String.prototype.concat() vs String.prototype.includes()
String.prototype.concat() and String.prototype.includes() are both JavaScript String APIs, and choosing between them is a common point of confusion. This page compares their syntax, parameters, return values, and typical use cases so you can pick the right one. String.prototype.concat(): Concatenates strings together. Its syntax is str.concat(str2, ..., strN). String.prototype.includes(): Checks if a string contains a specified substring. Its syntax is str.includes(searchString, position), and it returns a boolean. Example of String.prototype.concat(): "Hello ".concat("World") // "Hello World". Example of String.prototype.includes(): "Hello World".includes("World") // true. Read the full references for String.prototype.concat() and String.prototype.includes() to see edge cases, time complexity, browser support, and the mistakes developers most often make with each. Other String APIs worth comparing: String.prototype.charAt(), String.prototype.charCodeAt(), String.prototype.indexOf(), String.prototype.lastIndexOf(), String.prototype.match(), String.prototype.matchAll(). Both belong to the String category in the JS Reference documentation, which covers 35 entries in total. Try both in the JavaScript playground to see the behavioral difference live.