How to determine if a value is a numberThere are many situations in JavaScript where you want to determine if a value is a number. In business systems, handling numbers is frequent, such as amounts in ...
JavaScriptの `Date` コンストラクタにおいて、当月1日の0日前（`new Date (year, month, 0)`）を指定して前月の最終日を数学的に導き出し、ミリ秒単位で「前月初日 00:00:00」から「前月末日 23:59:59」を定義する厳密なカレンダー演算が必要です。
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