JavaScript

for...in

let data = {1: "a", 2: "b", 3: "c"};

let a = [];
for (let id in data) {
    a.push('<option value="'+id+'">'+data[id]+'</option>');
}

console.log(a);
console.log(a.join(""));

keys...map

let data = {1: "a", 2: "b", 3: "c"};

let a = Object.keys(data).map(function(id) {
    return '<option value="'+id+'">'+data[id]+'</option>';
});

console.log(a);
console.log(a.join(""));

Postman

function parse(obj) {
    let parsed = {};
    
    obj.forEach((item) => {
        parsed[item.key] = item.value;
    });
    
    return parsed;
}

function signature(str, secret) {
    str += "&key=" + secret;

    return CryptoJS.MD5(str).toString();
}

function signature(str, secret) {
    str += "&key=" + secret;

    return CryptoJS.MD5(str).toString().toUpperCase();
}

function signature(str, secret) {
    return CryptoJS.HmacSHA256(str, secret).toString();
}

function signature(str, secret) {
    const binary = CryptoJS.HmacSHA256(str, secret, true);
    
    return CryptoJS.enc.Base64.stringify(binary);
}

function buildQuery(obj) {
    let str = "";
    
    for (let key in obj) {
        str += key + "=" + obj[key] + "&";
    }

    return str.slice(0, -1);
}

function ksort(obj) {
    let keys = Object.keys(obj).sort();
    let sorted = {};
    
    for (let i in keys) {
        sorted[keys[i]] = obj[keys[i]];
    }
    
    return sorted;
}

let obj = parse(pm.request.body.urlencoded.all());
let str = buildQuery(ksort(obj));
let sign = signature(str);

pm.request.body.urlencoded.add({
    key:"sign",
    value: sign,
    type: "text"
});

pm.request.body.formdata.add({
    key:"sign",
    value: sign,
    type: "text"
});

pm.request.headers.add({
    key: "X-Signature",
    value: sign
});

const body = pm.request.body.raw;

const obj = JSON.parse(body);

const str = JSON.stringify(obj);