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This uses the apply() method with the constructor of the Function object to immediately invoke the function we defined. Immediately Invoked Constructor w/ apply() Hard to think of a practical application for this one, but it could be used in rare cases. The function can then be run with sum(1,2). This uses the apply() method to call the constructor of the Function object and create a function. However, it is useful in demonstrating that in JavaScript, functions are "just objects." There are several syntax variations- here's a good blog post that describes them all.ĭefining functions with the Function constructor is less performant than the other methods, so I wouldn't recommend this option. This example:Īrrow functions are anonymous by default, but when assigned to a variable, they assume the variable name.Īrrow functions are function expressions, and as such, they are usable in all the contexts shown above (variable assignment, passed as an argument, etc.). When using this shorthand, the defined function assumes the same name as the method.Īrrow (or "fat-arrow") functions are a shorthand definition that was introduced as part of the ES2015 specification. There is no anonymous version of this definition.
![javascript function javascript function](https://wwp.northeastern.edu/outreach/seminars/_utils/gfx/javascript_function_05.png)
It's functionally equivalent to this longer definition: This shorthand version of the method definition above was introduced in ES2015. Here's how we would call the methods defined in the examples: This is used in object-oriented programming, and is a practical way to namespace our functions and keep them out of the global scope. This can be done on both object literals and classes. Here's some more information on JavaScript closures, if you're curious.īy assigning functions (either named or unnamed) to properties of objects, we define methods on those objects. This lets us do clever things like the following (picking up from the example):Įach person can increment their own count variable despite the fact that it originated outside the scope of the count() function. This function-expression-as-argument is a common pattern for defining callbacks.įunctions can be returned from other functions.
![javascript function javascript function](https://www.tutorialsteacher.com/Content/images/oo-js/this-global.png)
While we could define the callback in a variable and pass it in, we can also use define it inline with a function expression. Like this: setTimeout(callback, interval) In this example, the setTimeout method takes two arguments: a callback function and a time interval. One neat application for the named variety of this is to make recursive functions more readable, by substituting arguments.callee with your function name. This is a combination of the variable assignment expression and the immediately invoked function (both demonstrated above). Immediately invoked functions can be used to encapsulate a program, preventing it from polluting the global namespace ( here's a simple example of that). The function's name is only available within its execution scope (defined by the parentheses), so it cannot be called later in the program. This function is immediately invoked, meaning that it is defined and called at the same time. The name of our function (add) cannot be called directly (see below), but it can come in handy when debugging: This is functionally equivalent to the Function Declaration above, except this variation is NOT hoisted. Also: here’s a great article on the differences between function declarations and expressions. This list aims to demonstrate that flexibility, and show use-cases for each example. Also, notice that this declaration does not need to be followed with a semicolon.įunction expressions are very flexible, and can used in many contexts. The declaration statement stands alone, and cannot be combined with other expressions. Functions declared like this are " hoisted", meaning, the JavaScript engine reads all these declarations first before executing any of the rest of the code. The "classic" declaration, which works well for simple uses. So without further ado, here are ALL the ways to define a JavaScript function. I could never seem to find one place that showed all of them, and explained why you might use each one. One small thing that confused me as I was learning JavaScript was the many different kinds of ways to define a JavaScript function.