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Selecting Items in array - Javascript - Using Filter method

Someone asked a question today about figuring out to select certain elements within an array from I to the end of the array and it made me wonder how to do that with the filter method.
One of the solutions that someone gave was to use slice and I understand that you're able to select from index to index, but how would you implement the filter method to do the same thing?
Example
let arr = ['bug', 'cat', 'dog', 'flea', 'bat', 'hat', 'rat'];
let newArr = arr.filter(element => element >= element.indexOf(3));
console.log(newArr);
This is what I came up with, it doesn't work, but the idea is to select all strings that have an index of 3 or greater and return them into another array.

The runtime passes the index to the filter callback:
let newArr = arr.filter((element, index) => index >= 3);
Performance-wise you're still making a new array and copying values, so it's about the same as .slice().

You should create a function with the filter criteria:
let arr = ['bug', 'cat', 'dog', 'flea', 'bat', 'hat', 'rat'];
function filterCriteria(item) {
return item >= someValue;
}
function someFunction() {
return arr.filter(filterCriteria);
}
"someFunction" will return the array filtered

While you are visiting every item, you could use a counter and decrement it until the counter reaches zero. Then take this values.
const
fromIndex = i => _ => !i || !i--,
array = ['bug', 'cat', 'dog', 'flea', 'bat', 'hat', 'rat'],
result = array.filter(fromIndex(3));
console.log(result);

the second argument in the filter callback is index. So You can do something like this
arr.filter((element,index) => index >= 3);

Convert array of strings into an array of objects

I have this JavaScript array:
[ "124857202", "500255104", "78573M104" ]
I want to convert this particular array into an array of objects as shown below:
[
{ name: "124857202" },
{ name: "500255104" },
{ name: "78573M104" }
]

Use Array#map to convert each value into a different value:
var newArr = arr.map(function(value) {
return {name: value};
});
Array#map applies the callback to each element in the array and returns a new array containing the return values of the callback.

I would take a look at the array.map function in javascript.
const mappedArr = arr.map(value => {
return {
name: value
}
})

I want to convert this particular array into an array of objects as
shown below
If you want to change the actual array in place (rather than creating a new array), you can use a for loop to iterate the indexes of your array. For each index, you can replace the value with an object {name: arr[i]}. This object has a name key, and takes a value which is the current element arr[i].
const arr = [ "124857202", "500255104", "78573M104" ];
for(let i = 0; i < arr.length; i++) {
arr[i] = {name: arr[i]};
}
console.log(arr);
Or, if you want to make a new array and leave the original untouched, you can use Felix's answer, here it can be re-written to use more modern ES6 features to make it more concise, such as an arrow function and shorthand property names:
const arr = [ "124857202", "500255104", "78573M104" ];
const res = arr.map(name => ({name}));
console.log(res);

Another approach - Array#reduce.
var arr = ["124857202", "500255104", "78573M104"];
var res = arr.reduce(function(s, a){
s.push({name: a});
return s;
}, [])
console.log(res);

You can use
var arrayOfStrings = ["124857202", "500255104", "78573M104"];
var arrayOfObjects = [];
arrayOfStrings.forEach(function (element, index) {
arrayOfObjects.push({
name: element,
})
});

Felix Kling' answer, gehsekky's answer and the second part of Nick Parsons' answer are the most correct. For completeness, here is a version that uses Underscore's _.map:
import { map } from 'underscore';
var result = map(array, name => ({name}));
For this particular use case, _.map doesn't buy you much compared to Array.prototype.map except for a little bit of added portability. Going the other way, however, is a bit easier on the brain with _.map because of Underscore's iteratee shorthands:
// Underscore map
var array = map(result, 'name');
// Array.prototype.map
var array = result.map(obj => obj.name);
Underscore's map and other collection functions really shine when you need to iterate over a plain object, since JavaScript's built-in methods don't support this at all:
var objectOfStrings = {
first: "124857202",
second: "500255104",
third: "78573M104"
};
// to array of strings, Underscore
var arrayOfStrings = map(objectOfStrings);
// to array of strings, vanilla JS
var arrayOfStrings = [], value;
for (key in objectOfStrings) {
arrayOfStrings.push(objectOfStrings[key]);
}
// to array of objects, Underscore
var arrayOfObjects = map(objectOfStrings, name => ({name}));
// to array of objects, vanilla JS
var arrayOfStrings = [], name;
for (key in objectOfStrings) {
name = objectOfStrings[key];
arrayOfStrings.push({name});
}
var objectOfObjects = {
first: {name: "124857202"},
second: {name: "500255104"},
third: {name: "78573M104"}
};
// to array of strings, Underscore
var arrayOfStrings = map(objectOfStrings, 'name');
// to array of strings, vanilla JS
var arrayOfStrings = [], value;
for (key in objectOfObjects) {
arrayOfStrings.push(objectOfObjects[key].name);
}
// to array of objects, Underscore
var arrayOfObjects = map(objectOfObjects);
// to array of objects, vanilla JS
var arrayOfObjects = [], value;
for (key in objectOfStrings) {
arrayOfObjects.push(objectOfStrings[key]);
}

Check if a value exist in a multidimensional array and remove it [duplicate]

I have an array like
vendors = [{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
} // and so on...
];
How do I check this array to see if "Magenic" exists? I don't want to loop, unless I have to. I'm working with potentially a couple thousand records.

