Predefined Functions & Methods?

Question

It's not clear to me when and if some functions are predefined or if their names are only examples given by the writer of the app. Which functions does the programmer need to name, which ones are predefined? How can I tell the difference?
In html the tags are clearly recognizable by the <> symbols and as far as I know the attributes cannot be defined by the programmer. However, in JavaScript it seems to me some functions and methods are given new names and others are given predefined names. Am I wrong?

Answer 1

there is a list of all predefined names in JavaScript, you can Google it. Other than that all the names are user defined

Answer 2

Coding today consists of many libraries(pre-defined code) meaning you could import some code to do a paticular task, in turn this reduces the amount of code YOU need to create .....Understanding your chosen language more and researching the available Libraries will help you understand more