Postman PreRequestScript to check if an OAuth token has expired and, if so, refreshes it by making a new authentication request.

refreshOAuthToken.js

const expiryDate = new Date(pm.environment.get('tokenExpiryDate'));
const currDate = new Date(Date.now());
const isTokenExpired = (isNaN(expiryDate) || expiryDate) < currDate;

if(isTokenExpired){
pm.sendRequest({
    url: pm.globals.get("oAuthUrl"),
    method: 'POST',
    header: {
        'Authorization': pm.globals.get("basicAuth"),
    },
    body:{
        mode: 'urlencoded',
        urlencoded: [
            { key: "grant_type", value: "password" },
            { key: "username", value: pm.globals.get("username") },
            { key: "password", value: pm.globals.get("password") }
        ]
    }
}, function (err, res) {
    if(err) return
    const response =  res.json();
    var authToken = response.access_token;
    const currDate = new Date();
    var tokenExpiry = new Date(currDate.getTime() + 1000 * (response.expires_in));
    pm.globals.set("tokenSetFrom", "PreRequestScript")
    pm.environment.set("tokenExpiryDate", tokenExpiry);
    pm.environment.set("authToken", authToken);
});
}