Anonymous Functions in JavaScript: Pass a Callback or Call It

The task

Create a badge formatter, send it to a callback runner and compare its reference with the text returned by invoking it.

Runnable Node 22 example

const assert = require('node:assert/strict');

const formatter = function (label) { return `[${label}]`; };
function run(callback, value) { return callback(value); }
assert.equal(typeof formatter, 'function');
assert.equal(run(formatter, 'new'), '[new]');
assert.equal(typeof formatter('new'), 'string');
assert.equal(formatter.name, 'formatter');
assert.throws(() => run(formatter('new'), 'again'), TypeError);
console.log(run(function (label) { return label.toUpperCase(); }, 'ready'));

Expected output and explanation

formatter is the callable value, while formatter("new") is the resulting string. The runner expects a callable value, so receiving that string produces the asserted TypeError. The inline expression passed in the final call is invoked by run and prints READY.

The name assertion demonstrates an inferred name from the assignment even though the expression has no explicit function name. “Anonymous syntax” does not guarantee that diagnostic tools show a blank name. This is a property of the actual assignment tested here, not a rule for every expression position.

Pitfall to check

A function expression and an arrow function are not interchangeable for every this or arguments requirement. Choose the form from the callback’s needs. Captured state is a separate closure topic; this formatter does not maintain private evolving state.