No need to reinvent the wheel loop, at least not explicitly (using arrow functions, modern browsers only):
if (vendors.filter(e => e.Name === 'Magenic').length > 0) {
/* vendors contains the element we're looking for */
}
or, better yet, use some as it allows the browser to stop as soon as one element is found that matches, so it's going to be faster:
if (vendors.some(e => e.Name === 'Magenic')) {
/* vendors contains the element we're looking for */
}
or the equivalent (in this case) find:
if (vendors.find(e => e.Name === 'Magenic')) {
/* same result as above, but a different function return type */
}
And you can even get the position of that element by using findIndex:
const i = vendors.findIndex(e => e.Name === 'Magenic');
if (i > -1) {
/* vendors contains the element we're looking for, at index "i" */
}
And if you need compatibility with lousy browsers then your best bet is:
if (vendors.filter(function(e) { return e.Name === 'Magenic'; }).length > 0) {
/* vendors contains the element we're looking for */
}

2018 edit: This answer is from 2011, before browsers had widely supported array filtering methods and arrow functions. Have a look at CAFxX's answer.
There is no "magic" way to check for something in an array without a loop. Even if you use some function, the function itself will use a loop. What you can do is break out of the loop as soon as you find what you're looking for to minimize computational time.
var found = false;
for(var i = 0; i < vendors.length; i++) {
if (vendors[i].Name == 'Magenic') {
found = true;
break;
}
}

No loop necessary. Three methods that come to mind:
Array.prototype.some()
This is the most exact answer for your question, i.e. "check if something exists", implying a bool result. This will be true if there are any 'Magenic' objects, false otherwise:
let hasMagenicVendor = vendors.some( vendor => vendor['Name'] === 'Magenic' )
Array.prototype.filter()
This will return an array of all 'Magenic' objects, even if there is only one (will return a one-element array):
let magenicVendors = vendors.filter( vendor => vendor['Name'] === 'Magenic' )
If you try to coerce this to a boolean, it will not work, as an empty array (no 'Magenic' objects) is still truthy. So just use magenicVendors.length in your conditional.
Array.prototype.find()
This will return the first 'Magenic' object (or undefined if there aren't any):
let magenicVendor = vendors.find( vendor => vendor['Name'] === 'Magenic' );
This coerces to a boolean okay (any object is truthy, undefined is falsy).
Note: I'm using vendor["Name"] instead of vendor.Name because of the weird casing of the property names.
Note 2: No reason to use loose equality (==) instead of strict equality (===) when checking the name.

The accepted answer still works but now we have an ECMAScript 6 native methods [Array.find][1] and [Array.some][2] to achieve the same effect.
Array.some
Use some If you only want to determine if an element exists i.e. you need a true/false determination.
Quoting MDN:
The some() method tests whether at least one element in the array passes the test implemented by the provided function. It returns true if, in the array, it finds an element for which the provided function returns true; otherwise it returns false. It doesn't modify the array.
Array.find
Use find if you want to get the matched object from array else returns undefined.
Quoting MDN:
The find() method returns the value of the first element in the provided array that satisfies the provided testing function. If no values satisfy the testing function, undefined is returned.
var arr = [
{
id: 21,
label: 'Banana',
},
{
id: 22,
label: 'Apple',
}
]
/* note : data is the actual object that matched search criteria
or undefined if nothing matched */
var data = arr.find(function(ele) {
return ele.id === 21;
});
if (data) {
console.log('found');
console.log(data); // This is entire object i.e. `item` not boolean
}
/* note : doesExist is a boolean thats true or false depending on of whether the data was found or not */
var doesExist = arr.some(function(ele) {
return ele.id === 21;
});
See my jsfiddle link There is a polyfill for IE provided by mozilla

Here's the way I'd do it
const found = vendors.some(item => item.Name === 'Magenic');
array.some() method checks if there is at least one value in an array that matches criteria and returns a boolean.
From here on you can go with:
if (found) {
// do something
} else {
// do something else
}

Unless you want to restructure it like this:
vendors = {
Magenic: {
Name: 'Magenic',
ID: 'ABC'
},
Microsoft: {
Name: 'Microsoft',
ID: 'DEF'
} and so on...
};
to which you can do if(vendors.Magnetic)
You will have to loop

May be too late, but javascript array has two methods some and every method that returns a boolean and can help you achieve this.
I think some would be most appropriate for what you intend to achieve.
vendors.some( vendor => vendor['Name'] !== 'Magenic' )
Some validates that any of the objects in the array satisfies the given condition.
vendors.every( vendor => vendor['Name'] !== 'Magenic' )
Every validates that all the objects in the array satisfies the given condition.

