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console.log of array returns undefined

I am trying to take two pieces of data from an object and push it as a new object into an array. The data is being supplied by an API call to my SQL database. That API call is working correctly and displaying the object in a console table. When the script runs a forEach method to extract the data into its own object and then push that new object to a new array, the new array returns "undefined". Code below:
Example data (only one placeholder entry currently, the events array will be seeded with multiple examples in this format)
events = [{location: "Emergency Shelter", latitude: "37.5434", longitude: "-77.4435"}]
Empty arrays declared and API call functioning properly:
let events = [];
let locations = [];
$.get("/api/events", data => {
for (let i = 0; i < data.length; i++) {
events.push(data[i]);
}
});
console.table displays the object correctly and includes the keys "latitude" and "longitude" with the correct corresponding values
forEach method:
locations = events.forEach(location => {
const coords = {};
coords.latitude = location.latitude;
coords.longitude = location.longitude;
locations.push(coords);
});
console.log("Coordinates list: " + locations);
console.log displays "Coordinates list: undefined"
I feel like I may be missing a return somewhere, but I'm not sure where. I tried adding
return locations;
inside the forEach method but it doesn't change anything (and I believe that would exit my function prior to getting through the entire array). Any help is appreciated!

forEach returns nothing so locations should be undefined. You shouldn't pass return value of forEach to locations
events.forEach(location => {
const coords = {};
coords.latitude = location.latitude;
coords.longitude = location.longitude;
locations.push(coords);
});
console.log("Coordinates list: " + locations);
Also you can use map function.
const events = [
{ location: 'Emergency Shelter', latitude: '37.5434', longitude: '-77.4435' }
];
const locations = events.map(({ latitude, longitude }) => ({
latitude,
longitude
}));
console.log(locations);

Try a map based approach which also would condense your code to ...
const events = [{
location: "Emergency Shelter",
latitude: "37.5434",
longitude: "-77.4435"
}, {
location: "Peopl's Kitchen",
latitude: "36",
longitude: "-78"
}, {
location: "Salvation Army",
latitude: "38",
longitude: "-76"
}];
const locations = events.map(({ latitude, longitude }) => ({ latitude, longitude }));
console.log("Coordinates list: ", locations); // do not concatenate the result.
console.log("Coordinates list: " + locations);
console.log("Coordinates list: " + JSON.stringify(locations));
.as-console-wrapper { min-height: 100%!important; top: 0; }
map creates a new array by iterating another one where each item of the new array equals the return value of the mapping/transforming function which, at each iteration step, does process the current value of the original array.

Filtering GeoJSON data using array of values to create Leaflet map

I'm still new to coding so please forgive me if I'm not asking the right questions.
I have a JSON file with data from a number of countries and I want to use Leaflet to paint all the countries in that file on a map. To do so I'm using a GeoJSON file containing all the countries of the world, and I want to filter this GeoJSON file by "id" (which is a 3-letter country code) so that it only includes the countries in my array (also 3-letter country code).
I used D3.json to get an array of just the country codes from my JSON file, and then looped through that array to get each country in the list to filter my GeoJSON file:
d3.json("countryList.json", function(data) {
const countries = [...new Set(data.map(d => d.country))];
console.log(countries);
for (var i = 0; i < countries.length; i++) {
var countryCode = countries[i];
console.log(countryCode);
};
I tried this with no luck:
L.geoJson(data, {
filter: function(feature) }
if (feature.id === countryCode) return True
}
};

