Can I move JS code out of Svelte component file to other file js file?

As far as I know, this isn't possible right now.

What you could do is export everything you need from the js file, and then import them in the component: https://svelte.dev/repl/1d630ff27a0c48d38e4762cf6b0c2da5?version=3.7.1

<script>
 import { name } from './mycode.js'
</script>

<h1>Hello {name}!</h1>

export let name = 'World';

However that would only be a partial solution as any mutation of data occurring within the file would not trigger a re-render of the DOM, as Svelte would not compile the .js file and would not be able to add its own code invalidating the values: https://svelte.dev/repl/c4b41b769ed747acb01a02a9af33e545?version=3.7.1

<script>
 import { name, handleClick } from './mycode.js'
</script>

<h1 on:click={handleClick}>Hello {name}!</h1>

export let name = 'World';
export const handleClick = () => {
    name = 'Everyone';
}

But that doesn't mean you can't be tricky if you are willing to go the extra mile to achieve this: https://svelte.dev/repl/8e259df629614ac99cb14cfae2f30658?version=3.7.1

<script>
 import { name, handleClick } from './mycode.js'

 const onClick = () => {
     handleClick();
     name = name;
 }
</script>

<h1 on:click={onClick}>Hello {name}!</h1>

export let name = 'World';
export const handleClick = () => {
    name = 'Everyone';
}

The extra line name = name forcing the DOM update.

This is possible with the svelte.preprocess API, which you'd most commonly reach via the preprocess option in rollup-plugin-svelte or svelte-loader. Something like this should work (though I haven't tested it):

const path = require( 'path' )
const fs = require( 'fs' )
...

plugins: [
  svelte({
    // ...
    preprocess: {
      script: ({ content, attributes, filename }) => {
        if ( 'string' === typeof attributes.src ) {
          const file = path.resolve(path.dirname(filename), attributes.src);
          const code = fs.readFileSync(file, 'utf-8');
          return {code, dependencies: [file]};
        }
      }
    }
  })
]