[CoaPsyFactor]

Here is js code, you put this in your main js, or inside <script> tags on index (main) file.
Just replace "PUT_YOUR_URL_HERE" with your url

After request is finished successfully your data will be stored in "window.API_DATA"

NOTE: API Must return proper JSON format
NOTE: ES6 is supported on most current generation browsers, you can checkout compatibility table here

PHP код:

'use strict'
const API_ENDPOINT = 'PUT_YOUR_URL_HERE';
(() => {
    const STATUS = {
        OK: 200
    };
    const ERROR = {
        NONE: 'NONE',
        PARSE: 'INVALID_JSON',
        STATUS: 'INVALID_STATUS_CODE'
    };
    const loadData = (url, data = {}) => {
        return new Promise((resolve, reject) => {
            const xhr = new XMLHttpRequest();
            // Using traditional function definition
            // to prevent invalid context (this)
            xhr.onreadystatechange = function () {
                if (XMLHttpRequest.DONE !== this.readyState) {
                    // Not resolving nor rejecting as we need this promise to be done when request is
                    return;
                }
                if (STATUS.OK !== this.status) {
                    // In case that status code is not 200 (OK) we assume something went wrong
                    return reject({ error: ERROR.STATUS, url });
                }
                let data;
                try {
                    data = JSON.parse(this.responseText);
                } catch (error) {
                    // In case that server response was not valid json
                    // we reject this promise with proper error
                    return reject({ error: ERROR.PARSE, response: this.responseText, url });
                }
                return resolve({ error: ERROR.NONE, data });
            };
            xhr.open('GET', url, true);
            xhr.send();
        });
    };
    window.onload = () => {
        loadData(API_ENDPOINT)
            .then((data) => { window.API_DATA = data; })
            .catch((error) => { console.error(error); });
    };
})();