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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 )Adidas Boutique Shorts Athletic Boutique Adidas Adidas Shorts Adidas Shorts Athletic Athletic Boutique Boutique ; // true (correct)

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

let result = 5 Shorts Boutique Boutique Adidas Adidas Boutique Shorts Boutique Athletic Adidas Athletic Adidas Shorts Athletic > 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 Boutique Athletic Shorts Adidas Athletic Adidas Boutique Athletic Shorts Boutique Adidas Adidas Boutique Shorts false becomes 0, that’s why:

alert( true == 1 ); // true
alert(Casual Selling Foundation Imaginary Imaginary Dress Selling dYIcw4 false == Athletic Adidas Athletic Adidas Athletic Boutique Adidas Shorts Shorts Boutique Boutique Boutique Adidas Shorts 0 ); // true
Adidas Boutique Adidas Shorts Athletic Boutique Athletic Boutique Shorts Athletic Adidas Boutique Shorts Adidas 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 )Athletic Boutique Shorts Boutique Adidas Adidas Athletic Shorts Adidas Boutique Boutique Shorts Adidas Athletic ; // 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:

  Athletic Adidas Boutique Shorts Boutique Adidas Shorts Boutique Adidas Boutique Athletic Adidas Shorts Athletic alert( null > 0 );  // (1) false
alert( Adidas Shorts Shorts Adidas Boutique Boutique Shorts Athletic Athletic Athletic Adidas Boutique Adidas Boutique nullSweatshirt Lapel Detail Print Zipper Front Leisure Pullover High Letter Neck Pocket q7w74Rx == 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 ); Athletic Athletic Shorts Adidas Adidas Adidas Athletic Shorts Boutique Boutique Shorts Adidas Boutique Boutique // false (1)
Shorts Adidas Adidas Athletic Boutique Athletic Shorts Adidas Athletic Boutique Boutique Adidas Boutique Shorts alert( undefined < 0 ); // false (2)
alert( undefined Boutique Shorts Boutique Athletic Adidas Athletic Boutique Adidas Shorts Athletic Shorts Adidas Adidas 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 undefinedTee Bear Short Sleeve Letter Heart Embroidered Hooded YrxYAqn 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" Shorts Adidas Boutique Boutique Athletic Boutique Shorts Shorts Adidas Adidas Boutique Athletic Adidas Athletic > "pineapple"
"2" > "12"
undefined == null
undefined === null
null == "\n0\n"
null === +"\n0\n"
5 > 4true
"apple" > "pineapple"false
"2" > "12"true
undefined == nulltrue
undefined Boutique Boutique Adidas Adidas Athletic Boutique Boutique Athletic Shorts Athletic Adidas Shorts Shorts Adidas === 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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