To just merge the arrays (without removing duplicates)

ES5 version use Array.concat:

var array1 = ["Vijendra", "Singh"];
var array2 = ["Singh", "Shakya"];

array1 = array1.concat(array2);

console.log(array1);

2023 update

The original answer was from years ago. ES6 is fully supported and IE is finally dead. Here's the simplest way to merge primitive and object arrays:

const merge = (a, b, predicate = (a, b) => a === b) => {
    const c = [...a]; // copy to avoid side effects
    // add all items from B to copy C if they're not already present
    b.forEach((bItem) => (c.some((cItem) => predicate(bItem, cItem)) ? null : c.push(bItem)))
    return c;
}

merge(['a', 'b', 'c'], ['c', 'x', 'd']);
// => ['a', 'b', 'c', 'x', 'd']

merge([{id: 1}, {id: 2}], [{id: 2}, {id: 3}], (a, b) => a.id === b.id);
// [{id: 1}, {id: 2}, {id: 3}]

Original answer

ES6 version use destructuring

const array1 = ["Vijendra","Singh"];
const array2 = ["Singh", "Shakya"];
const array3 = [...array1, ...array2];

Since there is no 'built in' way to remove duplicates (ECMA-262 actually has Array.forEach which would be great for this), we have to do it manually. Note that this pollutes the Array prototype, use with caution.

Array.prototype.unique = function() {
    var a = this.concat();
    for(var i=0; i<a.length; ++i) {
        for(var j=i+1; j<a.length; ++j) {
            if(a[i] === a[j])
                a.splice(j--, 1);
        }
    }

    return a;
};

Then, to use it:

var array1 = ["Vijendra","Singh"];
var array2 = ["Singh", "Shakya"];
// Merges both arrays and gets unique items
var array3 = array1.concat(array2).unique(); 

This will also preserve the order of the arrays (i.e, no sorting needed).

Since many people are annoyed about prototype augmentation of Array.prototype and for in loops, here is a less invasive way to use it:

function arrayUnique(array) {
    var a = array.concat();
    for(var i=0; i<a.length; ++i) {
        for(var j=i+1; j<a.length; ++j) {
            if(a[i] === a[j])
                a.splice(j--, 1);
        }
    }

    return a;
}

var array1 = ["Vijendra","Singh"];
var array2 = ["Singh", "Shakya"];
    // Merges both arrays and gets unique items
var array3 = arrayUnique(array1.concat(array2));

For those who are fortunate enough to work with browsers where ES5 is available, you can use Object.defineProperty like this:

Object.defineProperty(Array.prototype, 'unique', {
    enumerable: false,
    configurable: false,
    writable: false,
    value: function() {
        var a = this.concat();
        for(var i=0; i<a.length; ++i) {
            for(var j=i+1; j<a.length; ++j) {
                if(a[i] === a[j])
                    a.splice(j--, 1);
            }
        }

        return a;
    }
});

With Underscore.js or Lo-Dash you can do:

console.log(_.union([1, 2, 3], [101, 2, 1, 10], [2, 1]));

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.15/lodash.min.js"></script>

http://underscorejs.org/#union

http://lodash.com/docs#union

First concatenate the two arrays, next filter out only the unique items:

var a = [1, 2, 3], b = [101, 2, 1, 10]
var c = a.concat(b)
var d = c.filter((item, pos) => c.indexOf(item) === pos)

console.log(d) // d is [1, 2, 3, 101, 10]

Edit

As suggested a more performance wise solution would be to filter out the unique items in b before concatenating with a:

var a = [1, 2, 3], b = [101, 2, 1, 10]
var c = a.concat(b.filter((item) => a.indexOf(item) < 0))

console.log(c) // c is [1, 2, 3, 101, 10]

[...array1,...array2] //   =>  don't remove duplication 

OR

[...new Set([...array1 ,...array2])]; //   => remove duplication

This is an ECMAScript 6 solution using spread operator and array generics.

