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I am reading a book which contains the following example:

var position1 = function(f, g) {
  return function(x) {
    return f(g(x));
  }
};

Then the author writes: "...naive implementation of position, because it does not take the execution context into account..."

So the preferred function is that one:

var position2 = function(f, g) {
  return function() {
    return f.call(this, g.apply(this, arguments));
  }
}; 

Followed by an entire example:

var position2 = function position2(f, g) {
    return function() {
        return f.call(this, g.apply(this, arguments));
    }
};

var addFour = function addFour(x) {
    return x + 4;
};

var timesSeven = function timesSeven(x) {
    return x * 7;
};

var addFourtimesSeven2 = position2(timesSeven, addFour);
var result2 = addFourtimesSeven2(2);
console.log(result2);

Could someone please explain to me why the position2 function is the preferred one (maybe with an example)?

EDIT:

In the meantime i have tried to use methods as arguments as suggested, but it did not work. The result was NaN:

var position1 = function position1(f, g) {
    return function(x) {
        return f(g(x));
    };
};

var position2 = function position2(f, g) {
    return function() {
        return f.call(this, g.apply(this, arguments));
    }
};

var addFour = {
    myMethod: function addFour(x) {
        return x + this.number;
    },
    number: 4
};

var timesSeven = {
    myMethod: function timesSeven(x) {
        return x * this.number;
    },
    number: 7
};

var addFourtimesSeven1 = position1(timesSeven.myMethod, addFour.myMethod);
var result1 = addFourtimesSeven1(2);
console.log(result1);

var addFourtimesSeven2 = position2(timesSeven.myMethod, addFour.myMethod);
var result2 = addFourtimesSeven2(2);
console.log(result2);

I am reading a book which contains the following example:

var position1 = function(f, g) {
  return function(x) {
    return f(g(x));
  }
};

Then the author writes: "...naive implementation of position, because it does not take the execution context into account..."

So the preferred function is that one:

var position2 = function(f, g) {
  return function() {
    return f.call(this, g.apply(this, arguments));
  }
}; 

Followed by an entire example:

var position2 = function position2(f, g) {
    return function() {
        return f.call(this, g.apply(this, arguments));
    }
};

var addFour = function addFour(x) {
    return x + 4;
};

var timesSeven = function timesSeven(x) {
    return x * 7;
};

var addFourtimesSeven2 = position2(timesSeven, addFour);
var result2 = addFourtimesSeven2(2);
console.log(result2);

Could someone please explain to me why the position2 function is the preferred one (maybe with an example)?

EDIT:

In the meantime i have tried to use methods as arguments as suggested, but it did not work. The result was NaN:

var position1 = function position1(f, g) {
    return function(x) {
        return f(g(x));
    };
};

var position2 = function position2(f, g) {
    return function() {
        return f.call(this, g.apply(this, arguments));
    }
};

var addFour = {
    myMethod: function addFour(x) {
        return x + this.number;
    },
    number: 4
};

var timesSeven = {
    myMethod: function timesSeven(x) {
        return x * this.number;
    },
    number: 7
};

var addFourtimesSeven1 = position1(timesSeven.myMethod, addFour.myMethod);
var result1 = addFourtimesSeven1(2);
console.log(result1);

var addFourtimesSeven2 = position2(timesSeven.myMethod, addFour.myMethod);
var result2 = addFourtimesSeven2(2);
console.log(result2);

• javascript
• function-position

Share Improve this question Follow edited Apr 16, 2016 at 13:16 JShinigami asked Apr 16, 2016 at 9:25 JShinigamiJShinigami 8,07533 gold badges1818 silver badges2222 bronze badges 4

• This actually has nothing to do with currying. – Bergi Commented Apr 16, 2016 at 10:01
• You are right, I changed the wording. – JShinigami Commented Apr 16, 2016 at 13:16
• It is called FUNCTION position! Why do you pose methods for heaven's sake? Function position means to bine pure functions in curried form without side effects in point-free style: const pose = f => g => x => f(g(x)). Curried functions except only a single argument (as our pose function). Function position only works with curried functions, because a function only returns a single value. – user6445533 Commented Jul 24, 2016 at 21:46
• Then the author writes: "...naive implementation of position, because it does not take the execution context into account..." I consider the author as naive. – user6445533 Commented Jul 24, 2016 at 21:55

