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It's not that simple. You've mentioned only one of the possible meanings of `this`: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators

by toomim 9y ago

It's not that simple. You've mentioned only one of the possible meanings of `this`: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe... In the global execution context (outside of any function), this refers to the global object whether in strict mode or not. // In web browsers, the window object is also the global object: console.log(this === window); // true a = 37; console.log(window.a); // 37 this.b = "MDN"; console.log(window.b) // "MDN" console.log(b) // "MDN" Inside a function, the value of this depends on how the function is called. == Simple call == Since the following code is not in strict mode, and because the value of this is not set by the call, this will default to the global object, which is "window" in a browser. function f1() { return this; } // In a browser: f1() === window; // true // In Node: f1() === global; // true In strict mode, however, the value of this remains at whatever it was set to when entering the execution context, so, in the following case, this will default to undefined: function f2() { 'use strict'; // see strict mode return this; } f2() === undefined; // true So, in strict mode, if this was not defined by the execution context, it remains undefined. == The bind method == ECMAScript 5 introduced Function.prototype.bind. Calling f.bind(someObject) creates a new function with the same body and scope as f, but where this occurs in the original function, in the new function it is permanently bound to the first argument of bind, regardless of how the function is being used. function f() { return this.a; } var g = f.bind({a: 'azerty'}); console.log(g()); // azerty var h = g.bind({a: 'yoo'}); // bind only works once! console.log(h()); // azerty == Arrow functions == In arrow functions, this retains the value of the enclosing lexical context's this. In global code, it will be set to the global object: var globalObject = this; var foo = (() => this); console.log(foo() === globalObject); // true Note: if thisArg is passed to call, bind, or apply on invocation of an arrow function it will be ignored. You can still prepend arguments to the call, but the first argument (thisArg) should be set to null. No matter what, foo's this is set to what it was when it was created. The same applies for arrow functions created inside other functions: their this remains that of the enclosing lexical context. == As an object method == When a function is called as a method of an object, its this is set to the object the method is called on. In the following example, when o.f() is invoked, inside the function this is bound to the o object. var o = { prop: 37, f: function() { return this.prop; } }; console.log(o.f()); // 37 Note that this behavior is not at all affected by how or where the function was defined. In the previous example, we defined the function inline as the f member during the definition of o. However, we could have just as easily defined the function first and later attached it to o.f. Doing so results in the same behavior: var o = {prop: 37}; function independent() { return this.prop; } o.f = independent; console.log(o.f()); // 37 This demonstrates that it matters only that the function was invoked from the f member of o. Similarly, the this binding is only affected by the most immediate member reference. In the following example, when we invoke the function, we call it as a method g of the object o.b. This time during execution, this inside the function will refer to o.b. The fact that the object is itself a member of o has no consequence; the most immediate reference is all that matters. == "this" on the object's prototype chain == The same notion holds true for methods defined somewhere on the object's prototype chain. If the method is on an object's prototype chain, this refers to the object the method was called on, as if the method were on the object. == "this" with a getter or setter == Again, the same notion holds true when a function is invoked from a getter or a setter. A function used as getter or setter has its this bound to the object from which the property is being set or gotten. == As a constructor == When a function is used as a constructor (with the new keyword), its this is bound to the new object being constructed. While the default for a constructor is to return the object referenced by this, it can instead return some other object (if the return value isn't an object, then the this object is returned). == As a DOM event handler == When a function is used as an event handler, its this is set to the element the event fired from (some browsers do not follow this convention for listeners added dynamically with methods other than addEventListener). == In an in–line event handler == When code is called from an in–line on-event handler, its this is set to the DOM element on which the listener is placed: <button onclick="alert(this.tagName.toLowerCase());"> Show this </button> The above alert shows button. Note however that only the outer code has its this set this way: <button onclick="alert((function() { return this; })());"> Show inner this </button> In this case, the inner function's this isn't set so it returns the global/window object (i.e. the default object in non–strict mode where this isn't set by the call).