Currently it only works with Firefox, and possibly Internet Explorer Technical Preview.

But if you use Babel, you can have it now.

const input = [
  [1, 2, 3],
  [101, 2, 1, 10],
  [2, 1]
];
const mergeDedupe = (arr) => {
  return [...new Set([].concat(...arr))];
}

console.log('output', mergeDedupe(input));

Using a Set (ECMAScript 2015), it will be as simple as that:

const array1 = ["Vijendra", "Singh"];
const array2 = ["Singh", "Shakya"];
console.log(Array.from(new Set(array1.concat(array2))));

You can do it simply with ECMAScript 6,

var array1 = ["Vijendra", "Singh"];
var array2 = ["Singh", "Shakya"];
var array3 = [...new Set([...array1 ,...array2])];
console.log(array3); // ["Vijendra", "Singh", "Shakya"];

• Use the spread operator for concatenating the array.
• Use Set for creating a distinct set of elements.
• Again use the spread operator to convert the Set into an array.

Here is a slightly different take on the loop. With some of the optimizations in the latest version of Chrome, it is the fastest method for resolving the union of the two arrays (Chrome 38.0.2111).

JSPerf: "Merge two arrays keeping only unique values" (archived)

var array1 = ["Vijendra", "Singh"];
var array2 = ["Singh", "Shakya"];
var array3 = [];

var arr = array1.concat(array2),
  len = arr.length;

while (len--) {
  var itm = arr[len];
  if (array3.indexOf(itm) === -1) {
    array3.unshift(itm);
  }
}

while loop: ~589k ops/s
filter: ~445k ops/s
lodash: 308k ops/s
for loops: 225k ops/s

A comment pointed out that one of my setup variables was causing my loop to pull ahead of the rest because it didn't have to initialize an empty array to write to. I agree with that, so I've rewritten the test to even the playing field, and included an even faster option.

JSPerf: "Merge two arrays keeping only unique values" (archived)

let whileLoopAlt = function (array1, array2) {
    const array3 = array1.slice(0);
    let len1 = array1.length;
    let len2 = array2.length;
    const assoc = {};

    while (len1--) {
        assoc[array1[len1]] = null;
    }

    while (len2--) {
        let itm = array2[len2];

        if (assoc[itm] === undefined) { // Eliminate the indexOf call
            array3.push(itm);
            assoc[itm] = null;
        }
    }

    return array3;
};

In this alternate solution, I've combined one answer's associative array solution to eliminate the .indexOf() call in the loop which was slowing things down a lot with a second loop, and included some of the other optimizations that other users have suggested in their answers as well.

The top answer here with the double loop on every value (i-1) is still significantly slower. lodash is still doing strong, and I still would recommend it to anyone who doesn't mind adding a library to their project. For those who don't want to, my while loop is still a good answer and the filter answer has a very strong showing here, beating out all on my tests with the latest Canary Chrome (44.0.2360) as of this writing.

Check out Mike's answer and Dan Stocker's answer if you want to step it up a notch in speed. Those are by far the fastest of all results after going through almost all of the viable answers.

I simplified the best of this answer and turned it into a nice function:

function mergeUnique(arr1, arr2){
    return arr1.concat(arr2.filter(function (item) {
        return arr1.indexOf(item) === -1;
    }));
}

The ES6 offers a single-line solution for merging multiple arrays without duplicates by using destructuring and set.

const array1 = ['a','b','c'];
const array2 = ['c','c','d','e'];
const array3 = [...new Set([...array1,...array2])];
console.log(array3); // ["a", "b", "c", "d", "e"]

I know this question is not about array of objects, but searchers do end up here.

so it's worth adding for future readers a proper ES6 way of merging and then removing duplicates

array of objects:

var arr1 = [ {a: 1}, {a: 2}, {a: 3} ];
var arr2 = [ {a: 1}, {a: 2}, {a: 4} ];

var arr3 = arr1.concat(arr2.filter( ({a}) => !arr1.find(f => f.a == a) ));

