303 lines
8.1 KiB
Markdown
303 lines
8.1 KiB
Markdown
# Logical operators
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There are three logical operators in JavaScript: `||` (OR), `&&` (AND), `!` (NOT).
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Although they are called "logical", they can be applied to values of any type, not only boolean. Their result can also be of any type.
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Let's see the details.
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## || (OR)
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The "OR" operator is represented with two vertical line symbols:
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```js
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result = a || b;
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```
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In classical programming, the logical OR is meant to manipulate boolean values only. If any of its arguments are `true`, it returns `true`, otherwise it returns `false`.
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In JavaScript, the operator is a little bit trickier and more powerful. But first, let's see what happens with boolean values.
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There are four possible logical combinations:
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```js run
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alert( true || true ); // true
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alert( false || true ); // true
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alert( true || false ); // true
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alert( false || false ); // false
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```
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As we can see, the result is always `true` except for the case when both operands are `false`.
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If an operand is not a boolean, it's converted to a boolean for the evaluation.
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For instance, the number `1` is treated as `true`, the number `0` as `false`:
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```js run
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if (1 || 0) { // works just like if( true || false )
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alert( 'truthy!' );
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}
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```
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Most of the time, OR `||` is used in an `if` statement to test if *any* of the given conditions is `true`.
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For example:
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```js run
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let hour = 9;
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*!*
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if (hour < 10 || hour > 18) {
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*/!*
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alert( 'The office is closed.' );
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}
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```
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We can pass more conditions:
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```js run
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let hour = 12;
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let isWeekend = true;
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if (hour < 10 || hour > 18 || isWeekend) {
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alert( 'The office is closed.' ); // it is the weekend
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}
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```
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## OR "||" finds the first truthy value
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The logic described above is somewhat classical. Now, let's bring in the "extra" features of JavaScript.
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The extended algorithm works as follows.
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Given multiple OR'ed values:
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```js
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result = value1 || value2 || value3;
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```
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The OR `||` operator does the following:
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- Evaluates operands from left to right.
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- For each operand, converts it to boolean. If the result is `true`, stops and returns the original value of that operand.
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- If all operands have been evaluated (i.e. all were `false`), returns the last operand.
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A value is returned in its original form, without the conversion.
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In other words, a chain of OR `"||"` returns the first truthy value or the last one if no truthy value is found.
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For instance:
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```js run
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alert( 1 || 0 ); // 1 (1 is truthy)
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alert( true || 'no matter what' ); // (true is truthy)
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alert( null || 1 ); // 1 (1 is the first truthy value)
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alert( null || 0 || 1 ); // 1 (the first truthy value)
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alert( undefined || null || 0 ); // 0 (all falsy, returns the last value)
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```
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This leads to some interesting usage compared to a "pure, classical, boolean-only OR".
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1. **Getting the first truthy value from a list of variables or expressions.**
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Imagine we have a list of variables which can either contain data or be `null/undefined`. How can we find the first one with data?
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We can use OR `||`:
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```js run
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let currentUser = null;
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let defaultUser = "John";
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*!*
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let name = currentUser || defaultUser || "unnamed";
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*/!*
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alert( name ); // selects "John" – the first truthy value
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```
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If both `currentUser` and `defaultUser` were falsy, `"unnamed"` would be the result.
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2. **Short-circuit evaluation.**
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Operands can be not only values, but arbitrary expressions. OR evaluates and tests them from left to right. The evaluation stops when a truthy value is reached, and the value is returned. This process is called "a short-circuit evaluation" because it goes as short as possible from left to right.
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This is clearly seen when the expression given as the second argument has a side effect like a variable assignment.
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In the example below, `x` does not get assigned:
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```js run no-beautify
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let x;
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*!*true*/!* || (x = 1);
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alert(x); // undefined, because (x = 1) not evaluated
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```
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If, instead, the first argument is `false`, `||` evaluates the second one, thus running the assignment:
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```js run no-beautify
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let x;
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*!*false*/!* || (x = 1);
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alert(x); // 1
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```
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An assignment is a simple case. There may be side effects, that won't show up if the evaluation doesn't reach them.
