How to make javascript ignore escape ( \ ) character?

But the escape tag "\" disappears, but I need it there, what should I do?

You need to escape the backslash, i.e., use \\ instead of just \:

"[math]k: \\frac{(x+20)^{2}}{256}+\\frac{(y-15)^{2}}{81}=1[/math]"
          ^                       ^

Escape the escape character, like \\a.

You can use tagged template literals

var str = (s => s.raw)`[math]k: \frac{(x+20)^{2}}{256}+\frac{(y-15)^{2}}{81}=1[/math]`[0]

The anonymous arrow function will serve as tag and s.raw contains the original input

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