Steps to develop an NPM package without publishing it on NPM.

Creating a package

1. Create a folder name example-js
2. cd example-js
3. npm init --scope=@example-org
4. Create a file named index.js and add your function.

As an example, I created a function name sum

// ES6
export default function sum(x, y) {
    return x + y;
}

// CommonJS
module.exports = (x,y) => {
	return x + y;
}

// or
function sum(x, y) {
	return x + y;
}

module.exports = {
	sum,
}

Test out package

I use nuxtjs framework as an example. (read https://nuxt.com/docs/guide/concepts/esm)

1. Run npm init nuxt-app test-example-js
2. Run npm link ../example-js. (I put text-example-js and example-js on the same folder level)
3. As an example, edit index.vue and add the following lines

<script setup>
	// ES6
	import sum from '@example-org/example-js'
	// CommonJS
	const sum = require('@example-org/example-js')
	// or
	const {sum} = require('@example-org/example-js')

	const testSumJs = computed(() => {
		return sum(1, 2);
	})
</script>

Cleanup

Once you done testing the NPM package locally, remove the local link: npm unlink example-js