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By the end of this chapter you should be able to:
let
keywordSo far our functions have not been taking in any input; they simply are returning an output. So when would we want an input to our functions? Let's imagine we want to calculate the sum of two numbers. If our numbers are 5 and 5, well, that's easy.
5 + 5; // 10
But what happens if we don't know what our numbers are? How could we possibly do this? Better yet, what if we want to do 4 + 6, and 2 + 8 and 7 + 3...we would have to start writing a whole bunch of code that's does basically the same function. This is a great example of when we want to add inputs and use a function. Inputs to a function are called parameters or arguments.
So what does a function look like with parameters? Let's write a function called add
that takes in two parameters - number1
and number2
- and returns their sum.
function add(number1, number2){ return number1 + number2; }
Now our add function will work for any two numbers that we want to add together. It's important to note that the name of the parameters, number1
and number2
, are arbitrary names that we have chosen. If we change the names of the parameters to a
and b
, the function would do exactly the same thing:
// This function will do the same thing as our previous function function add(a, b){ return a + b; }
Now we can define a function with parameters. Let's see how we invoke that function:
add(4, 6); // returns 10 add(2, 8); // returns 10 add(7, 1); // returns 8
In the example above, we are now invoking the add function with parameters. A parameter can be a literal number like we have above, or we could even use variables:
let num1 = 5; let num2 = 8; add(num1, num2); // returns 13
It is important to understand that the variable names we are using when we invoke the function are not related at all to the variable names we have defined inside of the function. The values of num1
and num2
are being copied into the parameters number1
and number2
that are defined in the function.
Now that we have an idea of what functions do, let's talk about what happens when we define variables inside of functions. To do that, we first need to define what scope
is.
MDN defines scope
as "The context in which values and expressions are 'visible,' or can be referenced". In JavaScript (before ES2015, which is where we will start), there are only 2 kinds of scope: global scope and function scope.
The important takeaways here are
all variables that are defined outside of functions (and inside of functions without the let
keyword) are declared in the global scope, and
all variables defined inside of functions can only be accessed by those functions (and any inner functions).
Let's see an example.
let globalvariable = "I live in the global scope"; function makeNewScope(){ let functionScopevariable = "I live in the scope of the makeNewScope function"; } globalvariable; // "I live in the global scope"; makeNewScope(); // maybe this will define the functionScopevariable... functionScopevariable; // This gives us an error! To be specific, a ReferenceError because the functionScopevariable is not defined.
What happens when we remove the let
keyword?
// Since this variable declaration is in the global scope, it will // be a global variable with or without the let keyword. It is a best // practice to always use the let keyword though. globalvariable = "I live in the global scope"; function makeNewScope(){ // You do not want to do this in practice. You should // always defined your variables with the let keyword. functionScopevariable = "What happens now?"; } globalvariable; // "I live in the global scope" makeNewScope(); // now this will define the functionScopevariable! // The value of the variable will be "What happens now?" functionScopevariable;
If we omit the let
keyword inside of a function, we actually declare that variable in the global scope. While this may seem like the way to go, this is not best practice. If we need to change some variable in a function, we should at least declare it in the global scope and assign it in a function so that our code is more readable.
let globalvariable = "I live in the global scope"; // we are just declaring the variable now; its value will be set to undefined. let globalvariableToBeChanged; function makeNewScope() { // Here we are assigning the value "What happens now" to the // globalvariableToBeChanged variable. globalvariableToBeChanged = "Undefined no more!"; } globalvariable; // "I live in the global scope" makeNewScope(); // now this will assign a new value to the globalvariableToBeChanged! // The value of globalvariableToBeChanged is "Undefined no more!" globalvariableToBeChanged;
If you remember back in the section on if
and else
statements, we mentioned that if you declare a variable inside of a block using let
or const
, you can not access it outside of that block. This is another kind of scope that is called block
scope and when you use let
or const
inside of a block (if/else
statements and for
and while
loops), that variable is only accessible inside of the block.
Make a function for add
, subtract
, multiply
, and divide
. Each of these functions should accept two parameters and return the sum, difference, product and quotient.
add(2,2); // 4 subtract(2,2); // 0 multiply(2,2); // 4 divide(2,2); // 1
Write a function called sayHello
that takes in a string as a parameter. If the parameter passed to the function is your first name, it should return "Hello Boss", but if the parameter passed to the function is any other name, it should return the string "Hello" and the name
parameter.
