Changing the function that is called within a function in javascript

Question

I have different functions that run very simple mathematical equations. I want to have a function that is able to call any of these functions based on a variable that is set separately. I can't seem to get the function to recognize the variable as a "function" input.

When run in console, this gives me the result I'd expect:

function operate(whatOp, a, b){
  console.log(whatOp(a, b)}

So typing...

operate(divide, 3, 4)

...in console returns 0.75.

Yet, if I try to call it by having whatOp come from a variable, it doesn't work as...

let firstNumber = "999";
let secondNumber = "333";
let operation = "divide";;

operate(operation, firstNumber, secondNumber)

...in console returns undefined.

How do I get it to work based on this? I'm using a button to set the variable, but that's working and returns the value I want, so I think it has to do with the way I'm calling the variable?

operation is expected to be a function that accepts 2 arguments and not a string. — silentw
– silentw, Commented Aug 22, 2019 at 13:25
Just do let operation = divide. You ware passing divide, in your working example, not "divide" — VLAZ
– VLAZ, Commented Aug 22, 2019 at 13:25
Assuming divide is in e.g. window scope, then operate(window[operation], firstNumber, secondNumber) would work. — WuDo
– WuDo, Commented Aug 22, 2019 at 13:25
@VLAZ In my actual code I was giving it a value from the ID of a button I clicked (called divide, for instance), how would I make sure I pass that not as a string, but as simply divide — Billedale
– Billedale, Commented Aug 22, 2019 at 13:46

Dez · Accepted Answer · 2019-08-22 13:33:09Z

1

You can do something like:

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

function rest(a, b) {
  return a - b;
}

function divide(a, b) {
  return a / b;
}

function operate(whatOp, a, b){
  const operations = {
    add,
    rest,
    divide
  };

  return operations[whatOp](a, b);
}

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

function rest(a, b) {
  return a - b;
}

function divide(a, b) {
  return a / b;
}

function operate(whatOp, a, b){
  const operations = {
    add,
    rest,
    divide
  };

  return operations[whatOp](a, b);
}

console.log(operate('add', 1, 2));
console.log(operate('rest', 2, 1));
console.log(operate('divide', 4, 2));

answered Aug 22, 2019 at 13:33

Dez

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Comments

EddiGordo · Accepted Answer · 2019-08-22 13:43:24Z

1

You should check if the operation you need to execute is present in the global space. What i use to do is :

if (window[operation])
{
    operate(window[operation], firstNumber, secondNumber);
}
else
{
    console.error("Operation", operation, "not yet implemented !");
}

Hope it helps.

answered Aug 22, 2019 at 13:43

EddiGordo

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Comments

ChechoCZ · Accepted Answer · 2019-08-22 13:31:04Z

0

I had to do something similar, and I could do it by writing a simple switch statement:

function yourFunction(operation, a, b) {
  switch (operation) {
    case 'sum':
      sum(a, b);
      break;
  }
}

Hope it helps...

answered Aug 22, 2019 at 13:31

ChechoCZ

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Comments

Christian Vincenzo Traina · Accepted Answer · 2019-08-22 13:35:45Z

0

Some day, programming languages will be so intelligent that you will just say "divide" and they will divide two numbers. That day isn't arrived yet, so you have to specify what "divide" means. You can do it by defining a function:

function operate(whatOp, a, b){
  console.log(whatOp(a, b));
}

let firstNumber = "999";
let secondNumber = "333";
let operation = function divide(firstArgument, secondArgument) {
    return firstArgument / secondArgument; // Here is where you specify what "divide" is
}

operate(operation, firstNumber, secondNumber);

answered Aug 22, 2019 at 13:35

Christian Vincenzo Traina

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