As per ECMAScript 6 specification, you can use findIndex.
const magenicIndex = vendors.findIndex(vendor => vendor.Name === 'Magenic');
magenicIndex will hold either 0 (which is the index in the array) or -1 if it wasn't found.

As the OP has asked the question if the key exists or not.
A more elegant solution that will return boolean using ES6 reduce function can be
const magenicVendorExists = vendors.reduce((accumulator, vendor) => (accumulator||vendor.Name === "Magenic"), false);
Note: The initial parameter of reduce is a false and if the array has the key it will return true.
Hope it helps for better and cleaner code implementation

You cannot without looking into the object really.
You probably should change your structure a little, like
vendors = {
Magenic: 'ABC',
Microsoft: 'DEF'
};
Then you can just use it like a lookup-hash.
vendors['Microsoft']; // 'DEF'
vendors['Apple']; // undefined

const check = vendors.find((item)=>item.Name==='Magenic')
console.log(check)
Try this code.
If the item or element is present then the output will show you that element. If it is not present then the output will be 'undefined'.

Testing for array elements:
JS Offers array functions which allow you to achieve this relatively easily. They are the following:
Array.prototype.filter: Takes a callback function which is a test, the array is then iterated over with is callback and filtered according to this callback. A new filtered array is returned.
Array.prototype.some: Takes a callback function which is a test, the array is then iterated over with is callback and if any element passes the test, the boolean true is returned. Otherwise false is returned
The specifics are best explained via an example:
Example:
vendors = [
{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
} //and so on goes array...
];
// filter returns a new array, we instantly check if the length
// is longer than zero of this newly created array
if (vendors.filter(company => company.Name === 'Magenic').length ) {
console.log('I contain Magenic');
}
// some would be a better option then filter since it directly returns a boolean
if (vendors.some(company => company.Name === 'Magenic')) {
console.log('I also contain Magenic');
}
Browser support:
These 2 function are ES6 function, not all browsers might support them. To overcome this you can use a polyfill. Here is the polyfill for Array.prototype.some (from MDN):
if (!Array.prototype.some) {
Array.prototype.some = function(fun, thisArg) {
'use strict';
if (this == null) {
throw new TypeError('Array.prototype.some called on null or undefined');
}
if (typeof fun !== 'function') {
throw new TypeError();
}
var t = Object(this);
var len = t.length >>> 0;
for (var i = 0; i < len; i++) {
if (i in t && fun.call(thisArg, t[i], i, t)) {
return true;
}
}
return false;
};
}

Simplest method so far:
if (vendors.findIndex(item => item.Name == "Magenic") == -1) {
//not found item
} else {
//found item
}

My approach to solving this problem is to use ES6 and creating a function that does the check for us. The benefit of this function is that it can be reusable through out your project to check any array of objects given the key and the value to check.
ENOUGH TALK, LET'S SEE THE CODE
Array
const ceos = [
{
name: "Jeff Bezos",
company: "Amazon"
},
{
name: "Mark Zuckerberg",
company: "Facebook"
},
{
name: "Tim Cook",
company: "Apple"
}
];
Function
const arrayIncludesInObj = (arr, key, valueToCheck) => {
return arr.some(value => value[key] === valueToCheck);
}
Call/Usage
const found = arrayIncludesInObj(ceos, "name", "Tim Cook"); // true
const found = arrayIncludesInObj(ceos, "name", "Tim Bezos"); // false

2021 Solution*
Lodash .some (docs) is a clean solution, if you use the _matchesProperty (docs) shorthand:
_.some(VENDORS, ['Name', 'Magenic'])
Explanation
This will iterate through the VENDORS Array looking for an element Object with the Name key having a value of the String 'Magenic'. Once it finds this element, it returns true and stops iterating. If it doesn't find the element after looking through the entire Array, it returns false.
Code snippet
const VENDORS = [{ Name: 'Magenic', ID: 'ABC' }, { Name: 'Microsoft', ID: 'DEF' }];
console.log(_.some(VENDORS, ['Name', 'Magenic'])); // true
<script src="https://cdn.jsdelivr.net/npm/lodash#4.17.20/lodash.min.js"></script>
* Note that this uses the popular lodash library to achieve the simplest/shortest possible solution. I'm offering this as an alternative to the existing vanilla JS solutions, for those who are interested.

You have to loop, there is no way around it.
function seekVendor(vendors, name) {
for (var i=0, l=vendors.length; i<l; i++) {
if (typeof vendors[i] == "object" && vendors[i].Name === name) {
return vendors[i];
}
}
}
Of course you could use a library like linq.js to make this more pleasing:
Enumerable.From(vendors).Where("$.Name == 'Magenic'").First();
(see jsFiddle for a demo)
I doubt that linq.js will be faster than a straight-forward loop, but it certainly is more flexible when things get a little more complicated.

