209 lines
7.7 KiB
Markdown
209 lines
7.7 KiB
Markdown
# Comparisons
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We know many comparison operators from maths:
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- Greater/less than: <code>a > b</code>, <code>a < b</code>.
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- Greater/less than or equals: <code>a >= b</code>, <code>a <= b</code>.
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- Equals: `a == b` (please note the double equals sign `=`. A single symbol `a = b` would mean an assignment).
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- Not equals. In maths the notation is <code>≠</code>, but in JavaScript it's written as an assignment with an exclamation sign before it: <code>a != b</code>.
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## Boolean is the result
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Like all other operators, a comparison returns a value. In this case, the value is a boolean.
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- `true` -- means "yes", "correct" or "the truth".
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- `false` -- means "no", "wrong" or "not the truth".
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For example:
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```js run
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alert( 2 > 1 ); // true (correct)
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alert( 2 == 1 ); // false (wrong)
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alert( 2 != 1 ); // true (correct)
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```
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A comparison result can be assigned to a variable, just like any value:
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```js run
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let result = 5 > 4; // assign the result of the comparison
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alert( result ); // true
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```
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## String comparison
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To see whether a string is greater than another, JavaScript uses the so-called "dictionary" or "lexicographical" order.
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In other words, strings are compared letter-by-letter.
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For example:
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```js run
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alert( 'Z' > 'A' ); // true
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alert( 'Glow' > 'Glee' ); // true
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alert( 'Bee' > 'Be' ); // true
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```
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The algorithm to compare two strings is simple:
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1. Compare the first character of both strings.
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2. If the first character from the first string is greater (or less) than the other string's, then the first string is greater (or less) than the second. We're done.
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3. Otherwise, if both strings' first characters are the same, compare the second characters the same way.
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4. Repeat until the end of either string.
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5. If both strings end at the same length, then they are equal. Otherwise, the longer string is greater.
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In the examples above, the comparison `'Z' > 'A'` gets to a result at the first step while the strings `"Glow"` and `"Glee"` are compared character-by-character:
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1. `G` is the same as `G`.
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2. `l` is the same as `l`.
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3. `o` is greater than `e`. Stop here. The first string is greater.
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```smart header="Not a real dictionary, but Unicode order"
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The comparison algorithm given above is roughly equivalent to the one used in dictionaries or phone books, but it's not exactly the same.
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For instance, case matters. A capital letter `"A"` is not equal to the lowercase `"a"`. Which one is greater? The lowercase `"a"`. Why? Because the lowercase character has a greater index in the internal encoding table JavaScript uses (Unicode). We'll get back to specific details and consequences of this in the chapter <info:string>.
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```
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## Comparison of different types
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When comparing values of different types, JavaScript converts the values to numbers.
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For example:
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```js run
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alert( '2' > 1 ); // true, string '2' becomes a number 2
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alert( '01' == 1 ); // true, string '01' becomes a number 1
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```
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For boolean values, `true` becomes `1` and `false` becomes `0`.
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For example:
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```js run
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alert( true == 1 ); // true
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alert( false == 0 ); // true
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```
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````smart header="A funny consequence"
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It is possible that at the same time:
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- Two values are equal.
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- One of them is `true` as a boolean and the other one is `false` as a boolean.
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For example:
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```js run
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let a = 0;
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alert( Boolean(a) ); // false
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let b = "0";
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alert( Boolean(b) ); // true
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alert(a == b); // true!
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```
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From JavaScript's standpoint, this result is quite normal. An equality check converts values using the numeric conversion (hence `"0"` becomes `0`), while the explicit `Boolean` conversion uses another set of rules.
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````
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## Strict equality
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A regular equality check `==` has a problem. It cannot differentiate `0` from `false`:
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```js run
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alert( 0 == false ); // true
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```
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The same thing happens with an empty string:
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```js run
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alert( '' == false ); // true
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```
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This happens because operands of different types are converted to numbers by the equality operator `==`. An empty string, just like `false`, becomes a zero.
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What to do if we'd like to differentiate `0` from `false`?
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**A strict equality operator `===` checks the equality without type conversion.**
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In other words, if `a` and `b` are of different types, then `a === b` immediately returns `false` without an attempt to convert them.
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Let's try it:
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```js run
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alert( 0 === false ); // false, because the types are different
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```
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There is also a "strict non-equality" operator `!==` analogous to `!=`.
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The strict equality operator is a bit longer to write, but makes it obvious what's going on and leaves less room for errors.
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## Comparison with null and undefined
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There's a non-intuitive behavior when `null` or `undefined` are compared to other values.
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For a strict equality check `===`
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: These values are different, because each of them is a different type.
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```js run
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alert( null === undefined ); // false
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```
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For a non-strict check `==`
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: There's a special rule. These two are a "sweet couple": they equal each other (in the sense of `==`), but not any other value.
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```js run
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alert( null == undefined ); // true
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```
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For maths and other comparisons `< > <= >=`
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: `null/undefined` are converted to numbers: `null` becomes `0`, while `undefined` becomes `NaN`.
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Now let's see some funny things that happen when we apply these rules. And, what's more important, how to not fall into a trap with them.
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### Strange result: null vs 0
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Let's compare `null` with a zero:
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```js run
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alert( null > 0 ); // (1) false
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alert( null == 0 ); // (2) false
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alert( null >= 0 ); // (3) *!*true*/!*
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```
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Mathematically, that's strange. The last result states that "`null` is greater than or equal to zero", so in one of the comparisons above it must be `true`, but they are both false.
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The reason is that an equality check `==` and comparisons `> < >= <=` work differently. Comparisons convert `null` to a number, treating it as `0`. That's why (3) `null >= 0` is true and (1) `null > 0` is false.
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On the other hand, the equality check `==` for `undefined` and `null` is defined such that, without any conversions, they equal each other and don't equal anything else. That's why (2) `null == 0` is false.
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### An incomparable undefined
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The value `undefined` shouldn't be compared to other values:
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```js run
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alert( undefined > 0 ); // false (1)
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alert( undefined < 0 ); // false (2)
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alert( undefined == 0 ); // false (3)
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```
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Why does it dislike zero so much? Always false!
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We get these results because:
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- 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.
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- The equality check `(3)` returns `false` because `undefined` only equals `null`, `undefined`, and no other value.
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### Evade problems
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Why did we go over 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 problems with them:
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Just treat any comparison with `undefined/null` except the strict equality `===` with exceptional care.
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Don't use comparisons `>= > < <=` with a variable which may be `null/undefined`, unless you're really sure of what you're doing. If a variable can have these values, check for them separately.
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## Summary
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- Comparison operators return a boolean value.
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- Strings are compared letter-by-letter in the "dictionary" order.
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- When values of different types are compared, they get converted to numbers (with the exclusion of a strict equality check).
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- The values `null` and `undefined` equal `==` each other and do not equal any other value.
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- Be careful when using comparisons like `>` or `<` with variables that can occasionally be `null/undefined`. Checking for `null/undefined` separately is a good idea.
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