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functions are one of the fundamental building blocks in JavaScript. A function in JavaScript is similar to a procedure a set of statements that performs a task or calculates a value, but for a procedure to qualify as a function, it should take input and return an output where there is some obvious relationship between the input and the output. To use a function, you must define it somewhere in the scope from which you wish to call it.
Function declarations A function definition (also called a function declaration, or function statement) consists of the function keyword, followed by: The name of the function and a list of parameters to the function, enclosed in parentheses ; and separated by commas. The JavaScript statements that define the function, enclosed in curly brackets, {}.
example:
function printuserName(userName, surName){
console.log ('welcome ',userName, surName);
document.write(hello <b> ${userName} </b> ${surName} <br>)
} printuserName(userName, surName);