Comparing strings in JavaScript

This is extracted from the much larger post: The Complete Guide to Working With Strings in Modern JavaScript

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Equality

When you know you are comparing two string primitives, you can use either the == or === operators:

"abba" === "abba" // true
"abba" == "abba" // true

If you are comparing a string primitive to something that is not a string, == and === behave differently.

When using the == operator, the non-string will be coerced to a string. That means that JavaScript will try and make
it a string before comparing the values.

9 == "9"
// becomes 
"9" == "9" //true

For a strict comparison, where non-strings are not coerced to strings, use ===:

9 === "9" // false

The same is true of the not-equal operators, != and !==:

9 != "9" // false
9 !== "9" // true

If you're not sure what to use, prefer strict equality using ====.

When using String objects, two objects with the same value are not considered equal:

   new String("js") == new String("js") // false
   new String("js") === new String("js") // false

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Case sensitivity

For case-insensitive comparisons, do not convert both strings to upper or lowercase and then compare them, as this is unreliable with some characters.

Instead use localeCompare:

let a = 'Résumé';
let b = 'RESUME';

// Incorrect: returns 'false'
a.toLowerCase() === b.toLowerCase()

// Correct: returns '1'
a.localeCompare(b, undefined, { sensitivity: 'accent' })

localeCompare is supported in the following browsers

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Greater/less than

When comparing strings using the < and > operators, JavaScript will compare each character in 'lexicographical order'.

That means that they are compared letter by letter, in the order they would appear in a dictionary:

"aardvark" < "animal" // true
"gamma" > "zulu" // false

When comparing strings using < >, lowercase letters are considered larger than uppercase.

"aardvark" > "Animal" // true

This is because JavaScript is actually using each character's value in Unicode, where lowercase letters are after uppercase letters.

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True or false strings

Empty strings in JavaScript are considered false when compared with the == operator
(but not when using ===)

("" == false)  // true
("" === false) // false

Strings with a value are 'true', so you can do things like this:

if (someString) {
    // string has a value
} else {
    // string is empty or undefined
}