== vs === in JavaScript: loose vs strict equality

=== (strict equality) is true only when both operands have the same type and value. == (loose equality) first converts the operands to a common type, which produces results such as 0 == '' being true. Use === by default; the one common exception is x == null, which matches both null and undefined.

Side by side

node -e '
const cases = [["0", "\"\""], ["0", "\"0\""], ["\"\"", "\"0\""], ["null", "undefined"], ["NaN", "NaN"], ["[]", "false"], ["[1]", "1"]];
for (const [a, b] of cases) {
  const A = eval(a), B = eval(b);
  console.log(`${a.padEnd(9)} == ${b.padEnd(9)} ${String(A == B).padEnd(5)}   === ${A === B}`);
}'

0         == ""        true    === false
0         == "0"       true    === false
""        == "0"       false   === false
null      == undefined true    === false
NaN       == NaN       false   === false
[]        == false     true    === false
[1]       == 1         true    === false

Criterion by criterion

When == is acceptable

value == null is a compact test for "null or undefined":

node -e 'for (const v of [null, undefined, 0, "", false]) console.log(JSON.stringify(v) ?? "undefined", v == null)'

null true
undefined true
0 false
"" false
false false

For everything else, compare explicitly with ===. Use Number.isNaN(x) to detect NaN and Object.is(a, b) when -0 and +0 must be told apart.