What does define do in javascript

[Solved]-What is ‘define’ used for in JavaScript (aside from the obvious)?-require.js

I can’t say for sure without seeing the entire script, but it’s likely to be the define function from RequireJS, in particular the «define with dependencies» form of that function. It is used to define a «module»:

A module is different from a traditional script file in that it defines a well-scoped object that avoids polluting the global namespace. It can explicitly list its dependencies and get a handle on those dependencies without needing to refer to global objects, but instead receive the dependencies as arguments to the function that defines the module.

And the «define with dependencies» form of define is described as follows:

If the module has dependencies, the first argument should be an array of dependency names, and the second argument should be a definition function. The function will be called to define the module once all dependencies have loaded. The function should return an object that defines the module.

This is AMD pattern for writing modules which AMD stands for Asynchronous Module Definition for when you need to import modules async basically rather than something like commonJS.

define(['module1', 'module2'], function(module1, module2) < console.log(module1.sayHi()); >); 

Define takes an array of dependencies and once all those are loaded in the background (async) in a non-blocking way, define calls the callback which in turn accepts arguments (in this case the dependencies).

Another thing to note is that each one of those modules also needs to be defined using «define» keyword. So for instance module1 would be defined like below :

This way of writing modules (AMD) allows you to write with browser compatibility in mind (no require() like in nodeJS) and also you can define many formats including objects, JSON, etc while for instance commonJS needs modules to be objects.

Keep in mind, AMD has it’s own downfalls. Hope this helps someone.

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JavaScript: Define a variable in IF statement

Have you ever tried to write an IF statement to check a primitive/constant value against a JavaScript variable, but accidentally re-defined the variable?

 let state = "woosah"; if (state = "not-woosah") < console.log("not relaxed!"); >else

Accidents happen, what I really meant to write was:

This statement has 3 equals symbols ( === ) which means something completely different.

When I came across this problem for the first time, I asked myself..

Can you define variables inside an IF statement?

The answer is yes you can. Let’s look at the code example above again:

 let state = "woosah"; if (state = "not-woosah") < console.log("not relaxed!"); // This outputs! >else

What I did there, was re-define a JavaScript variable inside the IF statement.

But you can create a new variable right inside the IF statement parenthesis.

So my next question, which I’m sure your’s is too, should anyone be doing this?

Should you define a variable inside IF statement?

Honestly, there’s no right or wrong answer to this question. JavaScript allows it, so you can make your decision from there.

I haven’t come across a use-case where this makes sense for me to do.

And personally, I think this is prone to bugs. It may also make it difficult to be catch when debugging, and your peers may be speed reading your code and not catch that variable definition.

They may mistake it for an equals check.

To avoid this, there’s a writing style I use called Yoda conditions.

A guide to Yoda conditions

Yoda conditions is a programming style when writing expressions. It’s also known as Yoda notation.

Let’s look at the code example above. I’ve also corrected the statement.

 let state = "woosah"; if (state === "not-woosah") < console.log("not relaxed!"); >else < console.log("relaxed :)"); // This prints; relaxed >

In the code example above, I’m checking to see if the variable, state , equals the string «not-woosah» .

If that statement is true, than it will print, “Not relaxed!”. Otherwise it will print, “relaxed :)”.

Like I mentioned above, accidents happen, and you may accidentally forget to type the additional equals symbols ( === ).

To avoid this error, I use the Yoda condition programming style.

All I did above, was switch the constant, «not-woosah» , to the left-hand side.

So just in-case you’re typing really fast, and you write something like this:

Your console should yell at you with this error message:

 Uncaught SyntaxError: Invalid left-hand side in assignment 

Oh wow, you’ve made it this far! If you enjoyed this article perhaps like or retweet the thread on Twitter:

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Ruben Leija

I launched this blog in 2019 and now I write to 85,000 monthly readers about JavaScript. Say hi to me at Twitter, @rleija_.

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