Where camelCase is the convention
camelCase is the usual style for variables, functions and methods in JavaScript, TypeScript, Java, Kotlin and Swift, and for the properties of objects in those languages. Go uses it for names that stay inside a package. Many web APIs also use it for the keys in their JSON, and CSS in JavaScript uses it for properties such as backgroundColor.
Not every language agrees. Python, Ruby and Rust prefer snake_case, and CSS and HTML use kebab-case. If you are moving names between those worlds, the snake_case converter does the same job in the other direction, and the style row above covers the rest.
camelCase, lowerCamelCase and PascalCase
People use these names loosely. Strictly, lowerCamelCase starts with a small letter (userName) and UpperCamelCase, also called PascalCase, starts with a capital (UserName). This page produces the first. Classes, types and React components usually take the second, so switch to PascalCase in the style row for those.
How acronyms and numbers are handled
The converter splits the text into words, then writes the first in lower case and capitalizes only the first letter of each other word. Runs of capitals stay together until the capital that starts the next word, so HTTPServerError is split into HTTP, Server and Error and written as httpServerError.
Treating an acronym as a word is the rule in several style guides, including Google's Java style guide, and it is why the result reads xmlHttpRequest. Other code bases keep acronyms in capitals (the browser's own XMLHttpRequest is an example). If yours does, fix those spots by hand.
Digits stay with the word before them, so version2Update splits into version2 and Update. Apostrophes are dropped, so 'it's ready' becomes itsReady.
Converting a whole list of names
Paste a column of labels, one per line, and every line becomes one identifier. 'First Name', 'Last Name' and 'Date of Birth' become firstName, lastName and dateOfBirth. Empty lines stay empty, so the shape of your list is kept. This is handy for turning spreadsheet headers into object keys. Names must still be valid in your language: a name cannot start with a digit in most of them, and this page does not check that.