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Tramvai has polyfills integration:

  • there is a separate library @tinkoff/pack-polyfills that contains all the necessary polyfills
  • @tramvai/cli build polyfills in a separate file
  • @tramvai/module-render contains code that only loads polyfills where they are needed


Install polyfills pack

npm i --save @tinkoff/pack-polyfills

Create a file polyfill.ts

You need to create a file polyfill.ts inside your project, for example src/polyfill.ts and connect the polyfills inside:

import '@tinkoff/pack-polyfills';

Set up @tramvai/cli

After that, we need to tell @tramvai/cli that our project has polyfills. To do this, in tramvai.json we add for our project the line "polyfill: "src/polyfill.ts" in projects[APP_ID].polyfill example:

"projects": {
"pfphome": {
"name": "pfphome",
"root": "src",
"type": "application",
"polyfill": "src/polyfill.ts"

How polyfills loading works

On the @tramvai/cli side, we have configured to build the polyfills into a separate file, so it doesn't mix with the main code. On every build we will have a file with polyfills.

module-render if it finds polyfills in the build, then for each client embeds inline code that checks the availability of features in the browser and if the browser does not support any of the features, then we consider the browser is legacy and load polyfills. An example of a check: '!window.Promise.prototype.finally || !window.URL || !window.URLSearchParams || !window.AbortController || !window.IntersectionObserver || !Object.fromEntries || !window.ResizeObserver'

Replacing the polyfills loading check

Why would it be necessary?

If you do not fit the standard check for supported features in the browser and polyfills do not load in browsers where they should. In this case, it is better to create issue and we will update the check, or you can replace the check with another.

Important tips

  • POLYFILL_CONDITION should return true if the browser does not support some features
  • You should not load polyfiles into all browsers
  • It is better to extend DEFAULT_POLYFILL_CONDITION with additional checks, rather than replacing it

Replacing the check

To do this, we need to set provider POLYFILL_CONDITION, which is in import { POLYFILL_CONDITION } from '@tramvai/module-render' and pass a new line.

Example: This is a synthetic example, but suppose we want to additionally check for the presence of window.Promise in the browser, to do this we extend DEFAULT_POLYFILL_CONDITION string. The resulting expression should return true if the browsers do not support the feature.

import { POLYFILL_CONDITION, DEFAULT_POLYFILL_CONDITION } from '@tramvai/module-render';
import { provide } from '@tramvai/core';

const provider = provide({
useValue: `${DEFAULT_POLYFILL_CONDITION} || !window.Promise`,