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3 Answers 3

Sorted by: Reset to default 2

This just answers what position2 actually does:

position2 is used when you want to keep this as context in the functions itself. The following example shows that the result is 60 by using data.a and data.b:

'use strict';

var multiply = function(value) {
    return value * this.a;
}
var add = function(value) {
    return value + this.b;
}

var data = {
    a: 10,
    b: 4,
    func: position2(multiply, add)
};

var result = data.func(2);
// uses 'data' as 'this' inside the 'add' and 'multiply' functions
// (2 + 4) * 10 = 60

But yet, it still breaks the following example (unfortunately):

'use strict';

function Foo() {
    this.a = 10;
    this.b = 4;
}
Foo.prototype.multiply = function(value) {
    return value * this.a;
};
Foo.prototype.add = function(value) {
    return value + this.b;
};


var foo = new Foo();

var func = position2(foo.multiply, foo.add);
var result = func(2); // Uncaught TypeError: Cannot read property 'b' of undefined

Because the context of position2 (this) is undefined (and is not called in any other way, such as .apply, .call or obj.func()), you'd end up with this being undefined in the functions as well.

On the other hand, we can give it another context by using the following code:

'use strict';
var foo = new Foo();

var data = {
    a: 20,
    b: 8,
    func: position2(foo.multiply, foo.add)
}

var result = data.func(2); 
// uses 'data' as 'this'
// (2 + 8) * 10 = 200 :)

Or by explicitly setting the context:

'use strict';

var multiply = function(value) {
    return value * this.a;
};
var add = function(value) {
    return value + this.b;
};


var a = 20;
var b = 8;

var func = position2(multiply, add);

// All the same
var result1 = this.func(2);
var result2 = func.call(this, 2);
var result3 = func.apply(this, [2]);

position1 would not pass arguments other than the first to g()

If you do:

var position1 = function(f, g) {
  return function(x1, x2, x3) {
    return f(g(x1, x2, x3));
  }
};

the function will work for the first three arguments. If you however want it to work for an arbitrary number, you need to use Function.prototype.apply.

f.call(...) is used to set this as shown in Caramiriel's answer.

I disagree with the author.

Think of the use-case for function-position. Most of the time I utilize function-position for transformer-functions (pure functions; argument(s) in, result out and this is irrelevant).

2nd. Utilizing arguments the way he does it leads into a bad practice/dead end, because it implies that the function g() might depend on multiple arguments.

That means, that the position I create is not posable anymore, because it might not get all arguments it needs.
position that prevents position; fail

(And as a side-effect: passing the arguments-object to any other function is a performance no-go, because the JS-engine can't optimize this anymore)

Take a look at the topic of partial application, usually misreferenced as currying in JS, wich is basically: unless all arguments are passed, the function returns another function that takes the remaining args; until I have all my arguments I need to process them.

Then you should rethink the way you implement argument-order, because this works best when you define them as configs-first, data-last.
Example:

//a transformer: value in, lowercased string out
var toLowerCase = function(str){
    return String(str).toLowerCase();
}

//the original function expects 3 arguments, 
//two configs and the data to process.
var replace = curry(function(needle, heystack, str){
    return String(str).replace(needle, heystack);
});

//now I pass a partially applied function to map() that only 
//needs the data to process; this is really posable
arr.map( replace(/\s[A-Z]/g, toLowerCase) );

//or I create another utility by only applying the first argument
var replaceWhitespaceWith = replace(/\s+/g);
//and pass the remaining configs later
arr.map( replaceWhitespaceWith("-") );

A slightly different approach is to create functions that are, by design, not intended to get all arguments passed in one step, but one by one (or in meaningful groups)

var prepend = a => b => String(a) + String(b);  //one by one
var substr = (from, to) => value => String(str).substr(from, to);  //or grouped

arr.map( pose( prepend("foo"), substr(0, 5) ) );
arr.map( pose( prepend("bar"), substr(5) ) );
//and the `to`-argument is undefined; by intent

I don't intend to ever call such functions with all the arguments, all I want to pass them is their configs, and to get a function that does the job on the passed data/value.

Instead of substr(0, 5, someString), I would always write someString.substr(0, 5), so why take any efforts to make the last argument (data) applyable in the first call?

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