String .replace() / replaceAll()

// Replaces only the first match.
str.replace("search string", "replacement string")
str.replace(/regex/, "replacement string")

// Replaces all matches.
str.replaceAll("search string", "replacement string")

var str = "I like cats. cats are cute.";

// replace replaces only the first match.
console.log(str.replace("cats", "dogs")); // Outputs: "I like dogs. cats are cute."

// replaceAll replaces all matches.
console.log(str.replaceAll("cats", "dogs")); // Outputs: "I like dogs. dogs are cute."

// Using a regex with the g flag also replaces all occurrences.
console.log(str.replace(/cats/g, "dogs")); // Outputs: "I like dogs. dogs are cute."

// Using a regex with the i flag to replace case-insensitively.
var mixed = "Hello hello HELLO";
console.log(mixed.replace(/hello/gi, "Hi")); // Outputs: "Hi Hi Hi"

// The original string is not modified.
console.log(str); // Outputs: "I like cats. cats are cute."

string.replace() replaces only the first match. When passing a plain string, note that only the first occurrence is replaced. To replace all occurrences, use string.replaceAll() or pass a regular expression with the g flag to string.replace().

string.replaceAll() is a relatively new method added in ES2021 that replaces all matches at once. The result is equivalent to using a regex with the g flag, but string.replaceAll() is more readable if you are not familiar with regular expressions.

Both methods leave the original string unchanged and return a new string with the replacements applied. To use the result, assign it to a variable or use it directly.