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A JavaScript function is a block of code designed to perform a particular task. A JavaScript function is executed when "something" invokes it (calls it).
A function definition (also called a function declaration, or function statement) consists of the function keyword, followed by: The name of the function. A list of parameters to the function, enclosed in parentheses and separated by commas.
It's read as a function declaration (like function foo {}), rather than a function expression. So adding the ! before it, or wrapping it in parentheses, forces the parser to understand that it's an expression.
Functions are the main “building blocks” of the program. They allow the code to be called many times without repetition. We’ve already seen examples of built-in functions, like alert(message), prompt(message, default) and confirm(question). But we can create functions of our own as well.
Broadly speaking, JavaScript has four kinds of functions: Regular function: can return anything; always runs to completion after invocation; Generator function: returns a Generator object; can be paused and resumed with the yield operator; Async function: returns a Promise; can be paused and resumed with the await operator
JavaScript functions are defined with the function keyword. You can use a function declaration or a function expression.
Functions are one of the fundamental building blocks in JavaScript. A function is a JavaScript procedure—a set of statements that performs a task or calculates a value. To use a function, you must define it somewhere in the scope from which you wish to call it.
The Function object provides methods for functions. In JavaScript, every function is actually a Function object.
To avoid repeating the same code all over places, you can use a function to wrap that code and reuse it. JavaScript provides many built-in functions such as parseInt() and parseFloat(). In this tutorial, you will learn how to develop custom functions.
Functions are thought of as "first class" objects, meaning that despite their unique behavior, they can be used in all the same contexts as any other JavaScript object. For example, a function can be assigned to a variable, passed as an argument to other functions, and returned by other functions.