CODEWITHSUNDEEP
javascript
Latheesan
Latheesan

Reputation: 24116

Count the number of elements of an array in javascript

Consider the following:

var answers = [];

answers[71] = {
    field: 'value'
};

answers[31] = {
    field: 'value'
};

console.log(answers);

This outputs the length of the array as 72 but I was expecting it to return 2. Here's the output of the script from chrome console:

enter image description here

Any ideas why this is?

Upvotes: 18

Views: 46623

Answers (6)

SimpleJ
SimpleJ

Reputation: 14768

You can count the actual number of keys using Object.keys(array).length:

const answers = [];

answers[71] = {
    field: 'value'
};

answers[31] = {
    field: 'value'
};

console.log(Object.keys(answers).length); // prints 2

Upvotes: 39

user663031
user663031

Reputation:

Any ideas why this is?

Because that's how the language works.

From MDN:

The length property represents an unsigned, 32-bit integer that is always numerically greater than the highest index in the array.

From the spec:

Specifically, whenever an own property is added whose name is an array index, the value of the length property is changed, if necessary, to be one more than the numeric value of that array index;

Notice that it is not true that the length is always one greater than the index of the last populated item. In your case, if you did

answers[71] = {field: 'value'};
answers[31] = {field: 'value'};

The length would now be 72, but if you then did

delete answers[71];

the length would remain 72, and not be adjusted down to 32.

If your question was actually to find the actual number of populated slots in the array, see the other answers, or you could brute-force it, looping over all indexes up to the length and checking if a slot is populated using the in operator:

let count = 0;
for (let i = 0; i < arr.length; i++) count += i in arr;

Upvotes: 2

StephenTG
StephenTG

Reputation: 2647

By defining index 71, you've told the array that it needs to hold at least 72 entries, and any that you don't explicitly define will hold a value of undefined.

To find the number of actual elements, you can iterate through the array and check whether each element is undefined or not, and count how many are not undefined

Upvotes: 3

Pranav C Balan
Pranav C Balan

Reputation: 115222

You can simply use Array#filter method which doesn't iterate over deleted or undefined array elements.

answers.filter(function(v){ return true; }).length


var answers = [];

answers[71] = {
  field: 'value'
};

answers[31] = {
  field: 'value'
};

console.log(answers.filter(function(v) {
  return true;
}).length);

Upvotes: 3

Azamantes
Azamantes

Reputation: 1451

Just use object instead of array and check

Object.keys(your_object_like_array).length

to get the amount.

Upvotes: 1

Nina Scholz
Nina Scholz

Reputation: 386578

You could count it with Array#forEach or use Array#reduce

var answers = [], count = 0;

answers[71] = { field: 'value' };
answers[31] = { field: 'value' };
answers.forEach(_ => count++);

console.log(count);
console.log(answers.reduce(r => r + 1, 0));

Upvotes: 3

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