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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