How to implement Underscore's find array function

Question

I'm trying to implement the Underscore.js function _.find. I don't understand why my code below returns undefined for:

find([1, 2, 3, 4, 5, 6], function(num) {
  return num % 2 == 0;
});

Here is my attempt at the solution:

function find(arr,callback) {
  each(arr,item=>{
    if (callback(item)) {
      return item;
    };
  });
}
function each(arr,callback) {
  if (Array.isArray(arr)) {
    for (let i = 0; i < arr.length; i++) {
      callback(arr[i]);
    };
  } else{
    for(let key in arr) {
      callback(arr[key]);
    }
  };
}

Emile Bergeron · Accepted Answer · 2017-05-08 20:22:54Z

2

each(arr, item => {
    if (callback(item)) {
        return item;
    }
});

You're returning inside the inner function started with item => {. It doesn't short circuit the outer function. return only stops the function execution of the function you return from.

An each loop is not supposed to return anything, and an each loop is not supposed to short circuit and should run for every element in the collection. You shouldn't use each for a search function. I don't know how underscore's source does it, but a simple for loop here will allow you to return once something is found.

edited May 8, 2017 at 20:22

Emile Bergeron

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answered May 7, 2017 at 17:18

Andy Ray

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1 Comment

Adrian Over a year ago

Thank you very much Andy Ray. Very helpful.

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