esMain(import.meta) in Nue-js source code.

In this article, we will review a function, esMain(), in Nue.js source code. You will find the below code snippet at line 127 in packages/nuekit/cli.js.

// Only run main when called as real CLI
if (esMain(import.meta)) {

What caught my attention is this comment below:

// Only run main when called as real CLI

I was thinking how does this check happen? To understand that we need to see the esMain function code. It is located in packages/nuekit/src/util.js.

export function esMain(meta) {
  if (!meta || !process.argv[1]) return false
  return fileURLToPath(
    meta.resolve(process.argv[1])) === fileURLToPath(meta.url
  )
}

Let’s break this down. I would start with finding out what import.meta is.

import.meta

The import.meta meta-property exposes context-specific metadata to a JavaScript module. It contains information about the module, such as the module's URL.

Read more about import.meta.

TLDR: import.meta is created by the host environment and the object looks something like below:

{
  // In browsers, this is either the URL from which the script was obtained 
  // (for external scripts), or the URL of the containing document 
  // (for inline scripts). In Node.js, this is the file path (including the 
  // file:// protocol).
  "url": <value>, // ()
  // Resolves a module specifier to a URL 
  // using the current module's URL as base.
  "resolve": 
}

So if there is not meta or process.argv[1], this esMain function returns false.

if (!meta || !process.argv[1]) return false

otherwise, the below code is executed:

return fileURLToPath(
  meta.resolve(process.argv[1])
) === fileURLToPath(meta.url)

fileURLToPath is imported from “node:url” at the top of this file as shown below:

import { fileURLToPath, pathToFileURL } from 'node:url'

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Hey, my name is Ramu Narasinga. I study large open-source projects and create content about their codebase architecture and best practices, sharing it through articles, videos.

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References:

1. https://github.com/nuejs/nue/blob/master/packages/nuekit/src/cli.js#L126

2. https://github.com/nuejs/nue/blob/master/packages/nuekit/src/util.js#L16

3. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import.meta