> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# TypeScript Side Effect Import
A side-effect import is an import declaration that executes a module's code for its global side effects without binding any of its exported members to the current module's scope.
```typescript theme={"dark"}
import "module-name";
import "./local-module.js";
```
## Mechanics and Behavior
**Scope and Binding**
Unlike standard import declarations, a side-effect import does not introduce any local variables, functions, or types into the importing module's namespace. The target module is evaluated, but its `export` statements are ignored by the importing file.
**Compiler Emit and Elision**
The TypeScript compiler (`tsc`) performs import elision: it removes import statements from the emitted JavaScript if it determines the imported members are only used in type positions. However, side-effect imports are strictly exempt from this process. The compiler guarantees that a side-effect import is never elided and will always be preserved in the compiled JavaScript output.
**Type System Integration**
During the compilation phase, TypeScript parses the target module of a side-effect import. If the target module contains ambient declarations or global type augmentations, the compiler applies them to the global type environment.
```typescript theme={"dark"}
// global-types.ts
declare global {
interface Window {
customProperty: string;
}
}
export {};
// main.ts
import "./global-types.js"; // Side-effect import
// The compiler now recognizes 'customProperty' on the global Window interface
window.customProperty = "value";
```
**Evaluation Order**
In accordance with the ECMAScript Modules (ESM) specification, import declarations are hoisted. The JavaScript runtime resolves the module graph and evaluates the side-effect module *before* the importing module's own code begins executing. It is not evaluated inline at the lexical position of the `import` statement. Per the specification, the module is evaluated exactly once, regardless of how many times it is imported across the application.
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