JavaScript OR Operator: Returned Operands and Short-Circuit Calls

The task

Choose a display label from a supplied value or a fallback function. Compare an existing label, an empty string and a zero value while recording fallback calls.

Runnable Node 22 example

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

const calls = [];
function fallback() { calls.push('fallback'); return 'unassigned'; }
const present = 'desk-4' || fallback();
assert.equal(present, 'desk-4');
assert.deepEqual(calls, []);
const blank = '' || fallback();
assert.equal(blank, 'unassigned');
assert.deepEqual(calls, ['fallback']);
assert.equal(0 || 9, 9);
assert.equal(0 ?? 9, 0);
assert.equal(false || 'next', 'next');
console.log(`present=${present}; blank=${blank}; calls=${calls.length}`);

Expected output and explanation

The first nonempty label ends the evaluation before fallback runs. The empty string reaches the function, which returns unassigned and records one call. The operator result is that string, not the Boolean true.

Zero also triggers || in the numeric comparison, while ?? preserves it. That short contrast identifies the boundary; the dedicated nullish-coalescing guide supplies the complete fallback exercise. Pick the operator according to whether your application means “falsy” or specifically “nullish.”

Pitfall to check

Put required side effects outside an expression that may short-circuit. If a function must run for logging or validation, hiding it on the right of || can skip it for valid-looking input. Explicit control flow can make those requirements clearer.