- Concatenate strings with the
+operator - Interpolate variables and other JavaScript expressions inside template literals
- Read the MDN documentation on string methods and practice using a few
For this lab, you've just been onboarded to the dev team working on Flatbook, the world's premier Flatiron School-based social network. At the moment, the view that our users see upon logging in is pretty generic. We'd like to improve the user experience by adding some custom greeting capabilities.
To start off, let's run the test suite with the learn command. Our code is
currently failing all of the tests, but we expected that because we haven't done
anything yet. Let's get to work!
The first test is telling us that currentUser is not defined. Let's go to
index.js and write the following code:
const currentUser = 'Grace Hopper';Note: Generally, when the tests ask you to define something, you want to define it exactly as indicated in the test. But in this case, you don't have to write
'Grace Hopper', because the important part is the variable name:currentUser. You can use your own name, your pet's name, your favorite programmer's name — whatever you'd like.
Rerun the tests and you should see that the first one is passing.
The next failing test is similarly helpful, telling us exactly what we have to
fix: welcomeMessage contains "Welcome to Flatbook, ".
Let's return to index.js and define our second variable below where we
declared currentUser:
const currentUser = 'Grace Hopper';
const welcomeMessage = 'Welcome to Flatbook, ';Rerun the tests; you should see a second passing test.
The third test tells us that welcomeMessage should contain the value stored in
currentUser. This seems like it might contradict the second test a bit, but
let's try it out. Let's erase 'Welcome to Flatbook, ' and set welcomeMessage
equal to currentUser instead:
const currentUser = 'Grace Hopper';
const welcomeMessage = currentUser;When we rerun the tests, we still have two passing. But now the first and third tests are passing instead of the first and second! That doesn't seem quite right.
It turns out that the tests want welcomeMessage to include both 'Welcome to Flatbook, ' and the value stored in currentUser. Maybe we can include both of
them in a single string?
const currentUser = 'Grace Hopper';
const welcomeMessage = 'Welcome to Flatbook, currentUser';If we rerun the tests, we're once again passing the second test, but we're back to failing the third test. The new error message for the third test gives us a hint about what's happening:
AssertionError: expected 'Welcome to Flatbook, currentUser' to contain 'Grace Hopper'When JavaScript is expecting a variable to contain one thing, and it does not,
that is known as an AssertionError. The test suite looked at the value stored
in welcomeMessage and expected to find the string 'Grace Hopper', which is
the value stored in currentUser. Instead, welcomeMessage contains the
literal string "currentUser". It's important to understand the distinction:
currentUseris a variable that contains a string ('Grace Hopper'in our examples).'currentUser'is a string, not a variable.
The JavaScript engine sees a matching pair of single quotes (' '), creates a
new string, and assumes that everything in between the matching punctuation
marks is part of that string. For example, if we add quotation marks around the
first line of code that we wrote, it becomes a simple string consisting of 35
characters:
typeof "const currentUser = 'Grace Hopper';";
//=> "string"
"const currentUser = 'Grace Hopper';".length;
//=> 35
currentUser;
//=> Uncaught ReferenceError: currentUser is not definedAs demonstrated by the last line in that snippet, because we turned our code
into a string it no longer functions as JavaScript code for declaring and
assigning a currentUser variable.
Since we want welcomeMessage to contain both 'Welcome to Flatbook, ' and the
value stored in currentUser, we have two options: concatenation and
interpolation.
String concatenation is a way to take two strings and add one to the other,
creating a single, longer string. The easiest way to concatenate strings in
JavaScript is with the + operator, like so:
"High " + "five!";
//=> "High five!"
"We" + ' ' + `can` + " " + 'concat' + `enate` + " as many strings " + 'as our heart ' + `desires.`;
//=> "We can concatenate as many strings as our heart desires."Since our currentUser variable contains a string, we can concatenate it to the
end of 'Welcome to Flatbook, ' to dynamically create a new string based on
whatever value currentUser contains at a given moment:
const currentUser = 'Grace Hopper';
const welcomeMessage = 'Welcome to Flatbook, ' + currentUser;If we run the test suite with our updated code, we'll see both the second and third tests passing! However, before we move on, let's talk about interpolation.
String interpolation lets us dynamically insert values in the middle of a
string. To do this, we need to use template literals, which
are simply strings wrapped in backticks rather than single or double quotes.
Template literals enable us to interpolate the value of a variable into a string
by wrapping the variable in curly braces preceded by a dollar sign:
${yourVariable}. The ${}, when inside backticks, tells the JavaScript engine
that it needs to interpret the value of yourVariable and insert that value
into the string. If you forget to use the backticks and use single or double
quotes instead, the dollar sign, curly braces and variable name will all be
inserted into the string instead of the variable's value.
Unlike string concatenation, template literals will also allow you to use multi-line strings. Wrapping the string in backticks preserves any new lines when the string is returned or output.
const myString = 'template literal';
const myNumber = 10;
const myBoolean = false;
`Saying that interpolation with ${myString}s is better than concatenation ${90 + myNumber}% of the time is simply ${myBoolean}. But it is pretty cool!
Beware that new lines inside of a ${myString} will be preserved as new lines in the resulting ${typeof myString}!`;
//=> "Saying that interpolation with template literals is better than concatenation 100% of the time is simply false. But it is pretty cool!
// Beware that new lines inside of a template literal will be preserved as new lines in the resulting string!"Note that, in the example above, one of the things we interpolated into our
string is an arithmetic expression: ${90 + myNumber}. We aren't limited to
interpolating just variables — we can use any expression inside the
curly braces.