Correct me if i'm wrong..
i could have used forEach method like this,
var found=false;
vendors.forEach(function(item){
if(item.name === "name"){
found=true;
}
});
Nowadays i'm used to it ,because of it simplicity and self explanatory word.
Thank you.

Functions map, filter, find, and similar are slower than the simple loop.
For me they also less readable than the simple loop and harder to debug. Using them looks like a kind of irrational ritual.
Better have something like this:
arrayHelper = {
arrayContainsObject: function (array, object, key){
for (let i = 0; i < array.length; i++){
if (object[key] === array[i][key]){
return true;
}
}
return false;
}
};
And use it like this with given OP example:
vendors = [{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
}
];
let abcObject = {ID: 'ABC', Name: 'Magenic'};
let isContainObject = arrayHelper.arrayContainsObject(vendors, abcObject, 'ID');

if you're using jquery you can take advantage of grep to create array with all matching objects:
var results = $.grep(vendors, function (e) {
return e.Name == "Magenic";
});
and then use the results array:
for (var i=0, l=results.length; i<l; i++) {
console.log(results[i].ID);
}

You can use lodash. If lodash library is too heavy for your application consider chunking out unnecessary function not used.
let newArray = filter(_this.props.ArrayOne, function(item) {
return find(_this.props.ArrayTwo, {"speciesId": item.speciesId});
});
This is just one way to do this. Another one can be:
var newArray= [];
_.filter(ArrayOne, function(item) {
return AllSpecies.forEach(function(cItem){
if (cItem.speciesId == item.speciesId){
newArray.push(item);
}
})
});
console.log(arr);
The above example can also be rewritten without using any libraries like:
var newArray= [];
ArrayOne.filter(function(item) {
return ArrayTwo.forEach(function(cItem){
if (cItem.speciesId == item.speciesId){
newArray.push(item);
}
})
});
console.log(arr);
Hope my answer helps.

Many answers here are good and pretty easy. But if your array of object is having a fixed set of value then you can use below trick:
Map all the name in a object.
vendors = [
{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
}
];
var dirtyObj = {}
for(var count=0;count<vendors.length;count++){
dirtyObj[vendors[count].Name] = true //or assign which gives you true.
}
Now this dirtyObj you can use again and again without any loop.
if(dirtyObj[vendor.Name]){
console.log("Hey! I am available.");
}

To compare one object to another, I combine a for in loop (used to loop through objects) and some().
You do not have to worry about an array going out of bounds etc, so that saves some code. Documentation on .some can be found here
var productList = [{id: 'text3'}, {id: 'text2'}, {id: 'text4', product: 'Shampoo'}]; // Example of selected products
var theDatabaseList = [{id: 'text1'}, {id: 'text2'},{id: 'text3'},{id:'text4', product: 'shampoo'}];
var objectsFound = [];
for(let objectNumber in productList){
var currentId = productList[objectNumber].id;
if (theDatabaseList.some(obj => obj.id === currentId)) {
// Do what you need to do with the matching value here
objectsFound.push(currentId);
}
}
console.log(objectsFound);
An alternative way I compare one object to another is to use a nested for loop with Object.keys().length to get the amount of objects in the array. Code below:
var productList = [{id: 'text3'}, {id: 'text2'}, {id: 'text4', product: 'Shampoo'}]; // Example of selected products
var theDatabaseList = [{id: 'text1'}, {id: 'text2'},{id: 'text3'},{id:'text4', product: 'shampoo'}];
var objectsFound = [];
for(var i = 0; i < Object.keys(productList).length; i++){
for(var j = 0; j < Object.keys(theDatabaseList).length; j++){
if(productList[i].id === theDatabaseList[j].id){
objectsFound.push(productList[i].id);
}
}
}
console.log(objectsFound);
To answer your exact question, if are just searching for a value in an object, you can use a single for in loop.
var vendors = [
{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
}
];
for(var ojectNumbers in vendors){
if(vendors[ojectNumbers].Name === 'Magenic'){
console.log('object contains Magenic');
}
}

Alternatively you can do:
const find = (key, needle) => return !!~vendors.findIndex(v => (v[key] === needle));

var without2 = (arr, args) => arr.filter(v => v.id !== args.id);
Example:
without2([{id:1},{id:1},{id:2}],{id:2})
Result:
without2([{id:1},{id:1},{id:2}],{id:2})

You can try this its work for me.
const _ = require('lodash');
var arr = [
{
name: 'Jack',
id: 1
},
{
name: 'Gabriel',
id: 2
},
{
name: 'John',
id: 3
}
]
function findValue(arr,value) {
return _.filter(arr, function (object) {
return object['name'].toLowerCase().indexOf(value.toLowerCase()) >= 0;
});
}
console.log(findValue(arr,'jack'))
//[ { name: 'Jack', id: 1 } ]

const a = [{one:2},{two:2},{two:4}]
const b = a.filter(val => "two" in val).length;
if (b) {
...
}