// [ {a: 1}, {a: 2}, {a: 3}, {a: 4} ]

Just steer clear of nested loops (O(n^2)), and .indexOf() (+O(n)).

function merge(a, b) {
  var hash = {};
  var i;
  
  for (i = 0; i < a.length; i++) {
    hash[a[i]] = true;
  }
  for (i = 0; i < b.length; i++) {
    hash[b[i]] = true;
  }
  return Object.keys(hash);
}

var array1 = ["Vijendra", "Singh"];
var array2 = ["Singh", "Shakya"];

var array3 = merge(array1, array2);

console.log(array3);

Just throwing in my two cents.

function mergeStringArrays(a, b){
    var hash = {};
    var ret = [];

    for(var i=0; i < a.length; i++){
        var e = a[i];
        if (!hash[e]){
            hash[e] = true;
            ret.push(e);
        }
    }

    for(var i=0; i < b.length; i++){
        var e = b[i];
        if (!hash[e]){
            hash[e] = true;
            ret.push(e);
        }
    }

    return ret;
}

This is a method I use a lot, it uses an object as a hashlookup table to do the duplicate checking. Assuming that the hash is O(1), then this runs in O(n) where n is a.length + b.length. I honestly have no idea how the browser does the hash, but it performs well on many thousands of data points.

EDIT:

The first solution is the fastest only when there are few items. When there are over 400 items, the Set solution becomes the fastest. And when there are 100,000 items, it is a thousand times faster than the first solution.

Considering that performance is important only when there is a lot of items, and that the Set solution is by far the most readable, it should be the right solution in most cases

The perf results below were computed with a small number of items

Based on jsperf, the fastest way (edit: if there are less than 400 items) to merge two arrays in a new one is the following:

for (var i = 0; i < array2.length; i++)
    if (array1.indexOf(array2[i]) === -1)
      array1.push(array2[i]);

This one is 17% slower:

array2.forEach(v => array1.includes(v) ? null : array1.push(v));

This one is 45% slower (edit: when there is less than 100 items. It is a lot faster when there is a lot of items):

var a = [...new Set([...array1 ,...array2])];

And the accepted answer's is 55% slower (and much longer to write) (edit: and it is several order of magnitude slower than any of the other methods when there are 100,000 items)

var a = array1.concat(array2);
for (var i = 0; i < a.length; ++i) {
    for (var j = i + 1; j < a.length; ++j) {
        if (a[i] === a[j])
            a.splice(j--, 1);
    }
}

https://jsbench.me/lxlej18ydg

Array.prototype.merge = function(/* variable number of arrays */){
    for(var i = 0; i < arguments.length; i++){
        var array = arguments[i];
        for(var j = 0; j < array.length; j++){
            if(this.indexOf(array[j]) === -1) {
                this.push(array[j]);
            }
        }
    }
    return this;
};

A much better array merge function.

Performance

Today 2020.10.15 I perform tests on MacOs HighSierra 10.13.6 on Chrome v86, Safari v13.1.2 and Firefox v81 for chosen solutions.

Results

For all browsers

• solution H is fast/fastest
• solutions L is fast
• solution D is fastest on chrome for big arrays
• solution G is fast on small arrays
• solution M is slowest for small arrays
• solutions E are slowest for big arrays

Details

I perform 2 tests cases:

• for 2 elements arrays - you can run it HERE
• for 10000 elements arrays - you can run it HERE

on solutions A, B, C, D, E, G, H, J, L, M presented in below snippet

// https://stackoverflow.com/a/10499519/860099
function A(arr1,arr2) {
  return _.union(arr1,arr2)
}

// https://stackoverflow.com/a/53149853/860099
function B(arr1,arr2) {
  return _.unionWith(arr1, arr2, _.isEqual);
}

// https://stackoverflow.com/a/27664971/860099
function C(arr1,arr2) {
  return [...new Set([...arr1,...arr2])]
}

