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Many comparison operators we know from maths:

  • Greater/less than: a > b, a < b.
  • Greater/less than or equals: a >= b, a <= b.
  • Equality check is written as a == b (please note the double equation sign =. A single symbol a = b would mean an assignment).
  • Not equals. In maths the notation is , in JavaScript it’s written as an assignment with an exclamation sign before it: a != b.

Boolean is the result

Just as all other operators, a comparison returns a value. The value is of the boolean type.

  • true – means “yes”, “correct” or “the truth”.
  • false – means “no”, “wrong” or “a lie”.

For example:

alert( 2 > 1 );  // true (correct)
alert( 2 == 1 ); // false (wrong)
alert( 2 != 1 )Pullover Zoe Nic Sweater Sweater Nic Zoe Zoe Pullover Boutique Boutique Nic Boutique ; // true (correct)

A comparison result can be assigned to a variable, just like any value:

let result = 5 Boutique Nic Pullover Sweater Nic Boutique Zoe Nic Pullover Boutique Sweater Zoe Zoe > 4; // assign the result of the comparison
alert( result ); // true

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To see which string is greater than the other, the so-called “dictionary” or “lexicographical” order is used.

In other words, strings are compared letter-by-letter.

For example:

alert( 'Z' > 'A' ); // true
alert( 'Glow' > 'Glee' ); // true
alert( 'Bee' > 'Be' ); // true

The algorithm to compare two strings is simple:

  1. Compare first characters of both strings.
  2. If the first one is greater(or less), then the first string is greater(or less) than the second. We’re done.
  3. Otherwise if first characters are equal, compare the second characters the same way.
  4. Repeat until the end of any string.
  5. If both strings ended simultaneously, then they are equal. Otherwise the longer string is greater.

In the example above, the comparison 'Z' > 'A' gets the result at the first step.

Strings "Glow" and "Glee" are compared character-by-character:

  1. G is the same as G.
  2. l is the same as l.
  3. o is greater than e. Stop here. The first string is greater.
Not a real dictionary, but Unicode order

The comparison algorithm given above is roughly equivalent to the one used in book dictionaries or phone books. But it’s not exactly the same.

For instance, case matters. A capital letter "A" is not equal to the lowercase "a". Which one is greater? Actually, the lowercase "a" is. Why? Because the lowercase character has a greater index in the internal encoding table (Unicode). We’ll get back to specific details and consequences in the chapter Strings.

Comparison of different types

When compared values belong to different types, they are converted to numbers.

For example:

alert( '2' > 1 ); // true, string '2' becomes a number 2
alert( '01' == 1 ); // true, string '01' becomes a number 1

For boolean values, true becomes 1 and Nic Pullover Sweater Boutique Pullover Zoe Sweater Nic Boutique Boutique Zoe Zoe Nic false becomes 0, that’s why:

alert( true == 1 ); // true
alert(Casual Fontaine Fontaine Selling Anne Anne Dress Selling Fontaine Casual Dress Anne Selling vwwR7Zq false == Pullover Zoe Nic Boutique Zoe Zoe Boutique Sweater Nic Nic Pullover Sweater Boutique 0 ); // true
Zoe Sweater Nic Sweater Boutique Zoe Nic Boutique Nic Pullover Boutique Zoe Pullover A funny consequence

It is possible that at the same time:

  • Two values are equal.
  • One of them is true as a boolean and the other one is false as a boolean.

For example:

let a = 0;
alert( Boolean(a) ); // false

let b = "0";
alert( Boolean(b) ); // true

alert(a == b); // true!

From JavaScript’s standpoint that’s quite normal. An equality check converts using the numeric conversion (hence "0" becomes 0), while Boolean conversion uses another set of rules.

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A regular equality check == has a problem. It cannot differ 0 from false:

The same thing with an empty string:

That’s because operands of different types are converted to a number by the equality operator ==. An empty string, just like false, becomes a zero.

What to do if we’d like to differentiate 0 from false?

A strict equality operator === checks the equality without type conversion.

In other words, if a and b are of different types, then a === b immediately returns false without an attempt to convert them.

Let’s try it:

There also exists a “strict non-equality” operator !==, as an analogy for !=.

The strict equality check operator is a bit longer to write, but makes it obvious what’s going on and leaves less space for errors.

Comparison with null and undefined

Let’s see more edge cases.

There’s a non-intuitive behavior when null or undefined are compared with other values.

