Tree shaking — How to write code and reduce the bundle size in Webpack 4+


Tree shaking has become more popular during recent days and hence now its becomes a vital responsibility for a bundler to support tree shaking.

Okay why it is much more important?. The answer is below,

No user will wait for more than 3 seconds for a page to load.

So its important to keep the bundle size as small as possible for a web page to load fast. Webpack and Rollup are the two popular bundlers which supports tree shaking.

In this article, I am going to speak about some guidelines to write code that support tree shaking and also how to reduce bundle size in Webpack.

Write or compile code to ES6

Since tree shaking works by marking the export and import statements as unused, it is better to write or compile your code to ES6 format. Unused imports will be removed during minification process.

If your browsers does not support ES6 codes then there are many transpiler available to do the conversion (For example: Babel).

Use Feature Module (Loose coupling)

Segregate features into separate modules. Know your customer before planning your web site or library.

All users are not going to use all of your library features, so its always a good practice to segregate the features into separate modules and allowing the user to inject when needed.

The feature modules should be loosely coupled with main module and also ensure to avoid explicit usage of the feature module classes or functions inside main module. Create a pub-sub pattern for your website/library to perform communication between modules.

Learn to use functional programming paradigm

Well, writing code using functional programming paradigm allows you to easily maintain and deliver a bug free product. Below are some of the things, you need to know when using function programming.

  • Write pure class/functions.
  • Use array methods such as filter, some, map etc.
  • Avoid using shared state.
  • Avoid mutating state.

You can find a complete explanation about Function Programming in the Master the JavaScript Interview: What is Functional Programming? drafted by Eric Elliott.

Set sideEffects — Reduce bundle size in Webpack


When you have followed the above sections in your code then you are halfway away from enabling proper tree shaking capability for your code.

The last thing, I need to share is the use of settings sideEffects keyword as false in your package.json file. Settings this will inform the webpack that your code/project is safe to exclude the re-exported classes/functions.

Please go through the webpack official documentation for more information on marking your project with sideEffects key.


In this article, we have seen how to use write code that can be tree shaken properly and setting sideEffects keyword in package.json to mark the project as safe to exclude the re-exported codes.

Happy coding.. 🙂


Parameter handling in ES6

ECMA Script 6 provides the following extension support for the parameters of JavaScript methods.

  1. Default parameter values
  2. Rest parameter
  3. Spread operator

Default parameter values:

Example with ES5 & ES6


        function sum(a, b) {
            if (!b)
                b = 2;
            return a + b;
        var s = sum(5);
        console.log(s);//Result will be 7 as undefined variable 'b' assigned with value 2

In general we may pass lesser number of parameters instead of total capacity of parameters to the JavaScript methods. In this case the remaining parameters will be undefined.

This “undefined” scenario can be avoided in ES6 by initializing default values for function parameters. Have a look at below code snippets.


        function sum(a, b = 10) {
            return a + b;
        var s1 = sum(5);
        console.log(s1); //Result will be 15 due to default value
        var s2 = sum(5, 15);
        console.log(s2); //Result will be 20 due to passed value (Override default value of  “b”)

Note: The default values of function parameter will be overridden when we pass values to those parameters.

Rest parameter:

In ECMA Script 5, while we are calling the JavaScript functions with number of parameters that is greater than the actual parameters count of that method, it will ignore the “Rest” parameters and receive the actual parameters alone. So in this case we will use the “arguments” scope to get those “Rest” parameter values.


        function sum(a, b) {
            return a + b + arguments[2] + arguments[3];
        var s = sum(10, 30, 40, 20);
        console.log(s); //Result is 100

But in ES6, there is “Rest Parameter” (…parameterName) support for receiving those remaining parameter values. Have a look at the below code snippets.


        function sum(a, b, ...arg) {
            return a + b + arg[0] + arg[1];
        var s = sum(10, 30, 40, 20, 50, 60);
        console.log(s); //Result is 100

Note: Rest parameter must be last formal parameter

Spread operator:

The Spread operator support (…arrayName) in ES6 is enabled to iterate over the array values or string values and also to generate array function parameters.

Examples: (ES 5 vs ES 6)


        var arr1 = [4, 5, 6], arr2 = [1, 2, 3].concat(arr1);
        console.log(arr2); // [1, 2, 3, 4, 5, 6]

        var str = "HTML".split("");
        console.log(str); // ["H", "T", "M", "L"]


        var arr1 = [4,5,6], arr2 = [1,2,3, ...arr1];
        console.log(arr2); // [1, 2, 3, 4, 5, 6]

        var str = "HTML";
        console.log([...str]); // ["H", "T", "M", "L"]

Spread operator with function parameter:

In ES 5 array values can be spread to each parameter of the function with the help of “.apply()” predefined method like below code snippets.


        function sum(a, b, c, d) {
            return a + b + c + d;
        var s = sum.apply("", [5, 10, 15, 20]);
        console.log(s); // Result is 50

But in ES6 Spread operator can be used for spreading the parameter values to the functions. We can pass the parameter values in the following ways.

Example 1:

        function sum(a, b, c, d) {
            return a + b + c + d;
        var arr = [5, 10, 15, 20],
          s = sum(...arr);
        console.log(s); // Result is 50
Example 2:

        function sum(a, ...arg) {
            return a + arg[0] + arg[1] + arg[2];
        var arr = [10, 15, 20],
          s = sum(5, ...arr);
        console.log(s); // Result is 50

Note: Any type of parameter value collection can be passed to the “Rest parameters”.

ES6 const is not immutable

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Now most of the ES6 features can be used in modern browsers without transpiling. If you are using ES6 features in your project then you must have come across the const keyword.

Most of us know that the value assigned to the const cannot be changed. Yes, this is true when your are assigning primitive values.


But when comes with object type you can change the object’s property.


This proves that the const is not immutable and it cannot be reassigned or re-declared.

MDN says this clearly in their documentation.

The `const` declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned.

Happy Scripting…