You don't need d3 to filter your countries. Also, when you initialize a variable inside a for-loop, it's going to get overwritten unless you break from the loop. Anyways, take this example of filtering a single U.S. state out of 2 states and go from there:
var geojson_data = {"type":"FeatureCollection","features":[{"type":"Feature","id":"36","properties":{"name":"New York"},"geometry":{"type":"Polygon","coordinates":[[[-73.343806,45.013027],[-73.332852,44.804903],[-73.387622,44.618687],[-73.294514,44.437948],[-73.321898,44.246255],[-73.436914,44.043608],[-73.349283,43.769761],[-73.404052,43.687607],[-73.245221,43.523299],[-73.278083,42.833204],[-73.267129,42.745573],[-73.508114,42.08834],[-73.486206,42.050002],[-73.55193,41.294184],[-73.48073,41.21203],[-73.727192,41.102491],[-73.655992,40.987475],[-73.22879,40.905321],[-73.141159,40.965568],[-72.774204,40.965568],[-72.587988,40.998429],[-72.28128,41.157261],[-72.259372,41.042245],[-72.100541,40.992952],[-72.467496,40.845075],[-73.239744,40.625997],[-73.562884,40.582182],[-73.776484,40.593136],[-73.935316,40.543843],[-74.022947,40.708151],[-73.902454,40.998429],[-74.236547,41.14083],[-74.69661,41.359907],[-74.740426,41.431108],[-74.89378,41.436584],[-75.074519,41.60637],[-75.052611,41.754247],[-75.173104,41.869263],[-75.249781,41.863786],[-75.35932,42.000709],[-79.76278,42.000709],[-79.76278,42.252649],[-79.76278,42.269079],[-79.149363,42.55388],[-79.050778,42.690804],[-78.853608,42.783912],[-78.930285,42.953697],[-79.012439,42.986559],[-79.072686,43.260406],[-78.486653,43.375421],[-77.966344,43.369944],[-77.75822,43.34256],[-77.533665,43.233021],[-77.391265,43.276836],[-76.958587,43.271359],[-76.695693,43.34256],[-76.41637,43.523299],[-76.235631,43.528776],[-76.230154,43.802623],[-76.137046,43.961454],[-76.3616,44.070993],[-76.312308,44.196962],[-75.912491,44.366748],[-75.764614,44.514625],[-75.282643,44.848718],[-74.828057,45.018503],[-74.148916,44.991119],[-73.343806,45.013027]]]}},{"type":"Feature","id":"48","properties":{"name":"Texas"},"geometry":{"type":"Polygon","coordinates":[[[-101.812942,36.501861],[-100.000075,36.501861],[-100.000075,34.563024],[-99.923398,34.573978],[-99.698843,34.382285],[-99.57835,34.415147],[-99.260688,34.404193],[-99.189488,34.2125],[-98.986841,34.223454],[-98.767763,34.135823],[-98.570593,34.146777],[-98.488439,34.064623],[-98.36247,34.157731],[-98.170777,34.113915],[-98.088623,34.004376],[-97.946222,33.987946],[-97.869545,33.851022],[-97.694283,33.982469],[-97.458774,33.905791],[-97.371143,33.823637],[-97.256128,33.861976],[-97.173974,33.736006],[-96.922034,33.960561],[-96.850834,33.845545],[-96.631756,33.845545],[-96.423633,33.774345],[-96.346956,33.686714],[-96.149786,33.840068],[-95.936185,33.889361],[-95.8376,33.834591],[-95.602092,33.933176],[-95.547322,33.878407],[-95.289906,33.87293],[-95.224183,33.960561],[-94.966767,33.861976],[-94.868182,33.74696],[-94.484796,33.637421],[-94.380734,33.544313],[-94.183564,33.593606],[-94.041164,33.54979],[-94.041164,33.018527],[-94.041164,31.994339],[-93.822086,31.775262],[-93.816609,31.556184],[-93.542762,31.15089],[-93.526331,30.93729],[-93.630393,30.679874],[-93.728978,30.575812],[-93.696116,30.438888],[-93.767317,30.334826],[-93.690639,30.143133],[-93.926148,29.787132],[-93.838517,29.688547],[-94.002825,29.68307],[-94.523134,29.546147],[-94.70935,29.622824],[-94.742212,29.787132],[-94.873659,29.672117],[-94.966767,29.699501],[-95.016059,29.557101],[-94.911997,29.496854],[-94.895566,29.310638],[-95.081782,29.113469],[-95.383014,28.867006],[-95.985477,28.604113],[-96.045724,28.647929],[-96.226463,28.582205],[-96.23194,28.642452],[-96.478402,28.598636],[-96.593418,28.724606],[-96.664618,28.697221],[-96.401725,28.439805],[-96.593418,28.357651],[-96.774157,28.406943],[-96.801542,28.226204],[-97.026096,28.039988],[-97.256128,27.694941],[-97.404005,27.333463],[-97.513544,27.360848],[-97.540929,27.229401],[-97.425913,27.262263],[-97.480682,26.99937],[-97.557359,26.988416],[-97.562836,26.840538],[-97.469728,26.758384],[-97.442344,26.457153],[-97.332805,26.353091],[-97.30542,26.161398],[-97.217789,25.991613],[-97.524498,25.887551],[-97.650467,26.018997],[-97.885976,26.06829],[-98.198161,26.057336],[-98.466531,26.221644],[-98.669178,26.238075],[-98.822533,26.369522],[-99.030656,26.413337],[-99.173057,26.539307],[-99.266165,26.840538],[-99.446904,27.021277],[-99.424996,27.174632],[-99.50715,27.33894],[-99.479765,27.48134],[-99.605735,27.640172],[-99.709797,27.656603],[-99.879582,27.799003],[-99.934351,27.979742],[-100.082229,28.14405],[-100.29583,28.280974],[-100.399891,28.582205],[-100.498476,28.66436],[-100.629923,28.905345],[-100.673738,29.102515],[-100.799708,29.244915],[-101.013309,29.370885],[-101.062601,29.458516],[-101.259771,29.535193],[-101.413125,29.754271],[-101.851281,29.803563],[-102.114174,29.792609],[-102.338728,29.869286],[-102.388021,29.765225],[-102.629006,29.732363],[-102.809745,29.524239],[-102.919284,29.190146],[-102.97953,29.184669],[-103.116454,28.987499],[-103.280762,28.982022],[-103.527224,29.135376],[-104.146119,29.381839],[-104.266611,29.513285],[-104.507597,29.639255],[-104.677382,29.924056],[-104.688336,30.181472],[-104.858121,30.389596],[-104.896459,30.570335],[-105.005998,30.685351],[-105.394861,30.855136],[-105.602985,31.085167],[-105.77277,31.167321],[-105.953509,31.364491],[-106.205448,31.468553],[-106.38071,31.731446],[-106.528588,31.786216],[-106.643603,31.901231],[-106.616219,31.999816],[-103.067161,31.999816],[-103.067161,33.002096],[-103.045254,34.01533],[-103.039777,36.501861],[-103.001438,36.501861],[-101.812942,36.501861]]]}}]}
L.geoJSON(geojson_data, {
filter: function(feature) {
return feature.properties.name === "Texas";
}
}).addTo(map);

