What is the 'this' keyword in JavaScript?

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this is an implicit parameter every function receives. Its value is determined at call time , not at definition time, and it points to whichever object invoked the function. Understanding this means understanding the different binding rules that govern which object it ends up referencing. The four binding rules Default binding — a plain function call. In non-strict mode this is the global object ( window or globalThis ); in strict mode it is undefined . Implicit binding — called as a method. this is the object to the left of the dot: obj.fn() → this is obj . Explicit binding — call , apply , and bind force this to a specific object. new binding — when a function is invoked with new , this is the freshly created instance. Examples of each rule function show() { console.log(this); } show(); // global (or undefined in strict mode) const obj = { show }; obj.show(); // obj show.call({ x: 1 }); // { x: 1 } function Person(name) { this.name = name; } const p = new Person("Ada"); // p.name === "Ada" Arrow functions Arrow functions do not have their own this . They capture this from the surrounding lexical scope, which makes them ideal for callbacks that should preserve the outer context. class Counter { constructor() { this.n = 0; setInterval(() => { this.n++; }, 1000); // lexical this → Counter } } Losing this Passing a method as a callback strips its implicit binding. Fix it with .bind() or by wrapping in an arrow function. const user = { name: "Ada", greet() { console.log(this.name); } }; setTimeout(user.greet, 100); // undefined setTimeout(user.greet.bind(user), 100); // "Ada" setTimeout(() => user.greet(), 100); // "Ada" Precedence of the rules If multiple rules could apply, the engine picks in this order: new > explicit > implicit > default. This is why a bound function still ignores .call() and why new wins over bind for the instance's this . In modules and classes In ES modules, top-level this is undefined . Inside class methods this follows the same rules but almost always refers to the instance, since classes run in strict mode. Know the 4 rules in priority order: new > explicit (call/apply/bind) > implicit (obj.method) > default (global/undefined). The 'lost context' gotcha (assigning method to variable) is the most asked. Arrow functions inheriting this is the practical fix. Show call vs apply vs bind differences.