The Array.indexOf() method will replace the first instance. To get every instance use Array.map():

a = a.map(item => item == 3452 ? 1010 : item);

Of course, that creates a new array. If you want to do it in place, use Array.forEach():

a.forEach((item, i) => { if (item == 3452) a[i] = 1010; });

Answer from @gilly3 is great.

Replace object in an array, keeping the array order unchanged

I prefer the following way to update the new updated record into my array of records when I get data from the server. It keeps the order intact and quite straight forward one liner.

users = users.map(u => u.id !== editedUser.id ? u : editedUser);

var users = [
{id: 1, firstname: 'John', lastname: 'Ken'},
{id: 2, firstname: 'Robin', lastname: 'Hood'},
{id: 3, firstname: 'William', lastname: 'Cook'}
];

var editedUser = {id: 2, firstname: 'Michael', lastname: 'Angelo'};

users = users.map(u => u.id !== editedUser.id ? u : editedUser);

console.log('users -> ', users);

My suggested solution would be:

items.splice(start, deleteCount, item1);

Docs

The splice operation will start at index start, remove deleteCount item in the array , and will replace it with the new item item1.

Use indexOf to find an element.

var i = items.indexOf(3452);
items[i] = 1010;

First method

Best way in just one line to replace or update item of array

array.splice(array.indexOf(valueToReplace), 1, newValue)

Eg:

let items = ['JS', 'PHP', 'RUBY'];

let replacedItem = items.splice(items.indexOf('RUBY'), 1, 'PYTHON')

console.log(replacedItem) //['RUBY']
console.log(items) //['JS', 'PHP', 'PYTHON']

Second method

An other simple way to do the same operation is :

items[items.indexOf(oldValue)] = newValue

Easily accomplished with a for loop.

for (var i = 0; i < items.length; i++)
    if (items[i] == 3452)
        items[i] = 1010;

If using a complex object (or even a simple one) and you can use es6, Array.prototype.findIndex is a good one. For the OP's array, they could do,

const index = items.findIndex(x => x === 3452)
items[index] = 1010

For more complex objects, this really shines. For example,

const index = 
    items.findIndex(
       x => x.jerseyNumber === 9 && x.school === 'Ohio State'
    )

items[index].lastName = 'Utah'
items[index].firstName = 'Johnny'

You can edit any number of the list using indexes

for example :

items[0] = 5;
items[5] = 100;

ES6 way:

const items = Array(523, 3452, 334, 31, ...5346);

We wanna replace 3452 with 1010, solution:

const newItems = items.map(item => item === 3452 ? 1010 : item);

Surely, the question is for many years ago and for now I just prefer to use immutable solution, definitely, it is awesome for ReactJS.

For frequent usage I offer below function:

const itemReplacer = (array, oldItem, newItem) =>
  array.map(item => item === oldItem ? newItem : item);

A functional approach to replacing an element of an array in javascript:

const replace = (array, index, ...items) => [...array.slice(0, index), ...items, ...array.slice(index + 1)];

The immutable way to replace the element in the list using ES6 spread operators and .slice method.

const arr = ['fir', 'next', 'third'], item = 'next'

const nextArr = [
  ...arr.slice(0, arr.indexOf(item)), 
  'second',
  ...arr.slice(arr.indexOf(item) + 1)
]

Verify that works

console.log(arr)     // [ 'fir', 'next', 'third' ]
console.log(nextArr) // ['fir', 'second', 'third']

Replacement can be done in one line:

var items = Array(523, 3452, 334, 31, 5346);

items[items.map((e, i) => [i, e]).filter(e => e[1] == 3452)[0][0]] = 1010

console.log(items);

Or create a function to reuse:

Array.prototype.replace = function(t, v) {
    if (this.indexOf(t)!= -1)
        this[this.map((e, i) => [i, e]).filter(e => e[1] == t)[0][0]] = v;
  };

//Check
var items = Array(523, 3452, 334, 31, 5346);
items.replace(3452, 1010);
console.log(items);

Your question is vague. What exactly are you trying to replace? Are you asking how to replace all occurrences of a specific value (3452 in your question), or the first occurrence only, or the element of a specific index (1 in your question)? Do you want the replacing to be done in-place or in a copied array?

Anyways, I’d like to add some more information on top of the existing answers.

Replacing the element of a specific index…

in-place

with simple assignment.

const array = [523, 3452, 334, 31, 5346];

array[1] = 1010;
console.log(array);

in a copied array

with the Array.prototype.with method.

const array = [523, 3452, 334, 31, 5346];

const copied_and_replaced_array = array.with(1, 1010);
console.log(array, copied_and_replaced_array);

My suggested solution would be:

items = items.with(items.indexOf(oldValue),newValue)

Docs

This line of code using with operation will replace oldValue with newValue in the items array.

var items = Array(523,3452,334,31,5346);

If you know the value then use,

items[items.indexOf(334)] = 1010;

If you want to know that value is present or not, then use,

var point = items.indexOf(334);

if (point !== -1) {
    items[point] = 1010;
}

If you know the place (position) then directly use,

items[--position] = 1010;

If you want replace few elements, and you know only starting position only means,

items.splice(2, 1, 1010, 1220);

for more about .splice

The easiest way is to use some libraries like underscorejs and map method.