While, for most purposes, the choice of whether to use concatenation or string interpolation is primarily a matter of personal preference, JavaScript programmers tend to use string interpolation for all but the simplest of cases.
Let's rewrite our welcomeMessage to use a template literal:
const currentUser = 'Grace Hopper';
const welcomeMessage = `Welcome to Flatbook, ${currentUser}`;The first three tests are still passing, but the fourth wants our
welcomeMessage to end with an exclamation point. The fix is as simple as
adding a ! as the last character in the template literal:
const currentUser = 'Grace Hopper';
const welcomeMessage = `Welcome to Flatbook, ${currentUser}!`;Four tests down, six to go!
Sometimes we get so excited when someone logs into their Flatbook account that
we just want to shout out loud. We could copy over most of the code from
welcomeMessage and then change every character to its uppercase equivalent,
but as developers we try not to repeat ourselves. Instead, let's use the
.toUpperCase() string method:
const currentUser = 'Grace Hopper';
const welcomeMessage = `Welcome to Flatbook, ${currentUser}!`;
const excitedWelcomeMessage = welcomeMessage.toUpperCase();All strings in JavaScript have access to the same set of default methods, which
are common operations like changing a string and returning the new version,
searching through a string for specific character(s) and returning the match,
and so on. For example, we can use .toUpperCase() and
.toLowerCase() on a string to make the entire string uppercase
or lowercase. There are lots of other string methods that
you'll find useful at various points throughout your JavaScript programming
career.
Rerun the tests; you should see the first seven tests passing. Woohoo!
The mobile team at Flatbook is busy redesigning the site for smaller devices,
and they're a bit concerned about how much real estate the welcomeMessage
takes up on the screen. They want us to create a shorter version that truncates
the currentUser's name into just their first initial.
If you take a look at the first error, you'll see that the JavaScript engine is
telling us that it can't find shortGreeting:
shortGreeting
contains "Welcome, "
ReferenceError: shortGreeting is not definedOnce we define it in index.js:
...
const shortGreeting = '';we see a new error from the test suite:
shortGreeting
contains "Welcome, "
AssertionError: expected '' to contain 'Welcome, 'It expected shortGreeting to contain the string "Welcome, ", but
shortGreeting is currently an empty string, ''. We can fix that now:
...
const shortGreeting = 'Welcome, ';Next up is another AssertionError, this one checking that shortGreeting
contains the first letter from currentUser:
shortGreeting
contains the first initial of the name stored in the 'currentUser' variable
AssertionError: expected 'Welcome, ' to contain 'G'To get a sense of how specific the tests are, let's start by adding the entirety
of currentUser to shortGreeting:
const currentUser = 'Grace Hopper';
...
const shortGreeting = `Welcome, ${currentUser}`;Notice that we changed the single quotes to backticks, which allows us to
interpolate with ${ }.
The new error reads as follows:
shortGreeting
contains the first initial of the name stored in the 'currentUser' variable
AssertionError: expected 'Welcome, Grace Hopper' to not contain 'race Hopper'The test suite checks that shortGreeting contains the first character in
currentUser (G in our example) and that it doesn't contain the rest of the
string (race Hopper).
There are a few different ways we could get just the first character of
currentUser. The easiest would be to use bracket notation or the .charAt()
method to grab the character at index 0:
'Edsger Dijkstra'[0];
//=> "E"
'Edsger Dijkstra'.charAt(0);
//=> "E"However, it's a good practice to make our code flexible and future-proof it a
bit. What if our product team decides it would be better to shorten
currentName to two characters instead of one? Or three characters?
For the added flexibility, we're going to use .slice(), but you can always
explore the MDN documentation on string methods to pick out
your own strategy.
If you take a look at the documentation for .slice(), you will see
the following description:
The slice() method extracts a section of a string and returns it as a new string, without modifying the original string.
The method takes two arguments: the index at which the extraction should begin
and the index before which it should end. When we talk about indexes of a
string, we're talking about how to access specific characters at various points
within the string. Recall that computers start counting with 0. Because we start
at index 0 instead of 1, the index of each character in a string is always
one less than the character's place in the string. The second character is at
index 1, the fifth at index 4, the twelfth at index 11, and so on. The
index of the last character is always one less than the length of the
string:
'Edsger Dijkstra'.length;
//=> 15
'Edsger Dijkstra'[15];
//=> undefined
'Edsger Dijkstra'[14];
//=> "a"If we omit both arguments, .slice() will return a full copy of the original
string:
'Edsger Dijkstra'.slice();
//=> "Edsger Dijkstra"If we provide a single argument, .slice() will return a copy from that index
to the end of the string. For example, to grab Dijkstra's last name, we could
start the slice on index 7:
'Edsger Dijkstra'.slice(7);
//=> "Dijkstra"If we wanted the first three characters of Dijkstra's name, we would specify 0
as the first argument, the index at which to start, and 3 as the second
argument, the index before which to end:
'Edsger Dijkstra'.slice(0, 3);
//=> "Eds"To satisfy our team's current specifications for shortGreeting, we need to
start our slice at index 0 and end it before index 1:
currentUser.slice(0, 1);Now, when our product team asks us to use the first two characters of
currentUser, the change is as simple as currentUser.slice(0, 1) →
currentUser.slice(0, 2).
Add an exclamation point to the end, and the entire test suite should be passing:
const currentUser = 'Grace Hopper';
...
const shortGreeting = `Welcome, ${currentUser.slice(0, 1)}!`;Great work!