Think carbon, reduce Typescript bundle size

2020-02-28 - Thomas Queste

Reducing the size of JS compiled from Typescript is easy.

Here are some numbers and how to do it. It’s nearly free if you run on Node (!= browser).

TL;DR: enable tslib/importHelpers, reduce the size of the JS files, reduce storage and network, think carbon!

Size matters

Sizes after and before, with the difference in percent:

Before After Diff
line count *.js 4397 lines 4011 lines -9%
size in bytes *.js 186k 153k -18%


Install tslib, the library that contains the helper code:

npm i tslib

Enable the import of the helper functions:

  "compilerOptions": {
    "importHelpers": true

What does it do

Typescript will create helper functions for decorators, extends, assign, etc. in each compiled file.

But, Typescript can use a reference library, tslib and import the functions from there.

Sample of generated code without tslib/importHelpers:

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;

With tslib/importHelpers:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
const tslib_1 = require("tslib");


I measured the size before and after with the following commands:

# don't forget to do a clean-build
# npm run clean && npm run build

# count lines
( find ./dist -name '*.js' -print0 | xargs -0 cat ) | wc -l

# total size in bytes
{ find dist -type f -name "*.js" -printf "%s+"; echo 0; } | bc
Written by:

Thomas Queste

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