Pre-request for Postman to check if OAuth2 token expired and if so to refresh it.

Postman-pre-request.js

let currentTime = Math.floor(Date.now() / 1000);
let tokenExpiration = pm.environment.get("token_expiration");
if (tokenExpiration && tokenExpiration <= currentTime) {
    let apiUrl = pm.environment.get("api_url");
    let refreshToken = pm.environment.get("refresh_token");
    if (refreshToken) {
        pm.sendRequest(
            {
                url: 'http://' + apiUrl + '/oauth/v2/token',
                method: 'POST',
                header: 'Content-Type:application/json',
                body: {
                    mode: 'raw',
                    raw: JSON.stringify({
                      "client_id": "client_id",
                      "client_secret": "client_secret",
                      "refresh_token": refreshToken,
                      "grant_type": "refresh_token"
                    })
                }
            },
            function (err, response) {
                if (response.code == 200) {
                    let jsonData = response.json();
                    pm.environment.set("access_token", jsonData.access_token);
                    pm.environment.set("refresh_token", jsonData.refresh_token);
                    pm.environment.set("token_expiration", parseInt(jsonData.expires_in) + Math.floor(Date.now() / 1000));
                } else {
                    console.log({"error": err, "response": response});
                }
            }
        );
    } else {
        console.log("No refresh token");
    }
}