I would rather go with regex.
If your code is as follows,
vendors = [
{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
}
];
I would recommend
/"Name":"Magenic"/.test(JSON.stringify(vendors))

How to create a list of unique items in JavaScript? [duplicate]

This question already has answers here:
Get all unique values in a JavaScript array (remove duplicates)
(91 answers)
Closed 1 year ago.
In my CouchDB reduce function I need to reduce a list of items to the unique ones.
Note: In that case it's ok to have a list, it will be a small number of items of string type.
My current way is to set keys of a object, then return the keys of that object
since the place the code can't use things like _.uniq for example.
I'd like to find a more elegant way to spell it than this.
function(keys, values, rereduce) {
// values is a Array of Arrays
values = Array.concat.apply(null, values);
var uniq = {};
values.forEach(function(item) { uniq[item] = true; });
return Object.keys(uniq);
}

The best method seem to be using ES6 and Set. Single line and faster* than above according to fiddle
const myList = [1,4,5,1,2,4,5,6,7];
const unique = [...new Set(myList)];
console.log(unique);
*tested in safari

2021 answer:
const unique = (arr) => [...new Set(arr)];
unique([1, 2, 2, 3, 4, 4, 5, 1]); // [1, 2, 3, 4, 5]
Here you just create a set from the given array and then convert it back to the array.
I measured performance and it's almost twice faster now than the approach proposed in the old answer I posted before. Also, it's just a one-liner.
Updated fiddle
Old answer just for the record:
Commonly, the approach you used is a good idea.
But I could propose a solution that will make the algorithm a lot faster.
function unique(arr) {
var u = {}, a = [];
for(var i = 0, l = arr.length; i < l; ++i){
if(!u.hasOwnProperty(arr[i])) {
a.push(arr[i]);
u[arr[i]] = 1;
}
}
return a;
}
As you can see we have only one loop here.
I've made an example that is testing both your and my solutions. Try to play with it.

An alternative that's suitable for small lists would be to ape the Unix command line approach of sort | uniq:
function unique(a) {
return a.sort().filter(function(value, index, array) {
return (index === 0) || (value !== array[index-1]);
});
}
This function sorts the argument, and then filters the result to omit any items that are equal to their predecessor.
The keys-based approach is fine, and will have better performance characteristics for large numbers of items (O(n) for inserting n items into a hashtable, compared to O(n log n) for sorting the array). However, this is unlikely to be noticeable on small lists. Moreover, with this version you could modify it to use a different sorting or equality function if necessary; with hash keys you're stuck with JavaScripts notion of key equality.

This should work with anything, not just strings:
export const getUniqueList = (a: Array<any>) : Array<any> => {
const set = new Set<any>();
for(let v of a){
set.add(v);
}
return Array.from(set);
};
the above can just be reduced to:
export const getUniqueValues = (a: Array<any>) => {
return Array.from(new Set(a));
};
:)

To get unique objects, you can use JSON.stringify and JSON.parse:
const arr = [{test: "a"}, {test: "a"}];
const unique = Array.from(new Set(arr.map(JSON.stringify))).map(JSON.parse);
console.log(unique);

Using Object.keys will give you strings if you put in integer arguments (uniq([1,2,3]) => ['1','2','3']. Here's one with Array.reduce:
function uniq(list) {
return list.reduce((acc, d) => acc.includes(d) ? acc : acc.concat(d), []);
}

This is an old question, I know. However, it is at the top of some google searches, so I wanted to add that you can combine the answers from #RobHague and #EugeneNaydenov using the following:
function unique(arr) {
const u = {};
return arr.filter((v) => {
return u[v] = !u.hasOwnProperty(v);
});
};
You can also ignore undefined values (often handy) by adding:
function unique(arr) {
const u = {};
return arr.filter((v) => {
return u[v] = (v !== undefined && !u.hasOwnProperty(v));
});
};
You can play with this solution here: https://jsfiddle.net/s8d14v5n/

I find the other answers to be rather complicated for no gain that I can see.
We can use the indexOf method of the Array to verify if an item exists in it before pushing:
const duplicated_values = ['one', 'one', 'one', 'one', 'two', 'three', 'three', 'four'];
const unique_list = [];
duplicated_values.forEach(value => {
if (unique_list.indexOf(value) === -1) {
unique_list.push(value);
}
});
console.log(unique_list);
That will work with any type of variable as well, even objects (given the identifier actually reference the same entity, merely equivalent objects are not seen as the same).

what about
function unique(list) {
for (i = 0; i<list.length; i++) {
for (j=i+1; j<list.length; j++) {
if (list[i] == list[j]) {
list.splice(j, 1);
}
}
}
}

How to determine if Javascript array contains an object with an attribute that equals a given value?

I have an array like
vendors = [{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
} // and so on...
];
How do I check this array to see if "Magenic" exists? I don't want to loop, unless I have to. I'm working with potentially a couple thousand records.

