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June 18, 2014

JavaScript Function Terminology

Filed under: JavaScript @ 12:43 pm

I have been working on and off with JavaScript since 2001. But its one thing to use JavaScript functions and another thing entirely to know the vocabulary of JavaScript functions.

So here are some common JavaScript function terms:

Function Declaration

Defines a named function.

function onClick(buttonName) {

    return “Clicked: ” + buttonName;



  • onClick() – executes the function and returns the value.
  • onClick – references the function.

In JavaScript, function declarations are always moved (“hoisted“) to the top of their scope at runtime. This means that the function does not need to be declared before it is used.

So something like this is valid:


function onClick(buttonName) {

    return “Clicked: ” + buttonName;


Function Expression

Defines a function as part of a larger expression such as a variable assignment.

The function can be named, as with the onClick function below:

var buttonClick = function onClick(buttonName) {

    return “Clicked: ” + buttonName;


Or anonymous as with this function:

var buttonClick = function(buttonName) {

    return “Clicked: ” + buttonName;



  • buttonClick() – executes the function and returns the value.
  • buttonClick – references the function.
  • onClick() – generates an undefined error.

Function expressions are not “hoisted”. So attempting to use them before they are defined generates a type error:


var buttonClick = function(buttonName) {

    return “Clicked: ” + buttonName;


NOTE: The variable declaration (buttonClick) does get hoisted, but only the definition, not the assignment. So at runtime, var buttonClick = undefined is hoisted. But the assignment does not occur until the un-hoisted assignment statement is executed.

Function Statement

Basically a function declaration. (See above).


In general, guidance says to favor function expressions over function declarations (or function statements) for the following reasons:

  • The hoisting of function declarations can cause confusion or hard to find bugs.
  • Function expressions more clearly imply that the function itself is an object.

See this link for more examples and an enhanced discussion of function declarations vs function expressions.



  1.   John DeHope — June 19, 2014 @ 9:12 am    Reply

    I read your blog post first. Then I took the quiz in the blog post you referenced. I got all four questions right. You’re a good teacher!

  2.   DeborahK — June 19, 2014 @ 11:24 am    Reply

    Thanks John!

  3.   Ken Dason — June 20, 2014 @ 5:54 am    Reply

    Your have been a great help to the developer community since the early days of VB. Hopefully, this foray into JavaScript is an indication of more to come in this area! Thank you!!

  4.   DeborahK — June 20, 2014 @ 6:05 pm    Reply

    Thank you Ken! I am currently working on an AngularJS course for Pluralsight and having a great time! So yes, more JavaScript to come! 🙂

  5.   Josh Graber — October 21, 2014 @ 5:41 am    Reply

    What would be the point of naming a function expression? Is this similar to adding a comment? After all, you can’t call a function expression by name, so it serves no real purpose other than documentation, correct?

  6.   deborahk — October 27, 2014 @ 2:41 pm    Reply

    Hi Josh –
    With a named function expression, the function name *can* be used within the scope of the function.

    So this would be valid:
    var buttonClick = function onClick(buttonName) {

    return typeof onClick;


    More importantly, named functions can help with debugging. The function names will be used in the call stack.

  7.   Shawn Zhang — April 26, 2015 @ 7:10 pm    Reply

    I came from your video ‘AngularJS Line of Business Application’ to here.
    Thanks for the explanation of function terminology, I always saw people use IFFE in their angular code, but don’t know what happened behind it.
    Now I fully understand why the function been defined like that, Thanks.

    •   deborahk — June 25, 2015 @ 9:18 am    Reply

      Thank you for your comment. I appreciate the feedback!

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