RSTR-PERF-102 — `new Date()` inside a loop

Summary

new Date() allocates a Date object whose only use is usually getTime() or comparison. Doing it per iteration is gratuitous allocation; Date.now() returns the milliseconds directly without the object, and any of the comparisons/arithmetic you might want to do work on the number.

If the Date itself is needed (formatting, locale), hoist it out of the loop.

Severity

Low. Pure overhead; matters at hot-path scale.

Languages

JavaScript, TypeScript.

What rastray flags

for (const x of xs) {
    const t = new Date();                // ← flagged
    log(x, t.getTime());
}

What rastray deliberately does not flag

Date.now() calls.
new Date(value) with an argument — that's a parse / construction that genuinely needs the constructor.
Construction outside the loop.

How to fix it

// Need only the timestamp?
for (const x of xs) {
    log(x, Date.now());
}

// Need the Date for formatting? Hoist it if loop-invariant:
const now = new Date();
for (const x of xs) {
    log(x, now.toISOString());
}

If you really need a fresh Date per iteration (rare — you almost always want a single "now" for the entire batch), Date.now() plus a single new Date(ts) at the end is cheaper.

References

MDN: Date.now

rastray rules