// https://stackoverflow.com/a/48130841/860099
function D(arr1,arr2) {
  return Array.from(new Set(arr1.concat(arr2)))
}

// https://stackoverflow.com/a/23080662/860099
function E(arr1,arr2) {
  return arr1.concat(arr2.filter((item) => arr1.indexOf(item) < 0))
}


// https://stackoverflow.com/a/28631880/860099
function G(arr1,arr2) {
  var hash = {};
  var i;
  
  for (i = 0; i < arr1.length; i++) {
    hash[arr1[i]] = true;
  }
  for (i = 0; i < arr2.length; i++) {
    hash[arr2[i]] = true;
  }
  return Object.keys(hash);
}

// https://stackoverflow.com/a/13847481/860099
function H(a, b){
    var hash = {};
    var ret = [];

    for(var i=0; i < a.length; i++){
        var e = a[i];
        if (!hash[e]){
            hash[e] = true;
            ret.push(e);
        }
    }

    for(var i=0; i < b.length; i++){
        var e = b[i];
        if (!hash[e]){
            hash[e] = true;
            ret.push(e);
        }
    }

    return ret;
}



// https://stackoverflow.com/a/1584377/860099
function J(arr1,arr2) {
  function arrayUnique(array) {
      var a = array.concat();
      for(var i=0; i<a.length; ++i) {
          for(var j=i+1; j<a.length; ++j) {
              if(a[i] === a[j])
                  a.splice(j--, 1);
          }
      }

      return a;
  }

  return arrayUnique(arr1.concat(arr2));
}


// https://stackoverflow.com/a/25120770/860099
function L(array1, array2) {
    const array3 = array1.slice(0);
    let len1 = array1.length;
    let len2 = array2.length;
    const assoc = {};

    while (len1--) {
        assoc[array1[len1]] = null;
    }

    while (len2--) {
        let itm = array2[len2];

        if (assoc[itm] === undefined) { // Eliminate the indexOf call
            array3.push(itm);
            assoc[itm] = null;
        }
    }

    return array3;
}

// https://stackoverflow.com/a/39336712/860099
function M(arr1,arr2) {
  const comp = f => g => x => f(g(x));
  const apply = f => a => f(a);
  const flip = f => b => a => f(a) (b);
  const concat = xs => y => xs.concat(y);
  const afrom = apply(Array.from);
  const createSet = xs => new Set(xs);
  const filter = f => xs => xs.filter(apply(f));

  const dedupe = comp(afrom) (createSet);

  const union = xs => ys => {
    const zs = createSet(xs);  
    return concat(xs) (
      filter(x => zs.has(x)
       ? false
       : zs.add(x)
    ) (ys));
  }

  return union(dedupe(arr1)) (arr2)
}



// -------------
// TEST
// -------------

var array1 = ["Vijendra","Singh"];
var array2 = ["Singh", "Shakya"];

[A,B,C,D,E,G,H,J,L,M].forEach(f=> {
  console.log(`${f.name} [${f([...array1],[...array2])}]`);
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.20/lodash.min.js" integrity="sha512-90vH1Z83AJY9DmlWa8WkjkV79yfS2n2Oxhsi2dZbIv0nC4E6m5AbH8Nh156kkM7JePmqD6tcZsfad1ueoaovww==" crossorigin="anonymous"></script>
  
This snippet only presents functions used in performance tests - it not perform tests itself!

And here are example test run for chrome

UPDATE

I remove cases F,I,K because they modify input arrays and benchmark gives wrong results

Why don't you use an object? It looks like you're trying to model a set. This won't preserve the order, however.

var set1 = {"Vijendra":true, "Singh":true}
var set2 = {"Singh":true,  "Shakya":true}

// Merge second object into first
function merge(set1, set2){
  for (var key in set2){
    if (set2.hasOwnProperty(key))
      set1[key] = set2[key]
  }
  return set1
}

merge(set1, set2)

// Create set from array
function setify(array){
  var result = {}
  for (var item in array){
    if (array.hasOwnProperty(item))
      result[array[item]] = true
  }
  return result
}

For ES6, just one line:

a = [1, 2, 3, 4]
b = [4, 5]
[...new Set(a.concat(b))]  // [1, 2, 3, 4, 5]

The best solution...

