javascript - How to make a string automatically add 0 in front of a single digit and only retain two digits of a three-digit number

Question

Get data from the background. If the data is "4", the front desk will display "04". If the background is "04", the front desk will display "04"

My code:
$(".ball_1").html(data.ball_1>=10||data.ball_1.length=3data.ball_1:'0' data.bal_1||data.ball_1.substring (1));
Report an error directly

$(".ball_1").html(data.ball_1>=10?data.ball_1:'0' data.ball_1&&data.ball_1.length>=3?data.ball_1.substring(1)?data.ball_1 );

If the data is "4", it will display 4. How should this function be implemented?

Answer 1

If the data is "4", the front desk will display "04". If the background is "04", the front desk will display "04"

For example
"1" => "01"
"4" => "04"
"99" => "99"

---

Use slices

var addPrefix = str => ('00' + str).slice(-2); 

This can do what you need: pad zeros for one digit and retain only two digits for three digits

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Answer 2

var num = '1';

num = Number(num);

num = num<10? '0'+num : num;

//What does it mean to keep two digits in a three-digit number? ? ? ?
//num = (num==100 || num>100)? ... : ...

Answer 3

function x(d){
    D=d<10?`0${d}`:`${d}`.match(/^\d{2}/)[0];return D
}

Answer 4

parseInt('04')<10?'0'+parseInt('04'):'04'