// for this example, my first name is Tim sayHello("Tim"); // "Hello Boss" sayHello("Janey"); // "Hello Janey" sayHello("Elie"); // "Hello Elie"
Write a function called average
which accepts an array as a parameter. The function should return the average of all of the numbers in the array (you can assume that the array passed to the function will contain only numbers)
average([1,2,3,4,5]); // 3 average([1,2,3,4,5,6]); // 3.5 average([10,20]); // 15
Write a function called createStudent
, which accepts two parameters both of which are strings. The function should return an object with the keys firstName
and lastName
and the values should be each of the
createStudent("Elie", "Schoppik"); /* { firstName: "Elie", lastName: "Schoppik" } */ createStudent("Tim", "Garcia"); /* { firstName: "Tim", lastName: "Garcia" } */
Using your createStudent
function, create three students and save them each in a variable. Then create a variable called students
, which is an array that will store your three students
let tim = createStudent("Tim", "Garcia"); let matt = createStudent("Matt", "Lane"); let elie = createStudent("Elie", "Schoppik"); let students = [tim, matt, elie]; // your students array should contain three objects each with the keys of firstName and lastName. If they do not - make sure you correctly implement the createStudent function from above!
Write a function called findStudentByFirstName
, which accepts one parameter, a string. This function should iterate through the students
array you just made and if the parameter passed to the function is the same as one of the first name's of the students, the function should return true
. Otherwise it should return false
. This function should be case insensitive so that you can search successfully regardless of capitalization
findStudentByFirstName('elie') // true findStudentByFirstName('Elie') // true findStudentByFirstName('Janey') // false findStudentByFirstName('Janey') // false findStudentByFirstName('TIM') // true findStudentByFirstName('MMMaaaTTTtttTTT') // false
Write a function called extractEveryThird
which accepts an array as a parameter. The function should iterate over the array and return a new array with every 3rd element in the array passed to the function.
extractEveryThird([1,2,3]); // [3] extractEveryThird([1,2,3,4,5,6]); // [3,6] extractEveryThird(["a","b","c","d"]); // ["c"] extractEveryThird(["first value", "second value", "third value"]); // ["third value"]
Write a function called countEvensAndOdds
which accepts an array as a parameter. This function should return an object with the count of even numbers and the count of odd numbers. The object returned should have the keys oddCount
and evenCount
.
countEvensAndOdds([1,2,3,4]); /* { oddCount:2, evenCount:2 } */ countEvensAndOdds([1,2,3,4,5,6,7]); /* { oddCount:4, evenCount:3 } */
In the following example, what will be printed in the console? Make sure you first try read this code before pasting it into the console :)
let mylet = "Hello from global"; function scopePractice() { let mylet = "Hello from function scope"; } scopePractice(); console.log(mylet); let tricky = "Hello from global"; function trickyScopePractice() { tricky = "Hello from function scope"; } console.log(tricky);
Optional Bonus
Write a function called onlyCapitalLetters
which accepts a string and returns a new string with only the capital letters passed to the string.
onlyCapitalLetters("Amazing") // "A" onlyCapitalLetters("nothing") // "" onlyCapitalLetters("EVERYTHING") // "EVERYTHING"
add
function add(num1,num2){ return num1 + num2; }
subtract
function subtract(num1,num2){ return num1 - num2; }
multiply
function multiply(num1,num2){ return num1 * num2; }
divide
function divide(num1,num2){ return num1 / num2; }
sayHello
let myFirstName = "Elie"; function sayHello(str){ if(str === myFirstName){ return "Hello Boss"; } return "Hello " + str; }
average
function average(arr){ let total = 0; for(let i = 0; i < arr.length; i++){ total += arr[i]; } return total / arr.length; }
createStudent
function createStudent(firstName, lastName){ return { firstName: firstName, lastName: lastName } }
findStudentByFirstName
// let's first create some students let tim = createStudent("Tim", "Garcia"); let matt = createStudent("Matt", "Lane"); let elie = createStudent("Elie", "Schoppik"); let students = [tim, matt, elie]; function findStudentByFirstName(name){ let lowerCasedName = name.toLowerCase(); for(let i = 0; i < students.length; i++){ if(students[i].firstName.toLowerCase() === lowerCasedName){ return true; } } return false; }
extractEveryThird
function extractEveryThird(arr){ let newArr = []; for(let i = 2; i < arr.length; i += 3){ newArr.push(arr[i]); } return newArr; }
countEvensAndOdds
function countEvensAndOdds(arr){ let countObj = { oddCount: 0, evenCount: 0 } for(let i = 0; i < arr.length; i++){ if(arr[i] % 2 === 0){ countObj.evenCount++; } else { countObj.oddCount++; } } return countObj; }
scope practice
// the first console.log(mylet) outputs "Hello from global" // the second console.log(mylet) also outputs "Hello from global" (the trickyScopePractice function was not called!)
onlyCapitalLetters
function onlyCapitalLetters(str){ let newStr = ''; for(let i = 0; i < str.length; i++){ if(str[i].charCodeAt(0) < 91 && str[i].charCodeAt(0) > 64 ){ newStr += str[i]; } } return newStr; }
When you're ready, move on to Anonymous Functions and IIFEs