If you want to use built-in code, you can use toLocaleString() with minimumFractionDigits.

Browser compatibility for the extended options on toLocaleString() was limited when I first wrote this answer, but the current status looks good. If you're using Node.js, you will need to npm install the intl package.

Use with AIvar value = (100000).toLocaleString(
  undefined, // leave undefined to use the visitor's browser 
             // locale or a string like 'en-US' to override it.
  { minimumFractionDigits: 2 }
);
console.log(value);

Number formatting varies between cultures. Unless you're doing string comparison on the output,¹ the polite thing to do is pick undefined and let the visitor's browser use the formatting they're most familiar with.²

Use with AI// Demonstrate selected international locales
var locales = [
  undefined,  // Your own browser
  'en-US',    // United States
  'de-DE',    // Germany
  'ru-RU',    // Russia
  'hi-IN',    // India
  'de-CH',    // Switzerland
];
var n = 100000;
var opts = { minimumFractionDigits: 2 };
for (var i = 0; i < locales.length; i++) {
  console.log(locales[i], n.toLocaleString(locales[i], opts));
}

If you are from a culture with a different format from those above, please edit this post and add your locale code.

¹ Which you shouldn't.
² Obviously do not use this for currency with something like {style: 'currency', currency: 'JPY'} unless you have converted to the local exchange rate. You don't want your website to tell people the price is ¥300 when it's really $300. Sometimes real e-commerce sites make this mistake.

You can use padStart() to make sure a string is a certain length. If it's not it will pad it with whatever you want. In this case 0:

const nf = (n, c) => n.toString().padStart(c, '0');

function convertSeconds(s) {
  var min = Math.floor(s / 60);
  var sec = (s % 60)
  return nf(min, 2) + ':' + nf(sec, 2);
}

console.log(convertSeconds(64))
console.log(convertSeconds(119))

Not sure if you want to pad the minutes of not.

There is also the Intl.NumberFormat() which has an option for minimum digits (though this seems like overkill for this):

console.log(new Intl.NumberFormat('en-US', { minimumIntegerDigits: 2}).format(2));

console.log(new Intl.NumberFormat('en-US', { minimumIntegerDigits: 2}).format(59));