Connect Salesforce REST API Using JavaScript

const requestData = {

    'transId': 'NEFT0123456789',

    'transAmount': 20500.50,

    'transDate': '25/10/2022',

    'transRemark': 'M400 Payment message',

};

const tokenURL = 'https://XXXXXXX.sandbox.my.salesforce.com/services/oauth2/token';

const payloadURL = 'https://XXXXXXX.sandbox.my.salesforce.com/services/apexrest/M400_SFReceiver';

const grant_type = 'password';

const client_id = 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX';

const client_secret = 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX';

const username = 'XXXXXXXXXXXXXXXXXXXXXXXXXX';

const password = 'XXXXXXXXXXXXXXXXXXXXX';

fetch(tokenURL + '?grant_type=' + grant_type + '&client_id=' + client_id + '&client_secret=' + client_secret + '&username=' + username + '&password=' + password, {

    method: 'POST'

})

    .then(resp => resp.json())

    .then(json => {

        sfdcToken = json.access_token;

        this.doM400_APICall(sfdcToken, requestData);

    })

    .catch(err => {

        console.log(err);

        const sfdcResponse = document.getElementById('sfdcResponse');

        sfdcResponse.innerHTML = err;

    });

function doM400_APICall(accessToken, requestBody) {

    var xmlhttp = new XMLHttpRequest();

    xmlhttp.open("POST", payloadURL, false);

    xmlhttp.setRequestHeader("Content-Type", "application/json");

    xmlhttp.setRequestHeader('Authorization', 'Bearer ' + accessToken);

    xmlhttp.send(JSON.stringify(requestBody));

    document.getElementById("sfdcStatus").innerHTML = "<font color='green'>Status: " + xmlhttp.status + " " + xmlhttp.statusText + "</font>";

    document.getElementById("sfdcResponse").innerText = xmlhttp.responseText;

}