Postman Script (Pre-request)

pre-request.js

pm.test("Check for collection's variables..", function () {
    let vars = ['client_id', 'client_secret', 'grant_type'];
    vars.forEach(function (item, index, array) {
        console.log(item, index);
        pm.expect(pm.collectionVariables.get(item), item + " variable not set").to.not.be.undefined;
        pm.expect(pm.collectionVariables.get(item), item + " variable not set").to.not.be.empty; 
    });

    if ((!pm.collectionVariables.get("access_token") && pm.collectionVariables.get("IS_TESTING")) 
            || Date.now() > new Date(pm.collectionVariables.get("expires_in") * 1000)) {

        var baseUrl = pm.collectionVariables.get("IS_TESTING") == "true" ? pm.collectionVariables.get("TESTING_URI") : pm.collectionVariables.get("PRODUCTION_URI");
        var uploadBaseUrl = pm.collectionVariables.get("IS_TESTING") == "true" ? pm.collectionVariables.get("UPLOAD_TESTING_URI") : pm.collectionVariables.get("UPLOAD_PRODUCTION_URI");


        pm.sendRequest({
            url: baseUrl + 'oauth/token',
            method: 'POST',
            header: 'Content-Type: application/x-www-form-urlencoded',
            body: {
                mode: 'urlencoded',
                urlencoded: [
                    { key: "client_id", value: pm.collectionVariables.get("client_id"), disabled: false },
                    { key: "client_secret", value: pm.collectionVariables.get("client_secret"), disabled: false },
                    { key: "grant_type", value: pm.collectionVariables.get("grant_type"), disabled: false },
                ]
            }
        }, function (err, res) {
            if (err) {
                console.log(err);
            } else {
                pm.test("Status code is 200", () => {
                    pm.expect(res).to.have.status(200);
                });
                let resJson = res.json();
                pm.collectionVariables.set("BASE_URL", baseUrl);
                pm.collectionVariables.set("UPLOAD_BASE_URL", uploadBaseUrl);

                pm.collectionVariables.set("expires_in", resJson.expires_in);
                pm.collectionVariables.set("access_token", resJson.access_token);
                pm.collectionVariables.set("scope", resJson.scope);
                pm.collectionVariables.set("token_type", resJson.token_type);
            }
        });
    }
});