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1. In JavaScript, it's generally a good idea to use the brace-on-same-line style, i.e.

    if( ... ) {
      ...
    } else {
      ...
    }
   

   because of javascript's policy of automatic semicolon insertion. While it's usually not a problem, it may bite you one day if you use the brace-on-new-line style.

2. field_vldn is a strange name - your other functions are named more descriptively, and uses the conventional CamelCase style - why doesn't field_vldn do the same?

3. Your validation is being done in two places (field_vldn and in blankOrSame). Combine that logic.

4. The logic for comparing the accounts is too complicated. Or, each part of it is simple, but there's too much repetition, and using the while loops (two of them even, instead of extracting a function) is very brute-force, when a more generic solution is possible.

For that last point, here's the deal: this is a simple math problem. An account's balance can be expressed as:

$$ f(x) = ax + b $$

where \$x\$ is the number of days, \$a\$ is the daily increment, and \$b\$ is the initial balance.

To figure out when - if ever - the two accounts, \$f{_1}\$ and \$f{_2}\$, will be equal, you simply isolate \$x\$ in \$f{_1}(x) = f{_2}(x)\$:

$$ x = \frac{b{_2} - b{_1}}{a{_1}-a{_2}} $$

If \$x\$ is zero, the accounts are already equal at day 0.
If \$x\$ is negative, the accounts won't ever be equal (unless you go back in time).
If \$x\$ is positive and an integer, the two accounts will end up being exactly equal after \$x\$ days.
If \$x\$ is positive but not an integer, you simply round up to the next integer, and that's the number of days it'll take for one account to "overtake" the other.
If \$x\$ is ±Infinity (which you can check with Number.isFinite()) the two balances will progress in parallel, and will never be equal or intersect.