Short-circuit evaluation is a technique in JavaScript that allows you to evaluate expressions in a way that minimizes the number of evaluations that take place. The idea behind short-circuit evaluation is that you can stop evaluating an expression as soon as you know the final value of the expression, without having to evaluate all of the terms in the expression.

There are two logical operators in JavaScript that support short-circuit evaluation: the `&&`

operator (and) and the `||`

operator (or).

When using the `&&`

operator, the expression on the left is evaluated first. If the left-hand expression is falsy, the expression on the right is not evaluated, and the result is the left-hand expression. If the left-hand expression is truthy, the expression on the right is evaluated, and the result is the expression on the right.

Here’s an example of how you can use short-circuit evaluation with the `&&`

operator in JavaScript:

```
let x = 10;
let y = 20;
let z = (x > 5) && (y > 15);
console.log(z);
```

In this example, the expression `(x > 5)`

is evaluated first. Since `x`

is `10`

, which is greater than `5`

, this expression is truthy, so the expression `(y > 15)`

is evaluated next. Since `y`

is `20`

, which is greater than `15`

, this expression is also truthy, so the result of the expression `z`

is `true`

.

When using the `||`

operator, the expression on the left is evaluated first. If the left-hand expression is truthy, the expression on the right is not evaluated, and the result is the left-hand expression. If the left-hand expression is falsy, the expression on the right is evaluated, and the result is the expression on the right.

Here’s an example of how you can use short-circuit evaluation with the `||`

operator in JavaScript:

```
let x = 10;
let y = 20;
let z = (x < 5) || (y > 15);
console.log(z);
```

In this example, the expression `(x < 5)`

is evaluated first. Since `x`

is `10`

, which is not less than `5`

, this expression is falsy, so the expression `(y > 15)`

is evaluated next. Since `y`

is `20`

, which is greater than `15`

, this expression is truthy, so the result of the expression `z`

is `true`

.

Short-circuit evaluation can be a useful technique in JavaScript for optimizing your code and avoiding unnecessary evaluations. However, it’s important to use it appropriately and not overuse it, as it can make your code more difficult to understand for other developers who are not familiar with this technique.

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