padStart を使うとよいです。
第一引数で何桁まで埋めるか指定し、第二引数で埋める文字を指定します。
> '1'.padStart(2, '0')
=> "01"
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JavaScriptで文字列のゼロパディング（C言語でいう%02d）
Last updated at Posted at 2020-12-12
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