How to get unique values in an array

Question

How can I get a list of unique values in an array? Do I always have to use a second array or is there something similar to java\'s hashmap in JavaScript?

I am going

Answer 1

You only need vanilla JS to find uniques with Array.some and Array.reduce. With ES2015 syntax it's only 62 characters.

a.reduce((c, v) => b.some(w => w === v) ? c : c.concat(v)), b)

Array.some and Array.reduce are supported in IE9+ and other browsers. Just change the fat arrow functions for regular functions to support in browsers that don't support ES2015 syntax.

var a = [1,2,3];
var b = [4,5,6];
// .reduce can return a subset or superset
var uniques = a.reduce(function(c, v){
    // .some stops on the first time the function returns true                
    return (b.some(function(w){ return w === v; }) ?  
      // if there's a match, return the array "c"
      c :     
      // if there's no match, then add to the end and return the entire array                                        
      c.concat(v)}),                                  
  // the second param in .reduce is the starting variable. This is will be "c" the first time it runs.
  b);                                                 

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce