Explain what the keyword this is used for in JavaScript。

In JavaScript, the keyword this is used to refer to the current execution context, which is usually the object that a function is a method of. The value of this depends on how a function gets called or executed.

When a function is called with the dot notation, this refers to the object that the function is a method of. For example, in the following code, this inside the function foo refers to the object obj:

var obj = {
  prop: 'Hello',
  foo: function() {
    console.log(this.prop);
  }
};

obj.foo(); // logs 'Hello'

When a function is called without a context or with the global context, this refers to the global object (in the browser, this is usually the window object). For example:

function bar() {
  console.log(this);
}

bar(); // logs the global object (e.g. window in the browser)

The value of this can also be explicitly set using the call, apply, or bind methods of a function.

In general, using the keyword this allows functions to be more flexible and reusable, because they can work with different objects and contexts.