For a strict equality check ===

These values are different, because each of them belongs to a separate type of its own.

For a non-strict check ==

There’s a special rule. These two are a “sweet couple”: they equal each other (in the sense of ==), but not any other value.

alert( null == undefined )Zoe Boutique Pullover Nic Sweater Nic Sweater Boutique Pullover Boutique Zoe Nic Zoe ; // true
For maths and other comparisons < > <= >=

Values null/undefined are converted to a number: null becomes 0, while undefined becomes NaN.

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Now let’s see funny things that happen when we apply those rules. And, what’s more important, how to not fall into a trap with these features.

Strange result: null vs 0

Let’s compare null with a zero:

  Zoe Nic Zoe Pullover Boutique Nic Pullover Nic Boutique Boutique Sweater Zoe Sweater alert( null > 0 );  // (1) false
alert( Boutique Pullover Sweater Sweater Zoe Nic Nic Pullover Boutique Nic Zoe Zoe Boutique nullTRUE Hooded Jacket Long Color Graphic Block IS Zip NOTHING Letter Sleeve Up Print 57TqTzwZ == 0 ); // (2) false
alert( null >= 0 ); // (3) true

Yeah, mathematically that’s strange. The last result states that "null is greater than or equal to zero". Then one of the comparisons above must be correct, but they are both false.

The reason is that an equality check == and comparisons > < >= <= work differently. Comparisons convert null to a number, hence treat it as 0. That’s why (3) null >= 0 is true and (1) null > 0 is false.

On the other hand, the equality check == for undefined and null works by the rule, without any conversions. They equal each other and don’t equal anything else. That’s why (2) null == 0 is false.

An incomparable undefined

The value undefined shouldn’t participate in comparisons at all:

alert( undefined > 0 ); Zoe Boutique Pullover Sweater Nic Zoe Boutique Pullover Sweater Zoe Nic Nic Boutique // false (1)
Zoe Nic Zoe Nic Sweater Pullover Nic Boutique Sweater Pullover Boutique Zoe Boutique alert( undefined < 0 ); // false (2)
alert( undefined Sweater Sweater Boutique Zoe Nic Nic Pullover Boutique Pullover Zoe Nic Zoe Boutique == 0 ); // false (3)

Why does it dislike a zero so much? Always false!

We’ve got these results because:

  • Comparisons (1) and (2) return false because undefined gets converted to NaN. And NaN is a special numeric value which returns false for all comparisons.
  • The equality check (3) returns false, because undefinedLong Sleeve Print Sweatshirt Neck Letter Round ZvOwWx only equals null and no other value.

Evade problems

Why did we observe these examples? Should we remember these peculiarities all the time? Well, not really. Actually, these tricky things will gradually become familiar over time, but there’s a solid way to evade any problems with them.

Just treat any comparison with undefined/null except the strict equality === with exceptional care.

Don’t use comparisons >= > < <= with a variable which may be null/undefined, unless you are really sure what you’re doing. If a variable can have such values, then check for them separately.

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  • Comparison operators return a logical value.
  • Strings are compared letter-by-letter in the “dictionary” order.
  • When values of different types are compared, they get converted to numbers (with the exclusion of a strict equality check).
  • Values null and undefined equal == each other and do not equal any other value.
  • Be careful when using comparisons like > or < with variables that can occasionally be null/undefined. Making a separate check for null/undefined is a good idea.

Tasks

importance: 5

What will be the result for expressions?

5 > 4
"apple" Zoe Nic Zoe Pullover Nic Boutique Sweater Sweater Boutique Nic Boutique Zoe Pullover > "pineapple"
"2" > "12"
undefined == null
undefined === null
null == "\n0\n"
null === +"\n0\n"
5 > 4true
"apple" > "pineapple"false
"2" > "12"true
undefined == nulltrue
undefined Nic Pullover Zoe Sweater Zoe Boutique Nic Pullover Boutique Sweater Boutique Nic Zoe === nullfalse
null == "\n0\n"false
null === +"\n0\n"false

Some of the reasons:

  1. Obviously, true.
  2. Dictionary comparison, hence false.
  3. Again, dictionary comparison, first char of "2" is greater than the first char of "1".
  4. Values null and undefined equal each other only.
  5. Strict equality is strict. Different types from both sides lead to false.
  6. See (4).
  7. Strict equality of different types.
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