ThisArg

ThisArg is a conventional name for a value that should be used as the this context when a function is invoked. It is not a language keyword, but a common parameter name used by JavaScript standard library methods that accept callbacks, such as array iteration or searching methods, as well as by the Function.prototype methods call, apply, and bind.

In many methods that take a callback, the second argument is thisArg. When the callback is invoked by the method, thisArg is used as the this value inside the callback. Arrow functions, which close over their surrounding this, ignore thisArg.

Function.prototype.call and Function.prototype.apply explicitly set a function’s this value by passing thisArg as the first argument. Function.prototype.bind returns a new function that, when called, uses thisArg as its this value. In both cases, if thisArg is a primitive value, it is boxed into an object; if it is null or undefined, the behavior depends on strict mode.

If no thisArg is provided, the this inside a regular function call depends on strict mode: undefined in strict mode, or the global object in non-strict mode. In module code or many environments, the default can be undefined.

This concept helps manage context and scope in JavaScript functional patterns. See also Function.prototype.call, Function.prototype.apply, Function.prototype.bind, and array methods that accept a callback with an optional thisArg.