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Adding a Leading Zero in 1 Line in JavaScript

At work, I recently ran into a scenario where we were capturing a user's date of birth (DOB). This information is collected in 3 fields day, month, and year. If your DOB is January 7, 1962, people were inputting 1 for the month, 7 for the day, and 1962 for the year. This sounds good but our back-end database required both the month and day to be 2 digits. So I needed to find a way to add a leading zero for both of these fields.

Starting Data

let month = 1;
let day = 7;

My simple solution in one line is:

month = `0${month}`.slice(-2)

Let's walk through this code. This combines 0 with the value the user inputs for their month. Then it takes the right two digits and assigns this to the month.

But what if the user inputs the value 10 for their month? It still works. It will combine 0 with 10 to produce 010 and then it takes the right two digits which is 10.

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by Jennifer Bland

I am a Google Developers Expert. Entrepreneur. Mountain Climber. Neil Diamond fanatic. World traveler. MBA grad. Software Engineer. Interested in hiring me? Contact me at

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