Can't declare variable as object property in JavaScript

Question

I am declaring a function that, when given a string as an argument, returns an object where keys are the words in the string. Values are the # of occurrences of that word in the string.

When I run the below code, I get {}.

function countWords(string) {
  var result = {};
  var words = string.split(' ');
  for (var i = 0; i < words.length; i++) {
    var currentElement = words[i];
    var prop = result[currentElement];
    if(prop) {
        prop += 1;
    } else {
        prop = 1;
    }
  }
  return result;
}

console.log(countWords('hello hello')); // => {'hello': 2}

However, replacing prop = 1 with result[CurrentElement] = 1 returns the expected answer.

Why is it wrong to use prop = 1 in this context?

prop is just a variable. If you assign it to some property, then write prop = 1, you just redefine prop to be 1. You don’t actually touch the property itself. — Sebastian Simon
– Sebastian Simon, Commented Apr 17, 2017 at 15:16
Possible duplicate of Are variables/objects passed by value and why can't I change object's property with variable in javascript? — Sebastian Simon
– Sebastian Simon, Commented Apr 17, 2017 at 15:23

Doqnach · Accepted Answer · 2017-04-17 15:18:09Z

3

var prop = result[currentElement]; This makes a copy of the value in result[currentElement] into prop, then you add one but never put it back into the array. The assignment is by value, not by reference.

answered Apr 17, 2017 at 15:18

Doqnach

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Comments

Nina Scholz · Accepted Answer · 2017-04-17 15:19:53Z

You need to use the reference to the object for checking and incrementing.

result[currentElement]
^^                      object
      ^^^^^^^^^^^^^^^^  property of object

If you just get the value of it, you get a primitive value and not the wanted reference to the object.

value = result[currentElement] // undefined in the first call
                               //         1 in the second call

but you do not have a reference to result anymore.

function countWords(string) {
  var result = {};
  var words = string.split(' ');
  for (var i = 0; i < words.length; i++) {
    var currentElement = words[i];
    if (result[currentElement]) {
      result[currentElement] += 1;
    } else {
      result[currentElement] = 1;
    }
  }
  return result;
}

console.log(countWords('hello hello')); // => {'hello': 2}

ibrahim mahrir · Accepted Answer · 2017-04-17 15:18:50Z

0

In this line:

var prop = result[currentElement];

you are copying the value of result[currentElement] (apparently a number) and then increment the copy, thus th original value (result[currentElement]) remains the same.

answered Apr 17, 2017 at 15:18

ibrahim mahrir

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Comments

Qwertiy · Accepted Answer · 2017-04-17 15:19:07Z

0

Because variables are almost never linked by reference with anything else.

There are only 2 cases when it goes in other way:

arguments in nonstrict mode.

function f(a) {
  console.log(a, arguments[0]);
  a = 10;
  console.log(a, arguments[0]);
}

function g(a) {
  'use strict';
  console.log(a, arguments[0]);
  a = 10;
  console.log(a, arguments[0]);
}

f(7);
g(7);

Left side of assignment including decomposition.

var a = 8, b = 10;
console.log(a, b);
[a, b] = [b, a];
console.log(a, b);

var x = [0, 1, 2];
console.log(x.join(" "));
[x[1], x[0], x[2]] = x;
console.log(x.join(" "));

answered Apr 17, 2017 at 15:19

Qwertiy

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user3742005 · Accepted Answer · 2017-04-17 15:32:17Z

0

you need to add it in the result like this :

<script>
    function countWords(string) {
  var result = {};
  var words = string.split(' ');
  for (var i = 0; i < words.length; i++) {
    var currentElement = words[i];
    var prop = result[currentElement];
    if(prop) {
        prop += 1;
    } else {
        prop = 1;
    }
    result[currentElement]=prop;
  }
  return result;
}

console.log(countWords('hello hello test khalil'));
</script>

answered Apr 17, 2017 at 15:32

user3742005

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