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Use import defer * as feature from "./feature.js" when a module is statically known but its synchronous initialization can wait until first use. The module graph is still fetched, parsed, and linked up front; what waits is synchronous evaluation, which begins when code accesses a property on the deferred namespace. This can keep callers synchronous, but it does not guarantee a performance improvement and it changes when module side effects run.
What import defer does
import defer is a static import form that postpones synchronous module evaluation. Unlike a dynamic import, it does not wait to fetch and link the graph until a function runs: dependencies are fetched, parsed, and linked as part of preparing the importing module. The deferred module’s synchronous top-level code runs only when the deferred namespace is accessed, subject to the top-level-await behavior described below. MDN documents the syntax and evaluation rules.
Think of it as delaying execution, not loading. Because the import remains a static dependency, missing modules, syntax errors, and invalid imports are not concealed until first use. What may be postponed is the CPU work of evaluating a synchronous portion of the graph. The TC39 proposal describes reduced unnecessary initialization work as a design goal, not a measured or universal speedup. The proposal discusses the motivation and semantics.
How to write it
The supported form uses a namespace import; there is no deferred named-import form:
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import defer * as compiler from "./compiler.js";
export function compile(path) {
return compiler.createProgram([path], {});
}The access to compiler.createProgram triggers synchronous evaluation of the deferred module and the synchronous dependencies that must run before its export can be used. If compile is never called and no other code accesses the namespace, a purely synchronous deferred portion may never be evaluated.
Be mindful that inspecting or destructuring an export is also an access. For example, reading compiler.createProgram to save it in a local variable can trigger evaluation just as calling it does. The deferred boundary is therefore about when the namespace is touched, not whether a particular function is ultimately invoked.
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Choose between static, deferred, and dynamic imports
| Import form | When the graph is fetched and linked | When code is evaluated | Caller and specifier | Good fit |
|---|---|---|---|---|
Ordinary static import |
During module loading | As part of module loading | Synchronous use; statically written specifier | The module is needed immediately or its initialization effects must happen early. |
import defer * as ns |
Up front, as with a static dependency | Synchronous evaluation waits until namespace property access, except where top-level await requires eager evaluation | Callers can remain synchronous; statically written specifier | The module is known in advance, but synchronous initialization can safely wait until first use. |
await import(specifier) |
When the dynamic import is reached | As part of loading the requested module | Returns a promise; the specifier can be computed | Loading should itself be conditional or on demand, or the path is dynamic. |
MDN’s dynamic import reference explains its promise-based behavior. The practical distinction is whether you want to delay graph loading, delay synchronous evaluation while keeping a static dependency, or load a module selected at runtime. Dynamic import requires promise handling; import defer does not turn every caller into an async function.
Top-level await changes what can be deferred
A module that directly uses top-level await cannot wait for a synchronous namespace access to begin evaluation, because its evaluation is asynchronous. Such a module is evaluated eagerly. Asynchronous dependencies also run when required, although independent synchronous portions of the graph may still be deferred. MDN’s reference and the TC39 proposal describe this constraint.
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So import defer is not a way to make an asynchronous module behave synchronously. If a dependency uses top-level await, assess the actual graph rather than assuming that every part waits for the first property read.
Side effects, errors, and shared module state
Move side effects only when their timing is safe
Deferral moves top-level effects later. Do not defer setup that must happen before the rest of the application proceeds—for example, installing a polyfill needed by code that runs earlier. Review initialization code for registrations, global changes, event handlers, and other work whose ordering matters. MDN warns that deferred imports change when side effects occur.
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Know which failures are early and which can wait
Fetch, parse, and link problems remain part of preparing the static import graph, so deferral does not hide a missing dependency or a syntax or linking error until first use. If evaluation of work that remained deferred fails, that failure surfaces synchronously at the operation that triggers evaluation.
Imports do not create a second copy
The modifier applies to this import’s evaluation timing, not to a separate instance of the module. A non-deferred import of the same module can cause it to evaluate earlier, and a module’s code executes at most once within the module system’s shared state.
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The then export exception
A deferred namespace does not expose an export named then. If a module needs to provide that export, use a regular import or re-export it under another name rather than relying on the deferred namespace to expose it.
Check support before using the syntax
MDN currently labels import defer experimental, of limited availability, and not Baseline because some widely used browsers do not support it. Check the actual browsers and server-side runtimes you target, as well as your transpilation or bundling pipeline and deployment mode, before making the syntax part of an application. MDN’s compatibility information is the relevant starting point; support should not be assumed across environments.
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When it is a good fit
- Use it for a statically known module whose synchronous initialization is unnecessary until a later interaction or code path.
- Keep ordinary static imports for modules needed immediately or for effects that must occur early.
- Use dynamic
import()when the module should not be loaded until a condition is met, or when the specifier is computed and promise-based loading is acceptable. - Measure your own application if startup performance is the goal. The proposal’s motivation is reducing unnecessary initialization CPU work; the cited sources establish no general benchmark or guaranteed savings.
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