Javascript - storing the index of all array values in a variable

Question

Say I have an array in JS: var fruits = [apple,orange,banana] I want to store the index of each fruit in variables such that at any point in time, if I add more stuff to the array, I will still know that the index of apple is X. So in this case, 0 is apple, but if I add something to the beginning of that away, the index of apple changes.

The more verbose way I can think of is to loop through the array

for (var i=0;i<fruits.length;i++) {
   switch(fruits[i]) {
    case:"apple"
      var indexApple = i;
      break;
    //etc 
   }
}

Another way I can think of is use the value of the arrays as the variable name.

for (var i=0;i<fruits.length;i++) {
  //psedo code
  var 'index' + fruits[i] = i;
}

So in the end I'd have var indexApple = 0, indexOrange = 1, etc. THe key to the second method is to be able to create a dynamic variable by concatenating the string 'index' and the value of the array to create that variable. Not sure how to do that.

Note: Ideally I want the variables that store the index to be dynamically generated. Such that I only I can modify/add to the fruits array, and a new variable will be generated to store the index.

Answer 1

The simplest solution is simply to build an Object which gives you nearly O(1) lookup time, and will scale with your array:

function LinkFruits(fruits) {
    FruitLookup = {}
    fruits.forEach((fruit,ind) => FruitLookup[fruit] = ind)
}

Now you can simply "lookup" your index from the FruitLookup table when needed like:

console.log("The index of apple is",FruitLookup.apple,"and for orange is",FruitLookup.orange)

Now if you modify your array you simply need to run LinkFruits(fruits) .

Technical Note: If you want to fully automate this process you can look into Array.observe() which is now deprecated . Or overload the push and pop methods of this array to trigger the update before falling back to the default prototype methods.

Answer 2

it seems like ensuring your the value of the index is legitimate will be difficult. i would include jquery and use the inArray method which returns the index of the item in the array.

function showIndexes() {
  var appleIndex = $.inArray(fruits, "Apple"); //returns 0
  var guavaIndex = $.inArray(fruits, "Guava"); //returns -1

  fruits.unshift("Guava");
  appleIndex = $.inArray(fruits, "Apple"); //returns 1
  guavaIndex = $.inArray(fruits, "Guava"); //returns 0
}