Suggest an edit

Improve this article

Refine the answer for “Increment and decrement”. Your changes go to moderation before they’re published.

Approval required

Content

What you’re changing

Title (EN)

Short answer (EN)

Shown above the full answer for quick recall.

Answer (EN)

The `++` and `--` operators are **increment** and **decrement**,
and they are often confused precisely because of the difference between the **prefix** and **postfix** forms.
Let's go through everything in detail.

---

## What the `++` and `--` operators do

| Operator | Action | Example |
|---|---|---|
| `++` | increases the variable's value by **1** | `i++` → `i = i + 1` |
| `--` | decreases the variable's value by **1** | `i--` → `i = i - 1` |

> But it's important: **depending on the placement (**`++i` **or** `i++`**)**,
> the result of the expression can be **different**.

---

## Prefix form (`++i`, `--i`)

First it **changes** the variable's value,
then it **returns** the new value.

```javascript
let i = 5;
let result = ++i; // first i = 6, then 6 is returned

console.log(i);      // 6
console.log(result); // 6
```

> In other words:
>
> - It increases/decreases the variable.
> - It returns the **new** value.

---

## Postfix form (`i++`, `i--`)

First it **returns the old value**,
then it **changes the variable**.

```javascript
let i = 5;
let result = i++; // first returns 5, then increases i to 6

console.log(i);      // 6
console.log(result); // 5
```

> In other words:
>
> - It returns the **old** value.
> - Only then does it change the variable.

---

## Difference in an example

```javascript
let a = 1;
console.log(a++); // 1  → first logs, then increases
console.log(a);   // 2

let b = 1;
console.log(++b); // 2  → first increases, then logs
console.log(b);   // 2
```

---

## In expressions

This difference is especially noticeable
when the operator is used **inside an expression**:

```javascript
let x = 10;
let y = x++ + 5; // y = 10 + 5 = 15, then x = 11

let a = 10;
let b = ++a + 5; // a = 11, b = 16
```

| Form | Variable change | Returned value |
|---|---|---|
| `++i` | Increases **immediately** | New value |
| `i++` | Increases **afterward** | Old value |
| `--i` | Decreases **immediately** | New value |
| `i--` | Decreases **afterward** | Old value |

---

## In loops

Usually the `i++` form is used in loops,
because the expression's result is not used explicitly:

```javascript
for (let i = 0; i < 5; i++) {
  console.log(i);
}
```

> There is no difference here between `i++` and `++i`,
> because the returned value is not used.

---

## Summary

| Form | When it changes | What it returns | Example result |
|---|---|---|---|
| `++i` | Immediately | New value | `let i=1; console.log(++i)` → `2` |
| `i++` | After | Old value | `let i=1; console.log(i++)` → `1` |
| `--i` | Immediately | New value | `let i=3; console.log(--i)` → `2` |
| `i--` | After | Old value | `let i=3; console.log(i--)` → `3` |

Markdown · drag & drop images · ⌘B / ⌘I shortcuts496 words

For the reviewer

Note to the moderator (optional)

Visible only to the moderator. Helps review go faster.