Set object.

MethodDescription##addcleardeleteforEachhas

Set对象和Map对象一样，都有一个size属性，他返回Set对象的值的个数。

方法二 分析

function unique(arr) {
    //通过Set对象，对数组去重，结果又返回一个Set对象
    //通过from方法，将Set对象转为数组
    return Array.from(new Set(arr))
}

总结

这次说的两个方法，真的很简单，主要就是靠ES6里的新东西，难度不大，代码简单，主要就是多用用就好了。

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经人提醒，再补充一种，[...new Set(arr)]  

不懂 ...  的朋友，可以看这里 js扩展运算符

更多编程相关知识，请访问：编程学习！！

Add someone value, returning the Set object itself.
Deletes all key/value pairs and returns no value.
Delete a key and return true. If deletion fails, return false.
Performs the specified operation on each element.
Returns a Boolean value indicating whether a key is in the current Set object.

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