No need to reinvent the wheel loop, at least not explicitly (using arrow functions, modern browsers only):
if (vendors.filter(e => e.Name === 'Magenic').length > 0) {
/* vendors contains the element we're looking for */
}
or, better yet, use some as it allows the browser to stop as soon as one element is found that matches, so it's going to be faster:
if (vendors.some(e => e.Name === 'Magenic')) {
/* vendors contains the element we're looking for */
}
or the equivalent (in this case) find:
if (vendors.find(e => e.Name === 'Magenic')) {
/* same result as above, but a different function return type */
}
And you can even get the position of that element by using findIndex:
const i = vendors.findIndex(e => e.Name === 'Magenic');
if (i > -1) {
/* vendors contains the element we're looking for, at index "i" */
}
And if you need compatibility with lousy browsers then your best bet is:
if (vendors.filter(function(e) { return e.Name === 'Magenic'; }).length > 0) {
/* vendors contains the element we're looking for */
}

2018 edit: This answer is from 2011, before browsers had widely supported array filtering methods and arrow functions. Have a look at CAFxX's answer.
There is no "magic" way to check for something in an array without a loop. Even if you use some function, the function itself will use a loop. What you can do is break out of the loop as soon as you find what you're looking for to minimize computational time.
var found = false;
for(var i = 0; i < vendors.length; i++) {
if (vendors[i].Name == 'Magenic') {
found = true;
break;
}
}

No loop necessary. Three methods that come to mind:
Array.prototype.some()
This is the most exact answer for your question, i.e. "check if something exists", implying a bool result. This will be true if there are any 'Magenic' objects, false otherwise:
let hasMagenicVendor = vendors.some( vendor => vendor['Name'] === 'Magenic' )
Array.prototype.filter()
This will return an array of all 'Magenic' objects, even if there is only one (will return a one-element array):
let magenicVendors = vendors.filter( vendor => vendor['Name'] === 'Magenic' )
If you try to coerce this to a boolean, it will not work, as an empty array (no 'Magenic' objects) is still truthy. So just use magenicVendors.length in your conditional.
Array.prototype.find()
This will return the first 'Magenic' object (or undefined if there aren't any):
let magenicVendor = vendors.find( vendor => vendor['Name'] === 'Magenic' );
This coerces to a boolean okay (any object is truthy, undefined is falsy).
Note: I'm using vendor["Name"] instead of vendor.Name because of the weird casing of the property names.
Note 2: No reason to use loose equality (==) instead of strict equality (===) when checking the name.

The accepted answer still works but now we have an ECMAScript 6 native methods [Array.find][1] and [Array.some][2] to achieve the same effect.
Array.some
Use some If you only want to determine if an element exists i.e. you need a true/false determination.
Quoting MDN:
The some() method tests whether at least one element in the array passes the test implemented by the provided function. It returns true if, in the array, it finds an element for which the provided function returns true; otherwise it returns false. It doesn't modify the array.
Array.find
Use find if you want to get the matched object from array else returns undefined.
Quoting MDN:
The find() method returns the value of the first element in the provided array that satisfies the provided testing function. If no values satisfy the testing function, undefined is returned.
var arr = [
{
id: 21,
label: 'Banana',
},
{
id: 22,
label: 'Apple',
}
]
/* note : data is the actual object that matched search criteria
or undefined if nothing matched */
var data = arr.find(function(ele) {
return ele.id === 21;
});
if (data) {
console.log('found');
console.log(data); // This is entire object i.e. `item` not boolean
}
/* note : doesExist is a boolean thats true or false depending on of whether the data was found or not */
var doesExist = arr.some(function(ele) {
return ele.id === 21;
});
See my jsfiddle link There is a polyfill for IE provided by mozilla

Here's the way I'd do it
const found = vendors.some(item => item.Name === 'Magenic');
array.some() method checks if there is at least one value in an array that matches criteria and returns a boolean.
From here on you can go with:
if (found) {
// do something
} else {
// do something else
}

Unless you want to restructure it like this:
vendors = {
Magenic: {
Name: 'Magenic',
ID: 'ABC'
},
Microsoft: {
Name: 'Microsoft',
ID: 'DEF'
} and so on...
};
to which you can do if(vendors.Magnetic)
You will have to loop

May be too late, but javascript array has two methods some and every method that returns a boolean and can help you achieve this.
I think some would be most appropriate for what you intend to achieve.
vendors.some( vendor => vendor['Name'] !== 'Magenic' )
Some validates that any of the objects in the array satisfies the given condition.
vendors.every( vendor => vendor['Name'] !== 'Magenic' )
Every validates that all the objects in the array satisfies the given condition.

As per ECMAScript 6 specification, you can use findIndex.
const magenicIndex = vendors.findIndex(vendor => vendor.Name === 'Magenic');
magenicIndex will hold either 0 (which is the index in the array) or -1 if it wasn't found.

