2. JavaScript Fundamentals
Sometimes, we need to perform different actions based on different conditions.
To do that, we can use the if statement and the conditional operator ?, that's also called a "question mark" operator.
The if(...) statement evaluates a condition in parentheses and, if the result is true, executes a block of code.
For example:
let year = prompt('In which year was ECMAScript-2015 specification published?', '');
*!*
if (year == 2015) alert( 'You are right!' );
*/!*
In the example above, the condition is a simple equality check (year == 2015), but it can be much more complex.
If we want to execute more than one statement, we have to wrap our code block inside curly braces:
if (year == 2015) {
alert( "That's correct!" );
alert( "You're so smart!" );
}
We recommend wrapping your code block with curly braces {} every time you use an if statement, even if there is only one statement to execute. Doing so improves readability.
The if (…) statement evaluates the expression in its parentheses and converts the result to a boolean.
Let's recall the conversion rules from the chapter info:type-conversions:
0, an empty string "", null, undefined, and NaN all become false. Because of that they are called "falsy" values.true, so they are called "truthy".So, the code under this condition would never execute:
if (0) { // 0 is falsy
...
}
...and inside this condition -- it always will:
if (1) { // 1 is truthy
...
}
We can also pass a pre-evaluated boolean value to if, like this:
let cond = (year == 2015); // equality evaluates to true or false
if (cond) {
...
}
The if statement may contain an optional "else" block. It executes when the condition is falsy.
For example:
let year = prompt('In which year was the ECMAScript-2015 specification published?', '');
if (year == 2015) {
alert( 'You guessed it right!' );
} else {
alert( 'How can you be so wrong?' ); // any value except 2015
}
Sometimes, we'd like to test several variants of a condition. The else if clause lets us do that.
For example:
let year = prompt('In which year was the ECMAScript-2015 specification published?', '');
if (year < 2015) {
alert( 'Too early...' );
} else if (year > 2015) {
alert( 'Too late' );
} else {
alert( 'Exactly!' );
}
In the code above, JavaScript first checks year < 2015. If that is falsy, it goes to the next condition year > 2015. If that is also falsy, it shows the last alert.
There can be more else if blocks. The final else is optional.
Sometimes, we need to assign a variable depending on a condition.
For instance:
let accessAllowed;
let age = prompt('How old are you?', '');
*!*
if (age > 18) {
accessAllowed = true;
} else {
accessAllowed = false;
}
*/!*
alert(accessAllowed);
The so-called "conditional" or "question mark" operator lets us do that in a shorter and simpler way.
The operator is represented by a question mark ?. Sometimes it's called "ternary", because the operator has three operands. It is actually the one and only operator in JavaScript which has that many.
The syntax is:
let result = condition ? value1 : value2;
The condition is evaluated: if it's truthy then value1 is returned, otherwise -- value2.
For example:
let accessAllowed = (age > 18) ? true : false;
Technically, we can omit the parentheses around age > 18. The question mark operator has a low precedence, so it executes after the comparison >.
This example will do the same thing as the previous one:
// the comparison operator "age > 18" executes first anyway
// (no need to wrap it into parentheses)
let accessAllowed = age > 18 ? true : false;
But parentheses make the code more readable, so we recommend using them.
In the example above, you can avoid using the question mark operator because the comparison itself returns `true/false`:
```js
// the same
let accessAllowed = age > 18;
```
A sequence of question mark operators ? can return a value that depends on more than one condition.
For instance:
let age = prompt('age?', 18);
let message = (age < 3) ? 'Hi, baby!' :
(age < 18) ? 'Hello!' :
(age < 100) ? 'Greetings!' :
'What an unusual age!';
alert( message );
It may be difficult at first to grasp what's going on. But after a closer look, we can see that it's just an ordinary sequence of tests:
age < 3.'Hi, baby!'. Otherwise, it continues to the expression after the colon '":"', checking age < 18.'Hello!'. Otherwise, it continues to the expression after the next colon '":"', checking age < 100.'Greetings!'. Otherwise, it continues to the expression after the last colon '":"', returning 'What an unusual age!'.Here's how this looks using if..else:
if (age < 3) {
message = 'Hi, baby!';
} else if (age < 18) {
message = 'Hello!';
} else if (age < 100) {
message = 'Greetings!';
} else {
message = 'What an unusual age!';
}
Sometimes the question mark ? is used as a replacement for if:
let company = prompt('Which company created JavaScript?', '');
*!*
(company == 'Netscape') ?
alert('Right!') : alert('Wrong.');
*/!*
Depending on the condition company == 'Netscape', either the first or the second expression after the ? gets executed and shows an alert.
We don't assign a result to a variable here. Instead, we execute different code depending on the condition.
It's not recommended to use the question mark operator in this way.
The notation is shorter than the equivalent if statement, which appeals to some programmers. But it is less readable.
Here is the same code using if for comparison:
let company = prompt('Which company created JavaScript?', '');
*!*
if (company == 'Netscape') {
alert('Right!');
} else {
alert('Wrong.');
}
*/!*
Our eyes scan the code vertically. Code blocks which span several lines are easier to understand than a long, horizontal instruction set.
The purpose of the question mark operator ? is to return one value or another depending on its condition. Please use it for exactly that. Use if when you need to execute different branches of code.