Скачать или смотреть How to Fix the Issue of Promises not Working in Script Tags

Learn why your `Promise` implementation in script tags is failing and discover a step-by-step solution to correct it.
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This video is based on the question https://stackoverflow.com/q/63870721/ asked by the user '0xAnon' ( https://stackoverflow.com/u/5956254/ ) and on the answer https://stackoverflow.com/a/63871106/ provided by the user 'Bergi' ( https://stackoverflow.com/u/1048572/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Using Promises in Script Tags

When it comes to JavaScript, managing asynchronous processes can be tricky, especially in environments like iframes or when working with external scripts. One particular challenge that developers face is ensuring that functions defined in dynamically appended scripts with Promises are correctly loaded and available for execution. This problem is often exacerbated by the way JavaScript handles asynchronous loading, which can leave methods undefined when they are called before they have had a chance to load.

In this post, we’ll take a look at a specific scenario where a JavaScript function is appended to the DOM using Promises but isn't behaving as expected. We’ll break down the solution to ensure that the intended functions are properly registered and available when needed.

The Code Setup

You have a function, appendScript, that appends scripts to the document head and manages asynchronous loading with Promises. Here’s a simplified overview of what that function does:

[[See Video to Reveal this Text or Code Snippet]]

The Execution Function

Next, there’s a function that invokes appendScript, as shown below:

[[See Video to Reveal this Text or Code Snippet]]

The Issue at Hand

Despite the correct implementation of Promises, when calling the function defined in your dynamically added code, you encounter undefined values for specific methods. For example, the function window.onGreenPlayButtonClicked returns undefined in case 1 due to the asynchronous nature of the setup, while it behaves correctly in subsequent cases.

Examples of Code Strings

Code String 1 - Causes issues (returns undefined):

[[See Video to Reveal this Text or Code Snippet]]

Code String 2 - Works properly (returns function signature):

[[See Video to Reveal this Text or Code Snippet]]

Solution: Ensuring Asynchronous Initialization Properly Resolves

To solve the problem of the undefined method, it is essential to ensure proper order of operations when working with asynchronous calls. Specifically, you need to wait for the dynamic script to fully load before calling any functions defined within it.

Proposed Solution

You can modify the callAppendScript function as follows to ensure that main() is not called until the script has fully loaded and the onGreenPlayButtonClicked method is available:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion: Managing Asynchronous Behavior in JavaScript

When working with Promises and dynamically loaded scripts in JavaScript, it’s crucial to understand and manage the timing of your function calls. By ensuring that your code properly waits for an asynchronous operation to complete, you can resolve common issues like undefined methods and ensure that your application behaves as expected.

With this adjusted approach, you should now be able to handle your Promises in script tags effectively, ensuring that all your functions are loaded correctly when needed.