For Loop With Two Array in JavaScript

forEach loop through two arrays at the same time in javascript

Use the second parameter forEach accepts instead, which will be the current index you're iterating over:

n = [1,2,3,5,7,8,9,11,12,13,14,16,17,18,20,21,22];
n.forEach((element, index) => {  console.log(element, index);});

For Loop with two array in javascript

You can do something like this:

var html ='';
var option = [  ['meal', 'big', 'medium', 'small'],  ['drink', 'big', 'medium' ]];
for (var i = 0; i < option.length; i++){    var option_type = option[i];        html += '<div>' + option_type[0] + '</div>';        for(var j = 1; j < option_type.length; j++) {         html += '<span>' + option_type[j] + '</span>';    } }
document.getElementById('container').innerHTML=html;

#container span{     display:inline-block;     margin-right:10px;     border:1px solid #333;     padding:10px;}

<div id="container"></div>

Combining two arrays in JavaScript with for loop

1) declare the two arrays, especially the numbers one (possibly with the dot operator?)

You mean create rather than declare (you don't declare arrays in JavaScript, because it's a loosely typed language). Yes, you can use an array literal:

var letters = ["a", "b", "c", "d"];

or split:

var letters = "abcd".split("");

...though whether split is "smarter" is a matter of opinion. :-)

You can also use the mapping feature of Array.from:

var letters = Array.from(Array(4), (_, index) => String.fromCharCode(97 + index));

But that might be a bit overdoing it. It makes more sense for the numbers one as you mentioned:

var numbers = Array.from(Array(6), (_, index) => index + 1); // Note I've renamed it to be plural

2) combine the string elements of these two arrays into the combined one without using an old fashioned for loop, perhaps with Array.map() or Array.from()?

A for loop is simple and clear, but for-of would probably be even simpler and clearer:

for (const letter of letters) {
    for (const number of numbers) { // Note I've renamed the `numbers` array to be plural
        combined.push(letter + number);
    }
}

Although you could shoehorn it into an Array.from by doing math on the index, to me it would be overcomplicated.

for-of and Array.from are both new(ish) as of ES2015 (aka "ES6"), so be sure to check your target environments for support, and/or use transpiling (for for-of) and a polyfill (for Array.from).

Looping over two arrays in javascript simultaneously

Turn the first array into an object indexed by id first, so you can look up the appropriate matching object in O(1) time, and then you can .map the selectedList:

const masterList=[{id:1,name:'Abc'},{id:2,name:'Def'},{id:3,name:'Ghi'}];const selectedList=[2,3];
const masterById = masterList.reduce((a, obj) => {  a[obj.id] = obj;  return a;}, {});
const desiredList = selectedList.map(id => masterById[id]);console.log(desiredList);

looping through 2 arrays

You don't want a nested loop - you simply need to link the ith item in links to the ith item of hrefLinks. With forEach, you can do this just by using the second argument to the callback, which will refer to the current index being iterated over:

const list = document.body.appendChild(document.createElement('ul'));

const links = ['Home', 'Contact', 'About'];const hrefLinks = ['/', 'contact', 'about'];
links.forEach((linkName, i) => {  const li = document.createElement('li');  const a = document.createElement('a');  a.href = hrefLinks[i];  a.textContent = linkName;  li.appendChild(a);  li.className = 'nav-link';  list.appendChild(li);});

Looping over two arrays of object to loop for matching ids. Then return a new array of custom objects based of custom values

You can map over the array and use Array.find to get the item in contacts whose phoneNumber property is either the current item's to or from property. If an item is found, assign the item's name property to the current item's displayName property:

To implement a default display name, we can remove the if statement and use a ternary operator to assign the name property if an item was found, and otherwise assign the default display name.

const myNumber = '69';

const contacts=[{phoneNumber:"420",name:"Mike Smith",favorite:!1,color:"orange"},{phoneNumber:"360",name:"John Smith",favorite:!0,color:"green"}];

const messages=[{to:"69",from:"360",isRead:!1,time:new Date,owner:0,message:"Hello there friend this is walker"},{to:"69",from:"360",isRead:!0,time:new Date,owner:1,message:"Hello there friend this is walker"},{to:"69",from:"720",isRead:!1,time:new Date,owner:0,message:"Hello there friend from random person"},{to:"69",from:"720",isRead:!0,time:new Date,owner:1,message:"Hello there friend from random person"}];

let defaultDisplayName = 'Spectric'

const result = messages.map(e => {
  let found = contacts.find(f => [e.to, e.from].includes(f.phoneNumber))
  e.displayName = found ? found.name : defaultDisplayName;
  return e;
})

console.log(result)