Straight-forward string.prototype format method for Javascript.

string.prototype.format.js

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain
// it`s monkey patching, breaks incapsulation and considered bad practice

String.prototype.format = function() {
    var s = this;
    for (var i = 0; i < arguments.length; i++) {       
        var reg = new RegExp("\\{" + i + "\\}", "gm");             
        s = s.replace(reg, arguments[i]);
    }
    return s;
}

// better to invert loop
// https://regex101.com/r/u4HkIt/1

String.prototype.format = function() {
    var reg = new RegExp('{\s*[^}]*\s*}', 'g'),
        found = this.match(reg),
        object = typeof  arguments[0] === 'object',
        s = this,
        sub, key;
    if (object) object = arguments[0];

    if (object) while (found.length) {
        sub = found.shift();
        key = sub.slice(1, -1).trim();
        if (key in object)
            s = s.replace(sub, object[key]);
    }
    else while (found.length) {
        sub = found.shift();
        key = parseInt(sub.slice(1, -1));
        s = s.replace(sub, arguments[key]);
    }
    return s;
}