How to implement array deduplication in es5 and es6

In ES6, you can use the reverse() method of the array object to achieve array reversal. This method is used to reverse the order of the elements in the array, putting the last element first and the first element last. The syntax "array.reverse()". The reverse() method will modify the original array. If you do not want to modify it, you need to use it with the expansion operator "...", and the syntax is "[...array].reverse()".

async is es7. async and await are new additions to ES7 and are solutions for asynchronous operations; async/await can be said to be syntactic sugar for co modules and generator functions, solving js asynchronous code with clearer semantics. As the name suggests, async means "asynchronous". Async is used to declare that a function is asynchronous; there is a strict rule between async and await. Both cannot be separated from each other, and await can only be written in async functions.

Steps: 1. Convert the two arrays to set types respectively, with the syntax "newA=new Set(a);newB=new Set(b);"; 2. Use has() and filter() to find the difference set, with the syntax " new Set([...newA].filter(x =>!newB.has(x)))", the difference set elements will be included in a set collection and returned; 3. Use Array.from to convert the set into an array Type, syntax "Array.from(collection)".

PHP's array_unique() function is used to remove duplicate elements from an array. By default, strict equality (===) is used. We can specify the basis for deduplication through a custom comparison function: create a custom comparison function and specify the deduplication standard (for example, based on element length); pass the custom comparison function as the third parameter to the array_unique() function. Remove duplicate elements based on specified criteria.

For browser compatibility. As a new specification for JS, ES6 adds a lot of new syntax and API. However, modern browsers do not have high support for the new features of ES6, so ES6 code needs to be converted to ES5 code. In the WeChat web developer tools, babel is used by default to convert the developer's ES6 syntax code into ES5 code that is well supported by all three terminals, helping developers solve development problems caused by different environments; only in the project Just configure and check the "ES6 to ES5" option.

In es5, you can use the for statement and indexOf() function to achieve array deduplication. The syntax "for(i=0;i<array length;i++){a=newArr.indexOf(arr[i]);if(a== -1){...}}". In es6, you can use the spread operator, Array.from() and Set to remove duplication; you need to first convert the array into a Set object to remove duplication, and then use the spread operator or the Array.from() function to convert the Set object back to an array. Just group.

In es6, the temporary dead zone is a syntax error, which refers to the let and const commands that make the block form a closed scope. Within a code block, before a variable is declared using the let/const command, the variable is unavailable and belongs to the variable's "dead zone" before the variable is declared; this is syntactically called a "temporary dead zone". ES6 stipulates that variable promotion does not occur in temporary dead zones and let and const statements, mainly to reduce runtime errors and prevent the variable from being used before it is declared, resulting in unexpected behavior.

The map is ordered. The map type in ES6 is an ordered list that stores many key-value pairs. The key names and corresponding values ​​support all data types; the equivalence of key names is determined by calling the "Objext.is()" method. Implemented, so the number 5 and the string "5" will be judged as two types, and can appear in the program as two independent keys.