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Learn/JavaScript/Objects, Classes, and Advanced Patterns

JavaScript•Objects, Classes, and Advanced Patterns

JavaScript Temporal Mistakes

Temporal is a powerful API, but it can be confusing at first.

Remember to use the correct Temporal type

Avoid implicit conversions

Always handle time zones correctly

Formula

Use compare() instead of < and >

Remember that Temporal Objects are immutable

Here are some common mistakes and how to avoid them. Missing UTC (Z) for Instant An Instant must always include UTC.

Formula

Wrong const instant = Temporal.Instant.from("2026 - 05 - 17T14:30:00");
Correct const instant = Temporal.Instant.from("2026 - 05 - 17T14:30:00Z");

Using PlainDateTime with Time Zone

PlainDateTime does not support time zones.

Formula

Wrong const date = Temporal.PlainDateTime.from("2026 - 05 - 17T14:30:00Z");
Correct const date = Temporal.ZonedDateTime.from("2026 - 05 - 17T14:30:00");

Comparing Different Types

Temporals can go wrong when comparing different types.

Wrong const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.Instant.from("2026-05-17T14:30Z");
Temporal.PlainDate.compare(d1, d2);
Correct const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.PlainDateTime.from("2026-05-17T14:30");
Temporal.PlainDate.compare(d1, d2);

Using equals() with Different Types

Temporals can go wrong when comparing different types.

Wrong const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.Instant.from("2026-05-17T14:30Z");
d1.equals(d2);
Correct const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.PlainDateTime.from("2026-05-17T14:30");
d1.equals(d2.toPlainDate());

Formula

Using ==, < or > for Comparison

Temporal objects cannot be compared with ==, ===, < or >.

Wrong const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.PlainDate.from("2026-05-17");
d1 === d2; // False d1 == d2;  // False d1 < d2;   // TypeError d1 > d2;   // TypeError
Correct const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.PlainDate.from("2026-05-17");
Temporal.PlainDate.compare(d1, d2);

Expecting Mutation

Temporal objects are immutable.

Wrong const date = Temporal.PlainDate.from("2026-05-17");
date.add({ months: 1 });
Correct const date = Temporal.PlainDate.from("2026-05-17");
const next = d.add({ months: 1 });

Using valueOf()

Temporal objects do not convert to numbers.

Example

const date = Temporal.PlainDate.from("2026-05-17");
try {
text = date.valueOf();
} catch (err) {
text = err.name;
}

Using Instant for Local Time

Instant is always UTC (not local time).

Wrong

Temporal.Now.instant(); // not local time

Correct

Use

Temporal.Now.zonedDateTimeISO() to get local time:

Temporal.Now.zonedDateTimeISO();

Using PlainDate for Time

PlainDate has no time.

Wrong

Formula

Temporal.PlainDate.from("2026 - 05 - 17T14:30");

Correct

Formula

Temporal.PlainDateTime.from("2026 - 05 - 17T14:30");

Forgetting Time Zone in ZonedDateTime

ZonedDateTime requires a time zone.

Wrong

Formula

Temporal.ZonedDateTime.from("2026 - 05 - 17T14:30:00");

Correct

Formula

Temporal.ZonedDateTime.from("2026 - 05 - 17T14:30:00 + 02:00[Europe/Oslo]");

Not Choosing the Right Type

Each Temporal type has a specific purpose.

Type

Use

JavaScript Object Accessors

JavaScript Object Management

Loading lesson path

Learn/JavaScript/Objects, Classes, and Advanced Patterns

JavaScript•Objects, Classes, and Advanced Patterns

JavaScript Temporal Mistakes

Temporal is a powerful API, but it can be confusing at first.

Remember to use the correct Temporal type

Avoid implicit conversions

Always handle time zones correctly

Formula

Use compare() instead of < and >

Remember that Temporal Objects are immutable

Here are some common mistakes and how to avoid them. Missing UTC (Z) for Instant An Instant must always include UTC.

Formula

Wrong const instant = Temporal.Instant.from("2026 - 05 - 17T14:30:00");
Correct const instant = Temporal.Instant.from("2026 - 05 - 17T14:30:00Z");

Using PlainDateTime with Time Zone

PlainDateTime does not support time zones.

Formula

Wrong const date = Temporal.PlainDateTime.from("2026 - 05 - 17T14:30:00Z");
Correct const date = Temporal.ZonedDateTime.from("2026 - 05 - 17T14:30:00");

Comparing Different Types

Temporals can go wrong when comparing different types.

Wrong const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.Instant.from("2026-05-17T14:30Z");
Temporal.PlainDate.compare(d1, d2);
Correct const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.PlainDateTime.from("2026-05-17T14:30");
Temporal.PlainDate.compare(d1, d2);

Using equals() with Different Types

Temporals can go wrong when comparing different types.

Wrong const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.Instant.from("2026-05-17T14:30Z");
d1.equals(d2);
Correct const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.PlainDateTime.from("2026-05-17T14:30");
d1.equals(d2.toPlainDate());

Formula

Using ==, < or > for Comparison

Temporal objects cannot be compared with ==, ===, < or >.

Wrong const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.PlainDate.from("2026-05-17");
d1 === d2; // False d1 == d2;  // False d1 < d2;   // TypeError d1 > d2;   // TypeError
Correct const d1 = Temporal.PlainDate.from("2026-05-17");
const d2 = Temporal.PlainDate.from("2026-05-17");
Temporal.PlainDate.compare(d1, d2);

Expecting Mutation

Temporal objects are immutable.

Wrong const date = Temporal.PlainDate.from("2026-05-17");
date.add({ months: 1 });
Correct const date = Temporal.PlainDate.from("2026-05-17");
const next = d.add({ months: 1 });

Using valueOf()

Temporal objects do not convert to numbers.

Example

const date = Temporal.PlainDate.from("2026-05-17");
try {
text = date.valueOf();
} catch (err) {
text = err.name;
}

Using Instant for Local Time

Instant is always UTC (not local time).

Wrong

Temporal.Now.instant(); // not local time

Correct

Use

Temporal.Now.zonedDateTimeISO() to get local time:

Temporal.Now.zonedDateTimeISO();

Using PlainDate for Time

PlainDate has no time.

Wrong

Formula

Temporal.PlainDate.from("2026 - 05 - 17T14:30");

Correct

Formula

Temporal.PlainDateTime.from("2026 - 05 - 17T14:30");

Forgetting Time Zone in ZonedDateTime

ZonedDateTime requires a time zone.

Wrong

Formula

Temporal.ZonedDateTime.from("2026 - 05 - 17T14:30:00");

Correct

Formula

Temporal.ZonedDateTime.from("2026 - 05 - 17T14:30:00 + 02:00[Europe/Oslo]");

Not Choosing the Right Type

Each Temporal type has a specific purpose.

Type

Use

JavaScript Object Accessors

JavaScript Object Management