You can check directly in the browser console by hitting...

Without duplicate

a = [1, 2, 3];
b = [3, 2, 1, "prince"];

a.concat(b.filter(function(el) {
    return a.indexOf(el) === -1;
}));

With duplicate

["prince", "asish", 5].concat(["ravi", 4])

If you want without duplicate you can try a better solution from here - Shouting Code.

[1, 2, 3].concat([3, 2, 1, "prince"].filter(function(el) {
    return [1, 2, 3].indexOf(el) === -1;
}));

Try on Chrome browser console

 f12 > console

Output:

["prince", "asish", 5, "ravi", 4]

[1, 2, 3, "prince"]

My one and a half penny:

Array.prototype.concat_n_dedupe = function(other_array) {
  return this
    .concat(other_array) // add second
    .reduce(function(uniques, item) { // dedupe all
      if (uniques.indexOf(item) == -1) {
        uniques.push(item);
      }
      return uniques;
    }, []);
};

var array1 = ["Vijendra","Singh"];
var array2 = ["Singh", "Shakya"];

var result = array1.concat_n_dedupe(array2);

console.log(result);

There are so many solutions for merging two arrays. They can be divided into two main categories(except the use of 3rd party libraries like lodash or underscore.js).

a) combine two arrays and remove duplicated items.

b) filter out items before combining them.

Combine two arrays and remove duplicated items

Combining

// mutable operation(array1 is the combined array)
array1.push(...array2);
array1.unshift(...array2);

// immutable operation
const combined = array1.concat(array2);
const combined = [...array1, ...array2];    // ES6

Unifying

There are many ways to unifying an array, I personally suggest below two methods.

// a little bit tricky
const merged = combined.filter((item, index) => combined.indexOf(item) === index);
const merged = [...new Set(combined)];

Filter out items before combining them

There are also many ways, but I personally suggest the below code due to its simplicity.

const merged = array1.concat(array2.filter(secItem => !array1.includes(secItem)));

you can use new Set to remove duplication

[...new Set([...array1 ,...array2])]

You can achieve it simply using Underscore.js's => uniq:

array3 = _.uniq(array1.concat(array2))

console.log(array3)

It will print ["Vijendra", "Singh", "Shakya"].

New solution ( which uses Array.prototype.indexOf and Array.prototype.concat ):

Array.prototype.uniqueMerge = function( a ) {
    for ( var nonDuplicates = [], i = 0, l = a.length; i<l; ++i ) {
        if ( this.indexOf( a[i] ) === -1 ) {
            nonDuplicates.push( a[i] );
        }
    }
    return this.concat( nonDuplicates )
};

Usage:

>>> ['Vijendra', 'Singh'].uniqueMerge(['Singh', 'Shakya'])
["Vijendra", "Singh", "Shakya"]

Array.prototype.indexOf ( for internet explorer ):

Array.prototype.indexOf = Array.prototype.indexOf || function(elt)
  {
    var len = this.length >>> 0;

    var from = Number(arguments[1]) || 0;
    from = (from < 0) ? Math.ceil(from): Math.floor(from); 
    if (from < 0)from += len;

    for (; from < len; from++)
    {
      if (from in this && this[from] === elt)return from;
    }
    return -1;
  };

It can be done using Set.

var array1 = ["Vijendra","Singh"];
var array2 = ["Singh", "Shakya"];

var array3 = array1.concat(array2);
var tempSet = new Set(array3);
array3 = Array.from(tempSet);

//show output
document.body.querySelector("div").innerHTML = JSON.stringify(array3);

