This may be a mixup of terminology, mixing terms like "nearest" and "truncate", neither of which quite describes what the example demonstrates.

The example you give always rounds down, never up, to the nearest custom value (in this case 50). To do that you can just subtract the result of % 50. For example:

const val = 498;
console.log(val - val % 50);

Even make it a re-usable function:

const Nearest = (val, num) => val - val % num;

console.log(Nearest(498, 50));

Divide by 50, do the operation, multiply by 50.

console.log(Math.floor(498 / 50) * 50);
console.log(Math.ceil(498 / 50) * 50);
console.log(Math.round(498 / 50) * 50);
console.log(Math.trunc(498 / 50) * 50);

You divide your number by 50, take the ceiling of that number and then multiply it by 50.

Math.ceil(value / 50) * 50;

A quick sidenote: truncate has a whole other meaning for numbers in Javascript: Math.trunc on MDN

Edit:

If you want other rounding semantics than ceil you can of course use floor (always goes to lowest multiple of 50):

Math.floor(451 / 50) * 50; // => 450