JavaScript Comparison Operators – Why [] == [] is False?

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I am working on a part of the code where I have an array which looks like [[data]]. The data is rendered on the server side through the Django template engine. So my code looks like this:

var data = {{ series|safe }}; 
// data will be [[]] if no data is present
if (data ==[[]])
  console.log('no data');

The if always returns false. That means in [[]] == [[]] is false and my test shows that []==[] is false as well.

Any descriptions would be appreciated.

Best Answer

Because == (and ===) test to see if two objects are the same object and not if they are identical objects.

Most test frameworks will include functions such as deepEqual if you want to see if two objects are identical.

Related Solutions

JavaScript Comparison Operators – Identity vs. Equality

The equality operator will attempt to make the data types the same before making the comparison. On the other hand, the identity operator requires both data types to be the same as a prerequisite.

There are quite a few other posts out there similar to this questions. See:

How do the PHP equality (== double equals) and identity (=== triple equals) comparison operators differ? (has a nice comparison chart)
Which equals operator (== vs ===) should be used in JavaScript comparisons?

In practice, the identity operator comes in really handy when you want to be certain that a boolean value is true or false since...

1 == true     => true
true == true  => true
1 === true    => false
true === true => true

JavaScript – Why Does ([] == false) is True but ([] || true) Results in []?

Type conversion is not related to falsy and truthy values.

What is truthy and what is falsy is defined by the ToBoolean function defined in the specs and [] is indeed truthy.

On the other hand, [] == false returns true because of the type conversion that happens during the evaluation of the expression.

The rules of type conversion say that for x == y

If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

ToNumber results in 0 for false so we're left with the evaluation of [] == 0. According to the same rules

If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.

ToPrimitive results in an empty string. Now we have "" == 0. Back to our type conversion rules

If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.

ToNumber results in 0 for "" so the final evaluation is 0 == 0 and that is true!

Best Answer

Related Solutions

JavaScript Comparison Operators – Identity vs. Equality

JavaScript – Why Does ([] == false) is True but ([] || true) Results in []?

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