<div style="width:100%;height:4rem;line-height:4rem;background-color:steelblue;color:#DDD;text-align:center;font-family:Calibri" > 
  temp text 
</div>

//Array.indexOf was introduced in javascript 1.6 (ECMA-262) 
//We need to implement it explicitly for other browsers, 
if (!Array.prototype.indexOf)
{
  Array.prototype.indexOf = function(elt, from)
  {
    var len = this.length >>> 0;

    for (; from < len; from++)
    {
      if (from in this &&
          this[from] === elt)
        return from;
    }
    return -1;
  };
}
//now, on to the problem

var array1 = ["Vijendra","Singh"];
var array2 = ["Singh", "Shakya"];

var merged = array1.concat(array2);
var t;
for(i = 0; i < merged.length; i++)
  if((t = merged.indexOf(i + 1, merged[i])) != -1)
  {
    merged.splice(t, 1);
    i--;//in case of multiple occurrences
  }

Implementation of indexOf method for other browsers is taken from MDC

const array3 = array1.filter(t=> !array2.includes(t)).concat(array2)

Array.prototype.add = function(b){
    var a = this.concat();                // clone current object
    if(!b.push || !b.length) return a;    // if b is not an array, or empty, then return a unchanged
    if(!a.length) return b.concat();      // if original is empty, return b

    // go through all the elements of b
    for(var i = 0; i < b.length; i++){
        // if b's value is not in a, then add it
        if(a.indexOf(b[i]) == -1) a.push(b[i]);
    }
    return a;
}

// Example:
console.log([1,2,3].add([3, 4, 5])); // will output [1, 2, 3, 4, 5]

array1.concat(array2).filter((value, pos, arr)=>arr.indexOf(value)===pos)

The nice thing about this one is performance and that you in general, when working with arrays, are chaining methods like filter, map, etc so you can add that line and it will concat and deduplicate array2 with array1 without needing a reference to the later one (when you are chaining methods you don't have), example:

someSource()
.reduce(...)
.filter(...)
.map(...) 
// and now you want to concat array2 and deduplicate:
.concat(array2).filter((value, pos, arr)=>arr.indexOf(value)===pos)
// and keep chaining stuff
.map(...)
.find(...)
// etc

(I don't like to pollute Array.prototype and that would be the only way of respect the chain - defining a new function will break it - so I think something like this is the only way of accomplish that)

DeDuplicate single or Merge and DeDuplicate multiple array inputs. Example below.

useing ES6 - Set, for of, destructuring

I wrote this simple function which takes multiple array arguments. Does pretty much the same as the solution above it just have more practical use case. This function doesn't concatenate duplicate values in to one array only so that it can delete them at some later stage.

SHORT FUNCTION DEFINITION ( only 9 lines )

/**
* This function merging only arrays unique values. It does not merges arrays in to array with duplicate values at any stage.
*
* @params ...args Function accept multiple array input (merges them to single array with no duplicates)
* it also can be used to filter duplicates in single array
*/
function arrayDeDuplicate(...args){
   let set = new Set(); // init Set object (available as of ES6)
   for(let arr of args){ // for of loops through values
      arr.map((value) => { // map adds each value to Set object
         set.add(value); // set.add method adds only unique values
      });
   }
   return [...set]; // destructuring set object back to array object
   // alternativly we culd use:  return Array.from(set);
}

USE EXAMPLE CODEPEN:

// SCENARIO 
let a = [1,2,3,4,5,6];
let b = [4,5,6,7,8,9,10,10,10];
let c = [43,23,1,2,3];
let d = ['a','b','c','d'];
let e = ['b','c','d','e'];

// USEAGE
let uniqueArrayAll = arrayDeDuplicate(a, b, c, d, e);
let uniqueArraySingle = arrayDeDuplicate(b);

// OUTPUT
console.log(uniqueArrayAll); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 43, 23, "a", "b", "c", "d", "e"]
console.log(uniqueArraySingle); // [4, 5, 6, 7, 8, 9, 10]