var items = Array(523,3452,334,31,...5346);

_.map(items, function(num) {
  return (num == 3452) ? 1010 : num; 
});
=> [523, 1010, 334, 31, ...5346]

If you want a simple sugar sintax oneliner you can just:

(elements = elements.filter(element => element.id !== updatedElement.id)).push(updatedElement);

Like:

let elements = [ { id: 1, name: 'element one' }, { id: 2, name: 'element two'} ];
const updatedElement = { id: 1, name: 'updated element one' };

If you don't have id you could stringify the element like:

(elements = elements.filter(element => JSON.stringify(element) !== JSON.stringify(updatedElement))).push(updatedElement);

var index = Array.indexOf(Array value);
        if (index > -1) {
          Array.splice(index, 1);
        }

from here you can delete a particular value from array and based on the same index you can insert value in array .

 Array.splice(index, 0, Array value);

Well if anyone is interresting on how to replace an object from its index in an array, here's a solution.

Find the index of the object by its id:

const index = items.map(item => item.id).indexOf(objectId)

Replace the object using Object.assign() method:

Object.assign(items[index], newValue)

 items[items.indexOf(3452)] = 1010

great for simple swaps. try the snippet below

const items = Array(523, 3452, 334, 31, 5346);
console.log(items)

items[items.indexOf(3452)] = 1010
console.log(items)

Here is the basic answer made into a reusable function:

function arrayFindReplace(array, findValue, replaceValue){
    while(array.indexOf(findValue) !== -1){
        let index = array.indexOf(findValue);
        array[index] = replaceValue;
    }
}

Here's a one liner. It assumes the item will be in the array.

var items = [523, 3452, 334, 31, 5346]
var replace = (arr, oldVal, newVal) => (arr[arr.indexOf(oldVal)] = newVal, arr)
console.log(replace(items, 3452, 1010))

First, rewrite your array like this:

var items = [523,3452,334,31,...5346];

Next, access the element in the array through its index number. The formula to determine the index number is: n-1

To replace the first item (n=1) in the array, write:

items[0] = Enter Your New Number;

In your example, the number 3452 is in the second position (n=2). So the formula to determine the index number is 2-1 = 1. So write the following code to replace 3452 with 1010:

items[1] = 1010;

I solved this problem using for loops and iterating through the original array and adding the positions of the matching arreas to another array and then looping through that array and changing it in the original array then return it, I used and arrow function but a regular function would work too.

var replace = (arr, replaceThis, WithThis) => {
    if (!Array.isArray(arr)) throw new RangeError("Error");
    var itemSpots = [];
    for (var i = 0; i < arr.length; i++) {
        if (arr[i] == replaceThis) itemSpots.push(i);
    }

    for (var i = 0; i < itemSpots.length; i++) {
        arr[itemSpots[i]] = WithThis;
    }

    return arr;
};

presentPrompt(id,productqty) {
    let alert = this.forgotCtrl.create({
      title: 'Test',
      inputs: [
        {
          name: 'pickqty',
          placeholder: 'pick quantity'
        },
        {
          name: 'state',
          value: 'verified',
          disabled:true,
          placeholder: 'state',

        }
      ],
      buttons: [
        {
          text: 'Ok',
          role: 'cancel',
          handler: data => {

            console.log('dataaaaname',data.pickqty);
            console.log('dataaaapwd',data.state);


          for (var i = 0; i < this.cottonLists.length; i++){

            if (this.cottonLists[i].id == id){
                this.cottonLists[i].real_stock = data.pickqty;

            }
          }

          for (var i = 0; i < this.cottonLists.length; i++){

            if (this.cottonLists[i].id == id){
              this.cottonLists[i].state = 'verified';   

          }
        }
            //Log object to console again.
            console.log("After update: ", this.cottonLists)
            console.log('Ok clicked');
          }
        },

      ]
    });
    alert.present();
  }

As per your requirement you can change fields and array names.
thats all. Enjoy your coding.

The easiest way is this.

var items = Array(523,3452,334,31, 5346);
var replaceWhat = 3452, replaceWith = 1010;
if ( ( i = items.indexOf(replaceWhat) ) >=0 ) items.splice(i, 1, replaceWith);

console.log(items);
>>> (5) [523, 1010, 334, 31, 5346]

When your array have many old item to replace new item, you can use this way:

function replaceArray(array, oldItem, newItem) {
    for (let i = 0; i < array.length; i++) {
        const index = array.indexOf(oldItem);
        if (~index) {
            array[index] = newItem;
        }
    }
    return array
}

console.log(replaceArray([1, 2, 3, 2, 2, 8, 1, 9], 2, 5));
console.log(replaceArray([1, 2, 3, 2, 2, 8, 1, 9], 2, "Hi"));

function replaceArray(index, value, arr) { 
        const index_num = arr.indexOf(value)
        const indexed_data = arr[index]
        arr[index] = value;
        arr[index_num] = indexed_data;
        return arr;
}

const arr = [523,3452,334,31, ...5346]
console.log("initial array", arr)
const changed_array = replaceArray(1, 523, arr) // output [3452, 523,334,31, ...5346]
console.log("changed array", changed_array)