Local variable name shadows variable in outer scope JS-0123

[philipjonsen]

'connection' is already declared in the upper scope on line 11 column 21.
[repl.js]

12 const config = mergeConfig(loadConfig('./config.js'));
13 const services = await createServices(config);
14
15 return withConnection(services.db, (connection) => {

Two variables can have the same name if they're declared in different scopes.
In the example below, the parameter x is said to "shadow" the variable x declared above it. The outer x can no longer be accessed inside the sum function.

const x = 1
function add(x, y) {
return x + y
}

While shadowing does not cause any problems most of the time, it does make the code harder to read and understand. We highly recommend against shadowing.

const r = repl.start();
Object.assign(r.context, {


BAD PRACTICE:

const file = "data.txt"

function readFile(file) {
// The parameter file shadows the toplevel variable file.
if (fs.existsSync(file)) {
return fs.readFileSync(file)
}
return null
}

RECOMMENDED:

// Prefer variable names that are distinct and convey as much
// meaning as possible.
const dataFile = "data.txt"

function readFile(filePath) {
if (fs.existsSync(filePath)) {
return fs.readFileSync(filePath)
}
return null
}