I have the a file index.js with these contents:
const test = function (cb) { cb(); };
test(myCallback = function () { console.log('done'); });
When I run index.js using Node v16.6.1 and CommonJS:
done
When I run index.js using Node v16.6.1 with "type": "module" in my package.json file:
ReferenceError: myCallback is not defined
Can you tell me whether and how this relates to ECMAScript modules?
This is a side effect from using ESM. Strict mode (use strict) is enable by default. Which cause the error, because: myCallback isn't declared anywhere.
let myCallback;
test(myCallback = function () { console.log('done'); }); // here we assign myCallback to a function. are you sure you want to actually do this?
Before, you were attempting to create a global variable.
First, strict mode makes it impossible to accidentally create global variables. [...] Assignments, which would accidentally create global variables, instead throw an error in strict mode
There are two ways to name functions. You can set a variable equal to a function. Or you can give a function a name. Here are two ways to solve your problem
Name the function
test(function myCallback () { console.log('done'); });
Set variable equal to function
const myCallback = function () { console.log('done'); }
test(myCallback);