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**In short:**
**Partial application** is a technique in which a function is called **not with all its arguments at once**; instead, **part of the arguments is "remembered"**, and a **new function** is returned that waits for the remaining arguments.

---

### Detailed explanation

> The idea: "fix" part of a function's parameters
> to get a new, more specific function.

Unlike **currying**, where a function takes **one argument at a time**,
partial application lets you pass **several at once**.

---

### Example 1 - a simple implementation

```javascript
function multiply(a, b, c) {
  return a * b * c;
}

// Partially apply: fix a = 2
function partialMultiply(a) {
  return function(b, c) {
    return multiply(a, b, c);
  };
}

const double = partialMultiply(2);
console.log(double(3, 4)); // 24
```

We "fixed" the first argument (`a = 2`)
and got a new function, `double`.

---

### Example 2 - with `bind()`

JavaScript already lets you do partial application using `Function.prototype.bind`:

```javascript
function add(a, b, c) {
  return a + b + c;
}

const add5 = add.bind(null, 5); // fix a = 5
console.log(add5(10, 20)); // 35
```

`bind` creates a new function with the arguments already substituted in.

---

### Example 3 - a universal partial function

```javascript
function partial(fn, ...fixedArgs) {
  return (...remainingArgs) => fn(...fixedArgs, ...remainingArgs);
}

function greet(greeting, name) {
  return `${greeting}, ${name}!`;
}

const sayHi = partial(greet, 'Hello');
console.log(sayHi('Alice')); // "Hello, Alice!"
```

---

### Difference from currying

| Technique | What it does |
|---|---|
| **Currying** | Transforms a function with several arguments into a chain of one-argument functions (`f(a)(b)(c)`) |
| **Partial application** | Substitutes part of the arguments and returns a new function (`f(a, b, c)` -> `g(b, c)`) |

---

### Why partial application is needed

Reusing functions with different contexts
Simplifying calls and configurations
Improving readability
Often used in **functional programming**, **React** and **middleware**

---

### Summary

| Property | Partial application |
|---|---|
| Essence | "Remembers" part of the arguments |
| Returns | A new function |
| Example | `add.bind(null, 5)` or `partial(add, 5)` |
| Difference from currying | Accepts several arguments at once |
| Benefit | Code reuse and simplification |

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