What really determines the order JavaScript modules are executed in the .html?

Question

I read that the module that appears first is loaded first. This wasn't true.

Before I bountied this question I learned:

1. Modules that have no imports (leaf modules) are executed first.
2. A module that imports will not execute until the module it imports from executes.

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This has allowed me to explain a basic scenario like this:

2.js logs 2 and exports functionTwo (which, when called, logs "2-export").
1.js logs 1 and imports and executes functionTwo from 2.js.

No matter the source-order of these modules, 2.js always executes first because 1.js can't execute before 2.js - it relies on something 2.js exports to it. The console always reads: '2, 1, 2-export'.

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However it is an incomplete answer for these 2 scenarios:

1. ModuleA imports from ModuleB. ModuleB imports from ModuleA.
2. ModuleA imports from ModuleB. ModuleC imports from ModuleD. I've been experimenting with source-order and I can't intuitively pick up the rule that determines the complete order of execution of modules. All I do know is that D or B has to execute first. (Sometimes DB or BD executes first; sometimes DC or BA executes first)

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P.s.

Is it true that the ruling we're talking about only applies to modules without the async attribute? Is it true modules with the async attribute are simply executed as soon as they are loaded?

Answer 1

The rule is pretty simple: the leafs of the dependency trees are imported first (their code is being executed), then all of the intermediate modules up to the root modules.

That's why you observed this behavior:

• 1 is a leaf module, it's being executed first (console.log('1') and the functionExport declaration)
• 2 imports 1 so it's being executed right after (console.log('2'))
• 2 eventually calls the function from 1 (console.log('1export')

The ES6 specification actually doesn't detail clearly whether the imports should be processed sequentially or not. Some browsers might have sequential imports while some other browsers might not.

If you want to guarantee a certain order of execution and have a consistent behavior across browsers, you have to specify a chain of imports accordingly. Two chains of imports are not guaranteed to execute separately (that's why you see DB first sometimes, instead of DC). Two chains of imports are not guaranteed to execute in a certain order (that's why you see sometimes DB, sometimes BD).

The only guaranteed thing is that a script executes after its imports have already executed.

Last word about the async attribute, it allows to defer the fetching and execution while the browser continues to parse the page. It applies to module scripts just like regular scripts, the only difference is that they also load their dependencies, to comply with the rule stated above.