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