Polyfill là gì trong khoa học lập trình?

// ES6 Code
var s = 'Tom and Jerry';

if (s.startsWith('Tom')) {
    console.log('Hello Tom');
}

// ES5 Code
if (!String.prototype.startsWith) {
    String.prototype.startsWith = function (searchString, position) {
        position = position || 0;
        return this.substr(position, searchString.length) === searchString;
    };
}
var s = 'Tom and Jerry';

if (s.startsWith('Tom')) {
    console.log('Hello Tom');
}

if (!Math.trunc) { // if no such method
    // implement it
    Math.trunc = function (number) {
        // Math.ceil and Math.floor exist even in ancient JavaScript engines
        // they are covered later in the tutorial
        return number < 0 ? Math.ceil(number) : Math.floor(number);
    };
}