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    Create a Pipe Function to Enable Function Composition

    Andy Van SlaarsAndy Van Slaars

    In this lesson, we’ll refactor a series of function calls into a functional pipeline, making the code more declarative and removing the need for intermediate variables. Rather than reaching for a utility library, we’ll start by building our own pipe function to gain a clear understanding of how left-to-right function composition is accomplished in JavaScript.

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    00:00 Looking at this handle toggle method we can see that find by ID is called and result of that is passed into toggle todo, and then the result of that is passed in as an argument to update todo. The constants todo and toggled are basically only there, so we can pass them along to the next line and then they're never used again.

    00:16 It would be nice if we could get rid of these intermediate values, since they're all useful until the next line of code runs. We could do this by nesting our calls. What I mean by that is I could take this call to find by ID with my ID and the todos list, cut that and paste it in place of its intermediate value in toggle todo.

    00:37 Then I could take that entire call to toggle todo, cut that and I could paste it in place of toggled. This works and it essentially allows us to get rid of these other two values. This is really messy and as soon as you add a fourth function or a fifth function this is going to get really hard to read.

    00:57 To accommodate this behavior, but make it much more readable we're going to define a pipe utility. Pipe is going to let us take the results of one function and pass them in to the next function.

    01:07 Then take the results of that function pass them into the next. Just like that messy nasty code we just saw, but we'll keep the mess in our utility function keeping our production code much easier to read. I'm going to start by opening the utils.test file.

    01:21 We'll define some test to make sure that our pipe utility is working as expected. I've pasted some code in. I have two new functions that I'm using just for test purposes increment and double to just each take a number in and either increment or double that number.

    01:35 Then I have two test functions. The first one make sure that I can pass the results of increment to double. Then the other one does the inverse just to make sure that our pipe is going in the order we expect. We're going to define pipe in our utils.js file.

    01:48 I'm going to update my import to also import pipe. Then I'm going to come over to the terminal and I'm going to run the test with npm test. Of course these are going to fail because we haven't defined pipe yet. I'm going to open utils.js and I'm going to export const pipe. We'll set that to equal a function and now let's define it.

    02:12 Let's take another look at our test to see how we want to define pipe. If we look at our test in both cases we're calling pipe, we're passing it two functions, and we're getting back another function that we're calling later with an argument.

    02:25 Let's start defining our pipe function. In pipe the first two arguments are going to be a function f and a function which we'll just call g. That needs to return another function, so that's going to look something like this. That second function is going to take our arguments whatever that is.

    02:45 In this case I'm going to just use the rest operator to take whatever arguments are passed in and wrap them up in an array called args. Now we need to get a result. A result is going to take our first function and it's going to call that first function with whatever arguments we have.

    03:05 I'll use the spread operator to break that back out into a list of arguments. Then it needs to pass that result into our second function which in this case we're calling g. We'll do that by wrapping the results of f with the arguments in a call to g.

    03:23 If I say this our test will rerun and now our tests are passing. I'm just going to paste in a new test, and this test is just going to verify that pipe works with more than two functions. Here I have a pipeline that's going to add a number, increase double, then increase again.

    03:40 The end result should look something like what would happen if we nested these functions like this. We're going to get back a function, call with the arguments one and two, get a result, and we expect that value to be nine.

    03:52 Now, if I save this our test will fail, but the interesting thing to note here is that we expected nine and the result was four. We did get a result back. The problem is that pipe took the first two functions and ignored the second two, so let's fix that. The existing pipe function isn't exactly what we need, but it's useful. We're going to keep it.

    04:13 What I'm going to do is I'm going to take the export off of this and I'm just going to rename this to be _pipe so that we can use it inside of utils.js but we're not going to export it. Then we're going to replace that with a new pipe function, so export const pipe, and then we'll define that.

    04:30 Pipe is going to take in a list of functions and instead of two specific functions now it's going to be a variable length. I'm going to use the rest operator to take in functions and wrap them up in an array we'll call functions.

    04:44 Now, we have an array of functions and we need to return a single function. The way you take an array of anything and return a singular item is using reduce. I'm going to call functions.reduce and then I'm going to pass in our internal _pipe function.

    05:05 By calling reduce without an initial value, it's going to take the first two items in our functions array, pass them to _pipe and pipe those together and return a function. Then it can take the next one along with the returned function from the first call _pipe, pass those into pipe and put them together.

    05:23 It will go on and on until it runs out of functions returning a single function at the end that's just waiting to receive arguments at which point it will execute and pipe the values all the way through those functions.

    05:35 With that defined I'm going to save my file and we can confirm that the test passed. I'll open up the browser and jump over to App.js. At the top of the file I'm going to import pipe as well as partial from lib/utils. Now I'm going to update the handle toggle method to use our pipe and partial functions.

    06:02 I'm going to start by defining get updated todos which is going to be a function that we're going to get back from a call to pipe. We basically want to use pipe to recreate this structure here. We're going to start by piping find by ID. We're going to take the results of that pipe that into toggle todo.

    06:23 Then the results of that can get piped into update todo. There's one small problem here update todo actually takes two arguments. The value coming out of toggle todo is the second argument. What we need to do to make this work is partially apply update todo making sure that it already has its first argument which in this case is state.todos.

    06:48 With that in place we can get rid of these two lines. We can update this line to just use get updated todos and it will accept the ID and this.state.todos. Our browser will reload and we can just verify that everything still works as expected allowing us to toggle our todos.