• To see whether a string is greater than another, JavaScript uses the so-called “dictionary” or “lexicographical” order.
  • In other words, strings are compared letter-by-letter.
  • The algorithm to compare two strings is simple:
  1. Compare the first character of both strings.
  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.
     • Otherwise, if both strings’ first characters are the same, compare the second characters the same way.
  3. If both strings end at the same length, then they are equal.
     • Otherwise, the longer string is greater.
• When comparing values of different types, JavaScript converts the values to numbers.

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

let a = 0;
console.log(Boolean(a)); // false
let b = "0";
console.log(Boolean(b)); // true
console.log(a == b); // true

Strict Equality

• A regular equality check == cannot differentiate 0 from false.
  • The same thing happens with an empty string.
• This happens because operands of different types are converted to numbers by the equality operator.

console.log(0 == false); // true
console.log('' == false); // true

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

console.log( 0 === false ); // false, because the types are different

Comparison with null and undefined

• There’s a non-intuitive behavior when null or undefined are compared to other values.
• null and undefined are equal to each other (in the sense of ==), but not any other value.

console.log(null == undefined); // true

• An equality check == and comparisons > < >= <= work differently.
  • When it comes to math comparisons, null/undefined are converted to numbers.
    • null becomes 0, while undefined becomes NaN.
    • That’s why null >= 0 is true and null > 0 is false.
  • On the other hand, the equality check == for undefined and null is predefined to be equal to each other and don’t equal anything else.
    • That’s why null == 0 is false.
• In general, the value undefined shouldn’t be compared to other values.
  • undefined gets converted to NaN and NaN returns false for all comparisons.
  • The equality check of undefined only equals to null and no other value.
• Generally, treat any comparison with undefined/null except the strict equality === with extra care.
  • 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.

console.log(NaN == NaN); // false
console.log(undefined == undefined); // true
console.log(null == null); // true
console.log(undefined == null); // true