Polyfills and Transpilers

JavaScript evolves every now and then. Regularly, new language proposals are submitted, analyzed, and added to https://tc39.github.io/ecma262/ and then incorporated into the specification. There may be differences in how it is implemented in JavaScript engines depending on the browser. Some may implement the draft proposals, while others wait until the whole specification is released. Backward compatibility issues arise as new things are introduced.

To support the modern code in old browsers we use two tools: transpilers and polyfills.

Transpilers

It is a program that translates modern code and rewrites it using older syntax constructs so, that the older engine can understand it. For example, "nullish coalescing operator" ?? was introduced in 2020, and outdated browsers can’t understand it.

Now, it’s the transpiler's job to make the nullish coalescing operator” ?? understandable to the old browsers.

// before running the transpiler
height = height ?? 200;

// after running the transpiler
height = (height !== undefined && height !== null) ? height: 200;

Polyfills

There are times when new functionality isn't available in outdated browser engines. In this case, the code that uses the new functionality won’t work. To fill the gaps, we add the missing functionality which is called a polyfill. For example, the filter() method was introduced in ES5 and is not supported in some of the old browsers. This method accepts a function and returns an array containing only the values of the original array for which the function returns true.

const arr = [1, 2, 3, 4, 5, 6];
const filtered = arr.filter((e) => e % 2 === 0); // filter outs the even number
console.log(filtered);

// [2, 4, 6]

The polyfill for the filter is.

Array.prototype.filter = function (callback) {
  // Store the new array
  const result = [];
  for (let i = 0; i < this.length; i++) {
    // call the callback with the current element, index, and context.
    //if it passes the test then add the element in the new array.
    if (callback(this[i], i, this)) {
      result.push(this[i]);
    }
  }
  //return the array
  return result
}