spion 9y ago
So theargument has a default value when in non strict mode， and like all other arguments you can partially apply it with bind. Got it. Everything else, still explained by ”argument passed left of the dot at call time“ Oh yeah and arrow functions dont have a this argument so whatever this was already in scope is the binding.
- jdmichal 9y ago
  I'm a little amazed that you read all that, and still think it's so simple. Here's a simple counter argument to your "left of the dot" hypothesis: setTimeout(foo.doStuff, 0); When `doStuff` executes, `this` will not point to `foo`, even though it was "left of the dot". This same counter argument applies to every usage of a function as a first-class value.
  - spion 9y ago
    Because I said left to the dot at "call time". More succintly: > When you call a function, whatever is left of the dot is passed as the `this` argument The analogy is pretty good - you can't pass an argument to a function until you actually call it. In your example, the function hasn't been called yet, so no argument has been passed. foo.doStuff // <-- syntax to access a field member foo.doStuff() // syntax to call a field member with a `this` argument passed left of the dot. Furthermore, on the receiving side, the setTimeout function is seeing: function setTimeout(fn, delay) { // just fn, but not the argument. } Which makes it clear that you can no longer call that function with the proper `this` argument. My point was that `this` is only as confusing as you make it, and there is a simple and straightforward way to teach it (function argument). If you do that, the dozens of seemingly special cases go down to just a couple. Its unfortunate we don't yet have the bind operator https://github.com/tc39/proposal-bind-operator https://github.com/tc39/proposal-bind-operator If we had it, the explanation could be made even clearer. You would be able to call free-standing functions with a `this` argument left of the double colon, i.e. `thisArgument::freeStandingFunction()` - then the dot method call syntax `thisArgument.method()` could be explained as a syntax sugar for `thisArgument::(thisArgument.method)()` - a syntax sugar to at the same time access a function attached as a member field of an object and call it with that object as the `this` argument. The bind operator would make all those other `this` cases seem less special too. Of course you can call various bits of code with whatever `this` argument you want. The event handler code? `someElement::eventHandlerCode(event)`. A constructor? `Object.create(constructor.prototype)::constructor()`. Nothing special here, just a function argument, can have different values in different functions, depending what was passed as `this`. Its bizarre that this operator got stuck in stage 0 and is being subsumed by the pipeline operator (a terrible match for JavaScript). Looks like TC39 isn't interested in untangling the mess.

by toomim 9y ago

spion 9y ago

So theargument has a default value when in non strict mode， and like all other arguments you can partially apply it with bind. Got it. Everything else, still explained by ”argument passed left of the dot at call time“ Oh yeah and arrow functions dont have a this argument so whatever this was already in scope is the binding.

jdmichal 9y ago
I'm a little amazed that you read all that, and still think it's so simple. Here's a simple counter argument to your "left of the dot" hypothesis: setTimeout(foo.doStuff, 0); When `doStuff` executes, `this` will not point to `foo`, even though it was "left of the dot". This same counter argument applies to every usage of a function as a first-class value.
- spion 9y ago
  Because I said left to the dot at "call time". More succintly: > When you call a function, whatever is left of the dot is passed as the `this` argument The analogy is pretty good - you can't pass an argument to a function until you actually call it. In your example, the function hasn't been called yet, so no argument has been passed. foo.doStuff // <-- syntax to access a field member foo.doStuff() // syntax to call a field member with a `this` argument passed left of the dot. Furthermore, on the receiving side, the setTimeout function is seeing: function setTimeout(fn, delay) { // just fn, but not the argument. } Which makes it clear that you can no longer call that function with the proper `this` argument. My point was that `this` is only as confusing as you make it, and there is a simple and straightforward way to teach it (function argument). If you do that, the dozens of seemingly special cases go down to just a couple. Its unfortunate we don't yet have the bind operator https://github.com/tc39/proposal-bind-operator https://github.com/tc39/proposal-bind-operator If we had it, the explanation could be made even clearer. You would be able to call free-standing functions with a `this` argument left of the double colon, i.e. `thisArgument::freeStandingFunction()` - then the dot method call syntax `thisArgument.method()` could be explained as a syntax sugar for `thisArgument::(thisArgument.method)()` - a syntax sugar to at the same time access a function attached as a member field of an object and call it with that object as the `this` argument. The bind operator would make all those other `this` cases seem less special too. Of course you can call various bits of code with whatever `this` argument you want. The event handler code? `someElement::eventHandlerCode(event)`. A constructor? `Object.create(constructor.prototype)::constructor()`. Nothing special here, just a function argument, can have different values in different functions, depending what was passed as `this`. Its bizarre that this operator got stuck in stage 0 and is being subsumed by the pipeline operator (a terrible match for JavaScript). Looks like TC39 isn't interested in untangling the mess.