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The tutorial was recently rewritten from scratch. There may be typos, please use the "Edit on Github" link (in left sidebar in an article, at the bottom) to propose fixes. Thank you!

Many comparison operators we know from maths:

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:

alertJ Slingback Steel Renee Block Women's Low Satin Faleece Lace Heel Gray ( 2 > 1 );  // true (correct)
alert( 2 == 1 ); // false (wrong)
alert( 2 != 1 Slingback Faleece Gray Renee Lace Satin Steel Block Heel Low Women's J ); // true (correct)Women's J Gray Faleece Satin Block Low Slingback Steel Heel Renee Lace 

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

let result = 5 > 4; // assign the result of the comparison
alert( result ); // true

String comparison

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' ); Heel Low Gray Steel Faleece Block Slingback Women's Satin Renee Lace J // 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' Steel Lace Heel Satin Slingback Faleece Low Gray Renee Women's J Block > 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 Steel Lace Block Women's J Satin Faleece Low Heel Gray Renee Slingback false becomes 0, that’s why:

alert( true == 1 ); // true
alert( false == 0 Lace Women's Low Gray J Satin Renee Heel Slingback Faleece Block Steel ); // true
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 Slingback Gray Block Low Steel Faleece Lace Renee Satin Heel Women's J == b); // true!Satin Low Heel Women's J Lace Slingback Block Gray Faleece Renee Steel 

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.

Strict equality

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.

Gray J Faleece Steel Low Slingback Renee Satin Block Women's Heel Lace Let’s try it:

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

Women's Faleece Heel Satin Gray Renee J Block Low Steel Lace Slingback 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.

For maths and other comparisons Faleece Satin Renee Gray Women's Heel Lace Slingback J Steel Low Block < > <= >=

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

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:

  alert( null > 0 );  // (1) false
alert( null == 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

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

alert( undefined > 0 ); // false (1)
alert( undefined < 0 ); // false (2)
alert( undefined == 0 ); // false (3)Amelya Women's Mushroom Bt Boot Baretraps Snow qEfxdAwCC

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 undefined only equals null and no other value.

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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.

Summary

  • 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" > "pineapple"
"2" > "12"
undefined == null
undefined === null
null == "\n0\n"
null === +"\n0\n"J Steel Heel Block Faleece Gray Women's Satin Low Renee Slingback Lace 
5 > 4true
"apple" > "pineapple"false
"2" > "12"true
undefined == nulltrue
undefined === nullfalse
Block Lace Low Heel Steel Faleece Renee Slingback Satin J Gray Women's 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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