coderef

Import assertionsLimited availability

Module import … assert { type: "json" } and import … assert { type: "css" } JavaScript statements load JSON and CSS data.

Not yet supported in all major browsers — check the table and provide a fallback.

Discouraged: Import assertions are superseded by import attributes. Use instead: CSS import attributes, JSON import attributes

How to use ↓Compare browsers

Browser support

DesktopChrome
✕Not supportedup to 155
Current: 155
Edge
✕Not supportedup to 154
Current: 154
Firefox
✕Not supportedup to 157
Current: 157
Safari
✕Not supportedup to 27
Current: 27
MobileChrome Android
✕Not supportedup to 155
Current: 155
Firefox Android
✕Not supportedup to 157
Current: 157
Safari iOS
✕Not supportedup to 27
Current: 27

How to use

Not supported in Chrome, Edge, Firefox, Safari, Chrome Android, Firefox Android, Safari iOS. Use feature detection below and provide a fallback (or a polyfill) for those browsers.

Example

Full syntax and examples are on MDN — see the links below.

Feature detection

JS · check before using
// Syntax can't be tested with an if-statement — an unsupported
// browser fails to parse the whole script. Either transpile for older
// targets (e.g. Babel/TypeScript/esbuild), or probe in isolation:
let supported = true;
try {
  new Function("/* code that uses the syntax */");
} catch {
  supported = false;
}

This is a language syntax feature.

Specification

Sub-features

Support for each part of the feature (3). The overall table above shows when the whole feature works.

Sub-featureChromeEdgeFirefoxSafariChrome AndroidFirefox AndroidSafari iOS
import.import_assertions✕✕✕✕✕✕✕
import.import_assertions.type_css✕✕✕✕✕✕✕
import.import_assertions.type_json✕✕✕✕✕✕✕