Using a loop to push to an array which is a value in a Map

I am currently trying to create a function which dynamically changes values of an array in a map. This function is intended to create a new element in the array on each full loop, to represent a new instance to be counted.
The function is meant to count how many visitors to a country there are every day. This is done throgh a map, with the keys being the country and the values being a list of visitors. Each element is how many visitors there were in a specific day.
Where Map:
var countries = new Map();
('England', [0]);
('France', [0]);
('Spain', [0]);
('Poland', [0]);
('Germany', [0]);
('Russia', [0]);
For the code:
for(var day = 0; day < daysBack; day++){
var dateToFind = (currentDate.date() + " / " +
currentDate.month() + " / " +
currentDate.year());
var detectionsOnDate = getActivity(dateToFind );
// Returns an array of locations visited, each visit is a new element
// eg. ["France", "France", "England", "France"]
for(var detectionLocation of detectionsOnDate){
for(var[location, visits] of countries ){
if(location == detectionLocation){
countries.set(location, visits[day]++);
}
}
}
currentDate.subtract(1, 'days');
}
The result I am looking for is a map similar to:
{
('England', [242,235,853,122,512]);
('France', [241,621,173,173,512]);
('Spain', [62,235,213,465,126]);
('Poland', [734,163,856,354,142]);
('Germany', [234,235,643,237,512]);
('Russia', [174,200,136,107,164]);
}
In my idea of the logic, this should create a new element in the value every time a new loop of 'day' is done. On each loop of day the same element in the map should be updated.
I'm relatively new to JS so I'm sorry if this is an easy fix, still finding my way around. Any help is appriciated. Thanks.

push returns the length of the array. So, a number will be set to each key. Instead, you can concat a new number to the value array and set it to the current key.
map.set(key, value.concat(value[i]+1))
and
map.set(key, value.concat(0))

How to populate array with multiple data types inside?

