Скачать или смотреть Understanding why curly brace imports stay intact when mutating a module object

Explore the reason behind `curly brace imports` not being affected by module mutations. Learn about import behaviors and best practices for testing in JavaScript.
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Why Are Curly Brace Imports Not Changed When Mutating a Module Object?

As developers, we often need to manipulate and test our code in different ways. One common scenario arises from direct module mutations, which can lead to unexpected behaviors. Let's consider a specific problem that many JavaScript developers face—understanding why curly brace imports don't change even after mutating the original module object.

The Setup: A Testing Scenario

Let's illustrate the issue with an example. Imagine you have a fundamental create-react-app setup, and you are trying to test your app by mutating certain objects directly. Here’s a simplified version of how it might look:

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Now, when you later reference these mutations:

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You’ll notice that React.useContext, React.foo, and lodash.drop print the jest mock function you defined, while useContext, foo, and drop still show the original object.

The Deep Dive: Understanding the Behavior

This behavior stems from when the variables and constants are created in JavaScript. Let's explore why this separation occurs.

Execution Order

Import Execution: When you use import React, { useContext } from 'react';, the imports are executed before your mutations. Thus, the value for useContext (and foo in your example) is assigned before any mutations to React.useContext or React.foo occur.

Memory Reference:

When you declare const a = obj.a;, this makes a reference to the value of obj.a at that specific time.

For instance:

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The variable a maintains its independent value of 1, and doesn’t change even when obj.a is modified.

Recommendation for Best Practices

Given this behavior, it's crucial to approach testing with careful consideration. Here are a couple of strategies you might employ:

Dependency Injection:

Pass your dependencies directly into your components as props, allowing you to mock them in tests easily. For example:

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Context Provider:

Another common solution is to use context to provide your dependencies. You could create a mocked context for tests and one for production:

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This approach not only keeps your tests clean but also encourages the separation of concerns which will make your code more maintainable.

Conclusion

In summary, the reason why curly brace imports remain unchanged lies in the timing of when those variables are created compared to when mutations occur. Understanding this detail can help you write better tests and design patterns that respect the power of JavaScript's function and scope. As you continue to explore JavaScript testing, embracing concepts like Dependency Injection and context-based solutions will pay dividends over time.

With these insights, you're now better equipped to navigate the complexities of module mutations in your JavaScript applications.