Guys,
I thought I understood the problem well and found the proper solution to it.
(JS)
At that, while my solution always passes complete set of "small" random tests, it fails the 1st "big" test.
Can there be something wrong with JS "big" tests or I am really so stupid on Friday morning :) ...

didn't work on the following argument array: 4ic6j - Expected: '3pen', instead got: 'invalid word'
didn't work on the following argument array: 4ty439 - Expected: '2ante-pen', instead got: 'invalid word'
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Sometimes I can catch the above error in random tests. The original solution seems to treat such cases wrong.

55 is a good number! Let us stop here!..

JS random tests sometimes fail with '-NaN' generated by test as a correct answer.
Some logs given below, my solutions are obviously correct (which can be checked with direct calculation):

• Testing: -1+4a and a-9a^3: expected '-36a^4+9a^3+4a^2-a' to equal '-NaN'
• Testing: -8+T and T+5T^3+T^4: expected 'T^5-3T^4-40T^3+T^2-8T' to equal '-NaN'
• Testing: -1+4Z and Z^3: expected '4Z^4-Z^3' to equal '-NaN'
• Testing: -2x and x^2: expected '-2x^3' to equal '-NaN'

This solution passes all the tests but is it yet correct?

Can You consider the following cases? Other solutions in top give all the same results but opposite to yours...

console.log(wordSquare("ABCDEFFGGHHIIJJJ")) // returns true - shouldn't be false?;

console.log(wordSquare('AAAAACCCCRRRRRRR')) // returns false - shouldn't be true?;

console.log(wordSquare('OOOOOOOOOOOOOOOO')) // returns false - shouldn't be true?;

This Regex is wrong.

It will not approve the valid password where there is only one digit standing on the first position - i.e. please check with 1abcDEFghiJKL

It will approve the invalid password where _ is standing on the first position - i.e. please check with _abcDEFghiJKL1.

Therefore, this solution fails rendom tests sometimes.

It is a Map preloaded by the kata author - You may use it not to draw the letters by yourself. Please check the Kata description attentively.