NodeJS autocomplete shadow. How to implement it?

How is autocomplete shadow in nodejs implemented as a console application?

By "autocomplete shadow" I mean ... as I type in the node REPL 'func' without hitting tab I get a shadow of the keyword 'function':

How can I do this in my own console app:

// Simple REPL with tab completion ... how to make it into a REPL with shadow completion?
const readline = require('readline')
const rl = readline.createInterface({
    input: process.stdin, 
    output: process.stdout, 
    completer: (line) => { // This will only give me tab completion
        const completions = ['function', 'let', 'const']
        const hits = completions.filter(completion => completion.startsWith(line))
        return [hits.length ? hits : completions, line] 
    }
})
rl.prompt()
rl.on('line', input => {
    console.log(input)
    rl.prompt()
})