How to solve _.some using _.every?

Question

Working on a programming challenge to re-implement the functionality of underscore.js in standard javascript. Specifically I am working on implementing the `_.some` function. (http://underscorejs.org/#some) The part I'm struggling with is it's asking me to find a way to solve it using `_.every` internally. (http://underscorejs.org/#every) I have already finished the `_.every` function earlier and it works as it should. Here is logically what I am wanting to do in sketched code: ``` _.some = function(collection, truthStatementFunction) { return !(_every(collection, !truthStatementFunction)) } ``` Or in english, flip the truth statement to test where the condition is false... and if the `_.every` test then returns true... then we know that _.some of the original truth statement is false (so flip the return of `_.every` to get the correct return for `_some`). Likewise if `_.every` returns false then flip that to get the correct return of true for `_.some`. Obviously the problem with this…

Accepted Answer

Pass `_every` a function that returns an inversion of the result from `truthStatementFunction`:

```
_.some = function(collection, truthStatementFunction) { 
    return !(_every(collection, function(v) {
        return !truthStatementFunction(v);
    }));
}
```

To answer the second part of your question:

and is there a better way to solve this using _.every?

There is definitely a better way to solve this than using `_.every`. Just iterate over the collection, and return true as soon as you find any element that matches. Much more efficient in cases where `_.some` is going to return true, since you don't really care how many elements in the collection satisfy the predicate, as long as there is at least one.

```
_.some = function(c, pred) { 
    for(var i = 0; i < c.length; i++) {
        if (pred(c[i])) return true;
    }
    return false;
}
```

How to solve _.some using _.every?

Solution

Problem