John Anisere - 5 months ago 31

Javascript Question

suppose we have this code

`function largestOfFour(arr) {`

return arr.map(Function.apply.bind(Math.max, null));

}

where arr is an array of arrays.

- first,why must i use apply()?

I understand that when using the method Math.max() to operate on an array i must add the apply() method also. So i'll have something like this

`Math.max.apply(null, arr)`

- In this code what does bind() really do?
`arr.map(Function.apply.bind(Math.max, null))`

Please give an explanation i can understand,i really appreciate this.

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Answer Source

Looking at the entire expression:

```
arr.map(Function.apply.bind(Math.max, null));
```

*map* expects its first argument to be a function, which is returned by:

```
Function.apply.bind(Math.max, null);
```

*Function.apply* is a shorter version of *Function.prototype.apply*.

Calling *bind* on it returns a special version of *apply* whose *this* is set to *Math.max* and when called, will have as it's first parameter (i.e. the value that would normally be used as *this*) set to *null*, since it's not going to be used.

So each element in *arr* will effectively be called using:

```
Math.max.apply(null, member);
```

The use of *apply* means the values in member are passed as parameters, as if:

```
Math.max(member[0],member[1] ... member[n]);
```

So the expression returns the maximum value in each array. Those values are returned to *map*, which puts them into a new array.

```
var arr = [[1,2,3],[4,5,6]];
console.log(
arr.map(Function.apply.bind(Math.max, null)) //[3, 6]
);
```

and is effectively the same as:

```
var arr = [[1, 2, 3],[4, 5, 6]];
console.log(
arr.map(function(a) {return Math.max.apply(null, a)}) //[3, 6]
);
```

Though using recent features you might use destructing with rest parameter syntax:

```
var arr = [[1, 2, 3],[4, 5, 6]];
console.log(
arr.map(a => Math.max(...a)) // [3, 6]
);
```

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