I want to populate "markers" array that contains object and string. I have another javascript object stored in obj variable and I want to extract data from it and store it into markers array.
I am populating array with following code
for (i = 0; i < obj.results.bindings.length; i++) {
markers[i].content = obj.results.bindings[i].placeName.value;
markers[i].coords.lat = Number(obj.results.bindings[i].lat.value);
markers[i].coords.lng = Number(obj.results.bindings[i].long.value);
}
and markers array looks like this (populated manually, not by for loop)
var markers = [
{
coords:{lat:37.8707,lng:32.5050},
content:'<h1>Mevlana museum</h1>'
},
{
coords:{lat:37.8699,lng:32.4993},
content:'<h1>Kapu Camii</h1>'
},
{
coords:{lat:37.8749,lng:32.4931},
content:'<h1>Karatay Madrasa</h1>'
},
{
coords:{lat:37.8749,lng:32.4931},
content:'<h1>Karatay Madrasa</h1>'
},
{
coords:{lat:37.8749,lng:32.4931},
content:'<h1>Karatay Madrasa</h1>'
}
];
Now when javascript object 'obj' have same number of nested objects as elements of markers it works fine but when 'obj' have more nested objects than markers array has elements problems. I just want to add rest of the nested objects from 'obj' into markers. Why doesn't markers array grow dinamically when I add new elements in the for loop?

How about this?
Array should not grow dynamically. You could use push/pop method.
markers = [];
for (i = 0; i < obj.results.bindings.length; i++) {
markers.push({
content : obj.results.bindings[i].placeName.value,
coords : {
lat : Number(obj.results.bindings[i].lat.value),
lng : Number(obj.results.bindings[i].long.value)
}
})
}
I try again with your code below:
for (i = 0; i < obj.results.bindings.length; i++) {
markers[i] = {};
markers[i].content = obj.results.bindings[i].placeName.value;
markers[i].coords= {};
markers[i].coords.lat = Number(obj.results.bindings[i].lat.value);
markers[i].coords.lng = Number(obj.results.bindings[i].long.value);
}
I guess that you didn't assign the type as object, so it went wrong.
Please let me know if this works too.

You can use forEach of your obj.results.bindings then construct each and every markers that you want and eventually push into the array as below
let obj = {
results: {
bindings: [
{ placeName: 'cali', lat: '123', long: '456' },
{ placeName: 'hawaii', lat: '555', long: '333'},
{ placeName: 'korea', lat: '777', long: '888' }]
}
};
let markers = [];
obj.results.bindings.forEach(item => {
let mar = { content: '', coords: {}};
mar.content = item.placeName;
mar.coords.lat = Number(item.lat);
mar.coords.lng = Number(item.long);
markers.push(mar)
})
console.log(markers)

Push object into an array

I have requirement like need to form object as below
[
{
"place": "Royal Palace, Oslo",
"latitude" : "59.916911"
},
{
"place": "Royal Palace, Oslo",
"latitude" : "59.916911"
}
]
the above place and latitude values are available within map function as
let sampleArray = [];
jsonresponse.map((item) => {
let place = item.place;
let latitude = {/*with other logic function we will get latitude value*/}
//need to send these both values into below array to form array shown as above.
sampleArray.push();
})
thanks in advance.

You are using the map function wrong.
In a map function, you create a new array in which for each value the return value replaces the current value.
Your function isn't returning new values, and isn't pushing anything to the array. So you have 2 options:
//FIRST OPTION
const sampleArray = jsonResponse.map(({ place } => ({
place,
latitude: [SOME_VALUE]
}))
//SECOND OPTION
const sampleArray = [];
jsonresponse.forEach(({ place }) => {
sampleArray.push({
place,
latitude: [SOME_VALUE]
})
})
Also, notice the es6 destructuring syntax, it could save you some code.

All you need to do with Array.prototype.map is:
let sampleArray = jsonresponse.map((item) => {
let place = item.place;
let latitude = {/*with other logic function we will get latitude value*/}
return {
place,
latitude
}
})

Is this what you want to accomplish ?
let sampleArray = []
jsonresponse.map(item => {
sampleArray.push({
place: item.place,
latitude: {/*with other logic function we will get latitude value*/}
})
})
console.log(sampleArray)