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As we can see, such a use case is a "shorter way of doing `if`". The first operand is converted to boolean. If it's false, the second one is evaluated.
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Most of time, it's better to use a "regular" `if` to keep the code easy to understand, but sometimes this can be handy.
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## && (AND)
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The AND operator is represented with two ampersands `&&`:
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```js
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result = a && b;
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```
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In classical programming, AND returns `true` if both operands are truthy and `false` otherwise:
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```js run
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alert( true && true ); // true
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alert( false && true ); // false
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alert( true && false ); // false
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alert( false && false ); // false
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```
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An example with `if`:
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```js run
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let hour = 12;
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let minute = 30;
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if (hour == 12 && minute == 30) {
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alert( 'The time is 12:30' );
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}
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```
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Just as with OR, any value is allowed as an operand of AND:
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```js run
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if (1 && 0) { // evaluated as true && false
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alert( "won't work, because the result is falsy" );
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}
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```
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## AND "&&" finds the first falsy value
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Given multiple AND'ed values:
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```js
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result = value1 && value2 && value3;
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```
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The AND `&&` operator does the following:
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- Evaluates operands from left to right.
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- For each operand, converts it to a boolean. If the result is `false`, stops and returns the original value of that operand.
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- If all operands have been evaluated (i.e. all were truthy), returns the last operand.
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In other words, AND returns the first falsy value or the last value if none were found.
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The rules above are similar to OR. The difference is that AND returns the first *falsy* value while OR returns the first *truthy* one.
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Examples:
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```js run
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// if the first operand is truthy,
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// AND returns the second operand:
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alert( 1 && 0 ); // 0
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alert( 1 && 5 ); // 5
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// if the first operand is falsy,
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// AND returns it. The second operand is ignored
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alert( null && 5 ); // null
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alert( 0 && "no matter what" ); // 0
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```
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We can also pass several values in a row. See how the first falsy one is returned:
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```js run
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alert( 1 && 2 && null && 3 ); // null
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```
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When all values are truthy, the last value is returned:
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```js run
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alert( 1 && 2 && 3 ); // 3, the last one
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```
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````smart header="Precedence of AND `&&` is higher than OR `||`"
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The precedence of AND `&&` operator is higher than OR `||`.
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So the code `a && b || c && d` is essentially the same as if the `&&` expressions were in parentheses: `(a && b) || (c && d)`.
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````
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Just like OR, the AND `&&` operator can sometimes replace `if`.
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For instance:
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```js run
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let x = 1;
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(x > 0) && alert( 'Greater than zero!' );
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```
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The action in the right part of `&&` would execute only if the evaluation reaches it. That is, only if `(x > 0)` is true.
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So we basically have an analogue for:
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```js run
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let x = 1;
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if (x > 0) {
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alert( 'Greater than zero!' );
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}
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```
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The variant with `&&` appears shorter. But `if` is more obvious and tends to be a little bit more readable.
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So we recommend using every construct for its purpose: use `if` if we want if and use `&&` if we want AND.
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## ! (NOT)
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The boolean NOT operator is represented with an exclamation sign `!`.
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The syntax is pretty simple:
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```js
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result = !value;
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```
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The operator accepts a single argument and does the following:
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1. Converts the operand to boolean type: `true/false`.
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2. Returns the inverse value.
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For instance:
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```js run
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alert( !true ); // false
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alert( !0 ); // true
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```
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A double NOT `!!` is sometimes used for converting a value to boolean type:
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```js run
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alert( !!"non-empty string" ); // true
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alert( !!null ); // false
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```
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That is, the first NOT converts the value to boolean and returns the inverse, and the second NOT inverses it again. In the end, we have a plain value-to-boolean conversion.
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There's a little more verbose way to do the same thing -- a built-in `Boolean` function:
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```js run
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alert( Boolean("non-empty string") ); // true
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alert( Boolean(null) ); // false
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```
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The precedence of NOT `!` is the highest of all logical operators, so it always executes first, before `&&` or `||`.
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