This workshop is relevant to developers because:
- You will use array and array manipulations daily.
- Understanding built-in methods and callbacks will increase the quality of your code.
After this workshop, developers will be able to:
- Create arrays and store them in variables
- Access and change information inside arrays with array methods
- Explain examples that use callback functions to create more flexible code
- Traverse arrays with
forloops and theforEachiterator array method
Before this workshop, developers should already be able to:
- Identify an array of elements
- Open, edit, and save a
.jsfile in Atom - Evaluate simple lines of code in the Chrome Developer Console or Node REPL
##Array Method Basics##
var fruits = ["Apple", "Banana", "Cherry", "Durian", "Elderberry",
"Fig", "Guava", "Huckleberry", "Ice plant", "Jackfruit"];Accessing the first element:
fruits[0];
//=> "Apple"Accessing the length:
fruits.length;
//=> 10Accessing the last element:
fruits[fruits.length-1];
//=> "JackfruitAdding an element to the front:
fruits.unshift("Apricot");
//=> 11Adding an element to the end:
fruits.push("Kiwi");
//=> 12Removing an element from the front:
fruits.shift();
//=> "Apricot"Removing an element from the end:
fruits.pop();
//=> "Kiwi"Finding the index of an element:
fruits.indexOf("Jackfruit");
//=> 9
fruits[9];
//=>"Jackfruit"Removing an element by index position:
var huckleBerryPos = fruits.indexOf("HuckleBerry");
var removedItem = fruits.splice(huckleBerryPos, 1);
//=> ["Apple", "Banana", "Cherry", "Durian", "Elderberry", "Fig", "Guava", "Ice plant", "Jackfruit"];
By now you've all used for to iterate over your arrays.
var fruits = ["Apple", "Banana", "Cherry", "Durian", "Elderberry",
"Fig", "Guava", "Huckleberry", "Ice plant", "Jackfruit"];
for (var i=0; i<fruits.length; i++) {
console.log("eating a", fruits[i]);
}Quick Exercise: Practice array methods with Challenge Set A!
When we call a function, we usually pass some arguments to it. For example here, we're passing a string Strawberry to the indexOf function.
fruits.indexOf("Strawberry");We can do this with our own functions as well.
function makeSandwich(meat, cheese) {
return "bread, " + meat + ", " + cheese + ", bread";
}
/* call makeSandwich and pass arguments to it */
makeSandwich("turkey", "provolone");
//=> "bread, turkey, provolone, bread"Arguments can also be passed in with their variable names.
var favoriteMeat = "ham";
/* call makeSandwich with favoriteMeat and "colby" */
makeSandwich(favoriteMeat, "colby");
//=> "bread, ham, colby, bread"We can pass other types of arguments too, including arrays and objects.
function makeBigSandwich(toppingsArray) {
return "bread," + toppingsArray.toString() + ", & bread";
}
makeBigSandwich(['ham', 'salami', 'provolone']);
//=> "bread, ham, salami, provolone, & bread"**We can also pass functions as arguments.** In fact, JavaScript treats functions a lot like any other data type; you can pass them into other functions as arguments, return functions from functions, and store them in variables.
When a function is passed in as an argument, it can be called within the function at whim.
First let's look at a function that just uses another function.
function add(a, b) {
var result = a + b;
console.log(result);
return result;
}
add(4, 6);
//=> 10What are the two functions that are executed above when we call `add`?
>The functions that are executed are `add` and `console.log`Now let's make our add function a little more flexible. We'd like the option to display the result in multiple ways instead of always using console.log. We'll remove console.log and pass in a more generic display function to call instead.
function toParagraph(input) {
return "<p>" + input + "</p>";
}
function toCurrency(input) {
return "$" + input.toFixed(2);
}
function add(a, b, formatFunction) {
var result = a + b;
return formatFunction(result);
}
add(2, 2, toParagraph);
//=> "<p>4</p>"
add(2, 2, toCurrency);
//=> "$4.00"When a function takes in another function and calls it, we say the second function is a "callback" function.
Which function above is a callback function?
The `add` function above requires three arguments to work: a number (`a`), another number (`b`), and a callback function (`formatFunction`). The 3rd argument, "formatFunction" is just a placeholder/slot for our two formatting functions: `toCurrency` and `toParagraph`, but we have the flexibility to create new formatting functions now (`toRomanNumeral`! `toFraction`!), without needing to modify `add`.We can also use a different function as the formatFunction. This gives us a lot of flexibility.
function toAlert(input) {
alert(input);
}
// then calling it:
add(8, 9, toAlert); // **alerts 17**
//=> undefinedA common way to write this is to define the displayFunction function in-line with the main function call.
// defining our specific display function as we call the add function ("in-line"):
add(8, 9, function toAlert(input) {
alert(input);
});
// **alerts 17**
//=> undefined
Let's have another example, a more delicious example:
function eatSandwich(topping1, topping2, topping3, sandwichMaker) {
console.log("I'm going to make and eat a sandwich with: " + topping1 + ', ' + topping2 + ' and ' + topping3);
var layers = [topping1, topping2, topping3];
var sandwich = sandwichMaker(layers);
return ("Finished eating my " + sandwich + " sandwich!");
}
function makeBigSandwich(toppingsArray) {
return "bread, " + toppingsArray.toString() + ", & bread";
}
/* call the function: */
eatSandwich('bacon', 'lettuce', 'tomato', makeBigSandwich);
//=> I'm going to make and eat a sandwich with bacon, lettuce, and tomato
//=> Finished eating my bread, bacon, lettuce, tomato, & bread sandwich!We passed the makeBigSandwich function to the eatSandwich function as an argument. eatSandwich calls makeBigSandwich.
And now we'll re-write this with the function definition in-line with the function call.
// eatSandwich stays the same
function eatSandwich(topping1, topping2, topping3, sandwichMaker) {
console.log("I'm going to make and eat a sandwich with: " + topping1 + ', ' + topping2 + ' and ' + topping3);
var layers = [topping1, topping2, topping3];
var sandwich = sandwichMaker(layers);
return ("Finished eating my " + sandwich + " sandwich!");
}
// then to call the function:
eatSandwich('bacon', 'lettuce', 'tomato', function makeBigSandwich(toppingsArray) {
return "bread," + toppingsArray.toString() + "&bread";
});Let's get back to arrays.
We've seen we can traverse an array of elements with a simple for loop, but this isn't the most streamlined approach to accessing and changing a list in Javascript.
Javascript has provided us with quite a few powerful built-in "iterator" methods that make it a breeze to do something to each element in an array. Plus, they give us great opportunities to practice using callbacks!
Being able to use these methods is one sign of a more mature developer. We encourage you to use iterators instead of traditional for loops wherever you can!
To loop through an array with the ability to alter each element, similar to a for loop traversal, JavaScript gives us an Array method called forEach().
forEach function skeleton:
array.forEach(function callBack(element, index) {
console.log(index + ". " + element);
});Fruity Example - Make a numbered list
fruits.forEach(function callBack(element, index) {
console.log(index + ". " + element);
});
// => 0. Apple
// => 1. Banana
// => 2. Cherry
// => 3. Durian
// => 4. Elderberry
// => 5. Fig
// => 6. Guava
// => 7. Huckleberry
// => 8. Ice plant
// => 9. JackfruitQuick Exercise: Practice forEach with Challenge Set B!
Check out Mozilla Developer Network's Array documentation for more information on arrays. All of the methods listed in the Array instances section are available to use with JavaScript arrays. Commonly used methods in this section include join, sort, and reverse.
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