As the OP has asked the question if the key exists or not.
A more elegant solution that will return boolean using ES6 reduce function can be
const magenicVendorExists = vendors.reduce((accumulator, vendor) => (accumulator||vendor.Name === "Magenic"), false);
Note: The initial parameter of reduce is a false and if the array has the key it will return true.
Hope it helps for better and cleaner code implementation

You cannot without looking into the object really.
You probably should change your structure a little, like
vendors = {
Magenic: 'ABC',
Microsoft: 'DEF'
};
Then you can just use it like a lookup-hash.
vendors['Microsoft']; // 'DEF'
vendors['Apple']; // undefined

const check = vendors.find((item)=>item.Name==='Magenic')
console.log(check)
Try this code.
If the item or element is present then the output will show you that element. If it is not present then the output will be 'undefined'.

Testing for array elements:
JS Offers array functions which allow you to achieve this relatively easily. They are the following:
Array.prototype.filter: Takes a callback function which is a test, the array is then iterated over with is callback and filtered according to this callback. A new filtered array is returned.
Array.prototype.some: Takes a callback function which is a test, the array is then iterated over with is callback and if any element passes the test, the boolean true is returned. Otherwise false is returned
The specifics are best explained via an example:
Example:
vendors = [
{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
} //and so on goes array...
];
// filter returns a new array, we instantly check if the length
// is longer than zero of this newly created array
if (vendors.filter(company => company.Name === 'Magenic').length ) {
console.log('I contain Magenic');
}
// some would be a better option then filter since it directly returns a boolean
if (vendors.some(company => company.Name === 'Magenic')) {
console.log('I also contain Magenic');
}
Browser support:
These 2 function are ES6 function, not all browsers might support them. To overcome this you can use a polyfill. Here is the polyfill for Array.prototype.some (from MDN):
if (!Array.prototype.some) {
Array.prototype.some = function(fun, thisArg) {
'use strict';
if (this == null) {
throw new TypeError('Array.prototype.some called on null or undefined');
}
if (typeof fun !== 'function') {
throw new TypeError();
}
var t = Object(this);
var len = t.length >>> 0;
for (var i = 0; i < len; i++) {
if (i in t && fun.call(thisArg, t[i], i, t)) {
return true;
}
}
return false;
};
}

Simplest method so far:
if (vendors.findIndex(item => item.Name == "Magenic") == -1) {
//not found item
} else {
//found item
}

My approach to solving this problem is to use ES6 and creating a function that does the check for us. The benefit of this function is that it can be reusable through out your project to check any array of objects given the key and the value to check.
ENOUGH TALK, LET'S SEE THE CODE
Array
const ceos = [
{
name: "Jeff Bezos",
company: "Amazon"
},
{
name: "Mark Zuckerberg",
company: "Facebook"
},
{
name: "Tim Cook",
company: "Apple"
}
];
Function
const arrayIncludesInObj = (arr, key, valueToCheck) => {
return arr.some(value => value[key] === valueToCheck);
}
Call/Usage
const found = arrayIncludesInObj(ceos, "name", "Tim Cook"); // true
const found = arrayIncludesInObj(ceos, "name", "Tim Bezos"); // false

2021 Solution*
Lodash .some (docs) is a clean solution, if you use the _matchesProperty (docs) shorthand:
_.some(VENDORS, ['Name', 'Magenic'])
Explanation
This will iterate through the VENDORS Array looking for an element Object with the Name key having a value of the String 'Magenic'. Once it finds this element, it returns true and stops iterating. If it doesn't find the element after looking through the entire Array, it returns false.
Code snippet
const VENDORS = [{ Name: 'Magenic', ID: 'ABC' }, { Name: 'Microsoft', ID: 'DEF' }];
console.log(_.some(VENDORS, ['Name', 'Magenic'])); // true
<script src="https://cdn.jsdelivr.net/npm/lodash#4.17.20/lodash.min.js"></script>
* Note that this uses the popular lodash library to achieve the simplest/shortest possible solution. I'm offering this as an alternative to the existing vanilla JS solutions, for those who are interested.

You have to loop, there is no way around it.
function seekVendor(vendors, name) {
for (var i=0, l=vendors.length; i<l; i++) {
if (typeof vendors[i] == "object" && vendors[i].Name === name) {
return vendors[i];
}
}
}
Of course you could use a library like linq.js to make this more pleasing:
Enumerable.From(vendors).Where("$.Name == 'Magenic'").First();
(see jsFiddle for a demo)
I doubt that linq.js will be faster than a straight-forward loop, but it certainly is more flexible when things get a little more complicated.

Correct me if i'm wrong..
i could have used forEach method like this,
var found=false;
vendors.forEach(function(item){
if(item.name === "name"){
found=true;
}
});
Nowadays i'm used to it ,because of it simplicity and self explanatory word.
Thank you.

