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The current default native baseline for the ES module shims polyfill mode is browsers supporting import maps. In browsers with variable feature support, sources are analyzed with module specifiers rewritten using the very fast Wasm / asm.js lexer while sharing the source network fetch cache with the native loader.
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In browsers with full feature support no further processing is done. In polyfill mode, feature detections are performed for ES modules features. Shim mode also provides some additional features that aren't yet natively supported such as supporting multiple import maps, external import maps with a "src" attribute, dynamically injecting import maps, and reading current import map state, which can be useful in certain applications. In shim mode, only the above importmap-shim and module-shim tags will be parsed and executed by ES Module Shims. Shim mode is an alternative to polyfill mode and doesn't rely on native modules erroring - instead it is triggered by the existence of any or, or when explicitly setting the shimMode init option. This is because the polyfill cannot disable the native loader - instead it will only execute modules that would otherwise fail resolving or parsing to avoid duplicate fetches or executions that would cause performance and reliability issues. The first import being a bare specifier in the pattern above is important to ensure this. This execution failure is a feature - it avoids the polyfill causing double execution. Relative references must start with either "/", "./", or "./". Uncaught TypeError: Failed to resolve module specifier "app". Include ES Module Shims with a async attribute on the script, then include an import map and module scripts normally: Circular references, with the execption that live bindings are disabled for the first unexecuted circular parent.Dynamic import expressions ( import('src/' + varname')).
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When running in shim mode, module rewriting is applied for all users and custom resolve and fetch hooks can be implemented allowing for custom resolution and streaming in-browser transform workflows.īecause we are still using the native module loader the edge cases work out comprehensively, including:
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When running in polyfill mode, the 67% of users with import maps entirely bypass the shim code entirely.įor the remaining 30% of users, the highly performant (see benchmarks) production and CSP-compatible shim kicks in to rewrite module specifiers driven by the Web Assembly ES Module Lexer. Shims modern ES Modules features like import maps on top of the baseline modules support in browsers supported by 95% of users.