This code helps to get the people who liked the modern site page based on the Library and Item ID parameters

spgetLikedpeople.js

//getRequest method reference
//https://gist.github.com/ktskumar/a9e9df497673e9fd26ead8532b9ff425
function getRequest(url) {
    var request = new XMLHttpRequest();
    return new Promise(function(resolve, reject) {
        request.onreadystatechange = function() {
            if (request.readyState !== 4) return;
            if (request.status >= 200 && request.status < 300) {
                resolve(request);
            } else {
                reject({
                    status: request.status,
                    statusText: request.statusText
                });
            }
        };

        request.open('GET', url, true);
        request.setRequestHeader("Content-Type", "application/json;charset=utf-8");
        request.setRequestHeader("ACCEPT", "application/json; odata.metadata=minimal");       
        request.setRequestHeader("ODATA-VERSION", "4.0");
        request.send();

    });
}

//Returns the user names liked the page. Page is identified based on the Id
//Also returns the User ID, Email, Liked Time, ...
getRequest("https://domain.sharepoint.com/sites/name/_api/web/lists/getbyTitle('Site Pages')/GetItemById(1)/likedByInformation?$expand=likedby").then(function(output) {
    var result = JSON.parse(output.response);
    var strMessage = "Total Likes: " + result.likedBy.length + "\r\n";
    for (var i = 0; i < result.likedBy.length; i++) {
        strMessage += result.likedBy[i].name + "\r\n";
    }
    alert(strMessage);
});