Functions map, filter, find, and similar are slower than the simple loop.
For me they also less readable than the simple loop and harder to debug. Using them looks like a kind of irrational ritual.
Better have something like this:
arrayHelper = {
arrayContainsObject: function (array, object, key){
for (let i = 0; i < array.length; i++){
if (object[key] === array[i][key]){
return true;
}
}
return false;
}
};
And use it like this with given OP example:
vendors = [{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
}
];
let abcObject = {ID: 'ABC', Name: 'Magenic'};
let isContainObject = arrayHelper.arrayContainsObject(vendors, abcObject, 'ID');

if you're using jquery you can take advantage of grep to create array with all matching objects:
var results = $.grep(vendors, function (e) {
return e.Name == "Magenic";
});
and then use the results array:
for (var i=0, l=results.length; i<l; i++) {
console.log(results[i].ID);
}

You can use lodash. If lodash library is too heavy for your application consider chunking out unnecessary function not used.
let newArray = filter(_this.props.ArrayOne, function(item) {
return find(_this.props.ArrayTwo, {"speciesId": item.speciesId});
});
This is just one way to do this. Another one can be:
var newArray= [];
_.filter(ArrayOne, function(item) {
return AllSpecies.forEach(function(cItem){
if (cItem.speciesId == item.speciesId){
newArray.push(item);
}
})
});
console.log(arr);
The above example can also be rewritten without using any libraries like:
var newArray= [];
ArrayOne.filter(function(item) {
return ArrayTwo.forEach(function(cItem){
if (cItem.speciesId == item.speciesId){
newArray.push(item);
}
})
});
console.log(arr);
Hope my answer helps.

Many answers here are good and pretty easy. But if your array of object is having a fixed set of value then you can use below trick:
Map all the name in a object.
vendors = [
{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
}
];
var dirtyObj = {}
for(var count=0;count<vendors.length;count++){
dirtyObj[vendors[count].Name] = true //or assign which gives you true.
}
Now this dirtyObj you can use again and again without any loop.
if(dirtyObj[vendor.Name]){
console.log("Hey! I am available.");
}

To compare one object to another, I combine a for in loop (used to loop through objects) and some().
You do not have to worry about an array going out of bounds etc, so that saves some code. Documentation on .some can be found here
var productList = [{id: 'text3'}, {id: 'text2'}, {id: 'text4', product: 'Shampoo'}]; // Example of selected products
var theDatabaseList = [{id: 'text1'}, {id: 'text2'},{id: 'text3'},{id:'text4', product: 'shampoo'}];
var objectsFound = [];
for(let objectNumber in productList){
var currentId = productList[objectNumber].id;
if (theDatabaseList.some(obj => obj.id === currentId)) {
// Do what you need to do with the matching value here
objectsFound.push(currentId);
}
}
console.log(objectsFound);
An alternative way I compare one object to another is to use a nested for loop with Object.keys().length to get the amount of objects in the array. Code below:
var productList = [{id: 'text3'}, {id: 'text2'}, {id: 'text4', product: 'Shampoo'}]; // Example of selected products
var theDatabaseList = [{id: 'text1'}, {id: 'text2'},{id: 'text3'},{id:'text4', product: 'shampoo'}];
var objectsFound = [];
for(var i = 0; i < Object.keys(productList).length; i++){
for(var j = 0; j < Object.keys(theDatabaseList).length; j++){
if(productList[i].id === theDatabaseList[j].id){
objectsFound.push(productList[i].id);
}
}
}
console.log(objectsFound);
To answer your exact question, if are just searching for a value in an object, you can use a single for in loop.
var vendors = [
{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
}
];
for(var ojectNumbers in vendors){
if(vendors[ojectNumbers].Name === 'Magenic'){
console.log('object contains Magenic');
}
}

Alternatively you can do:
const find = (key, needle) => return !!~vendors.findIndex(v => (v[key] === needle));

var without2 = (arr, args) => arr.filter(v => v.id !== args.id);
Example:
without2([{id:1},{id:1},{id:2}],{id:2})
Result:
without2([{id:1},{id:1},{id:2}],{id:2})

You can try this its work for me.
const _ = require('lodash');
var arr = [
{
name: 'Jack',
id: 1
},
{
name: 'Gabriel',
id: 2
},
{
name: 'John',
id: 3
}
]
function findValue(arr,value) {
return _.filter(arr, function (object) {
return object['name'].toLowerCase().indexOf(value.toLowerCase()) >= 0;
});
}
console.log(findValue(arr,'jack'))
//[ { name: 'Jack', id: 1 } ]

const a = [{one:2},{two:2},{two:4}]
const b = a.filter(val => "two" in val).length;
if (b) {
...
}

I would rather go with regex.
If your code is as follows,
vendors = [
{
Name: 'Magenic',
ID: 'ABC'
},
{
Name: 'Microsoft',
ID: 'DEF'
}
];
I would recommend
/"Name":"Magenic"/.test(JSON.stringify(vendors))