When working with Moment.js, it’s essential to know which methods change the original date object and which return new values. This can help you avoid unintended side effects in your code. Here’s a quick guide:

?Methods that Mutate the Original Date Object:

• startOf(unit) — Sets the date to the start of the specified unit (e.g., ‘year’, ‘month’, ‘day’).
• endOf(unit) — Sets the date to the end of the specified unit.
• add(value, unit) — Adds time to the original Moment object based on the specified value and unit.
• subtract(value, unit) — Subtracts time from the original Moment object based on the specified value and unit.
• set(unit, value) — Sets the specified unit of the original Moment object to the given value.

? Methods that Return a New Object:

• toISOString() — Converts the Moment object to an ISO 8601 string.
• toDate() — Converts the Moment object to a native JavaScript Date object.
• format() — Formats the Moment object as a string based on the provided • format string.
• clone() — Creates a copy of the Moment object.

Knowing the difference between mutating and non-mutating methods in Moment.js is crucial for writing clean and predictable code. Be mindful of these distinctions to avoid unexpected bugs and improve code maintainability.