Someone else suggested the answers to this question would also help, but that comment got deleted by the system when the question was closed. It's certainly relevant though.
but I dont just want to increment it, I need to call Trie function for each function inside for loop, so need to know how can I increment like ch++, inside the loop
JavaScript doesn't have a character type, just strings with one character. Its strings are UTF-16 code unit sequences (details in my blog post What is a String?), so you can start with n = "a".charCodeAt(0) (97) and loop while n <= "z".charCodeAt(0) (122) and output the result of String.fromCharCode(n). If you needed to do something similar but in the realm of characters that require multiple code units, the equivalent operations are .codePointAt and String.fromCodePoint.
n = "a".charCodeAt(0)(97) and loop whilen <= "z".charCodeAt(0)(122) and output the result ofString.fromCharCode(n). If you needed to do something similar but in the realm of characters that require multiple code units, the equivalent operations are.codePointAtandString.fromCodePoint.