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“This” in JavaScript

bjorn.tipling.com

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Re: “This” in JavaScript

#71

Post by ulisesrmzroche »

DonHopkins wrote: post #49
VieElm wrote: post #27

Earlier quoted context omitted.

>In JS this is just another object, like any other object. You are mistaken, "this" is not an object nor a variable, it's a keyword and behaves differently than variable scope. Consider closures for example.

And the tragic thing about "this" being a magic keyword that looks up a dynamic value in the runtime, instead of a normal lexically scoped variable like it appears, is that 90% of the time "this" is exactly the thing you want a closure to close over, so you have to do ridiculous stuff like "var self = this;". Even knowing this rule very well and being totally hard core OCO (Obsessive Compulsive ORDER, not Disorder) a…

You don't have to do that, you can tell a function what to use by using 'call' or 'apply'. Some people even use bind, but I think it adds too much line noise.

Re: “This” in JavaScript

#72

Post by vog »

ulisesrmzroche wrote: post #70
vog wrote: post #46

The "this" keyword in JavaScript is a well-known anti-pattern. Although it can be explained with a few rules, it becomes cumbersome quickly. The anti-pattern around "this" is well-known for decades. For example, SICP explicitly mentions that anti-pattern. Of course, it doesn't refer to JavaScript, but to some other old programming language which had a keyword with very similar issues. I think it's not a hyperbole to…

What? No, it's not an anti-pattern. It's about understanding functional scoping, JavaScript doesn't use lexical scoping.

Not sure where you got this from. JavaScript does use lexical scoping for almost everything, except for "this".

Re: “This” in JavaScript

#73

Post by romaniv »

xiaoma wrote: post #31
couchand wrote: post #16

The author critically misunderstands the way that `this` gets set, and in doing so has developed a significantly overcomplicated mental model. This is exemplified in the statement: You can use this in any function on an object to refer to other properties on that object. This is not the same as an instance created with new. ... Note, there was no use of new , no Object.create and no function called to create obj. Thi…

Even more simply, I'd just say: 1) The keyword "this" refers to whatever is left of the dot at call-time. 2) If there's nothing to the left of the dot, then "this" is the root scope (e.g. Window). 3) A few functions change the behavior of "this"— bind , call and apply 4) The keyword "new" binds this to the object just created So if you're using the value of "this" in someFunction... thing.someFunction(); // this ===…

I see that some people are slowly redefining the word "simple" as applied to programming languages. A 4-point bullet list that still requires knowing about typical caller behavior is hardly "simple".

Re: “This” in JavaScript

#74

Post by vcarl »

romaniv wrote: post #73
xiaoma wrote: post #31

Earlier quoted context omitted.

Even more simply, I'd just say: 1) The keyword "this" refers to whatever is left of the dot at call-time. 2) If there's nothing to the left of the dot, then "this" is the root scope (e.g. Window). 3) A few functions change the behavior of "this"— bind , call and apply 4) The keyword "new" binds this to the object just created So if you're using the value of "this" in someFunction... thing.someFunction(); // this ===…

I see that some people are slowly redefining the word "simple" as applied to programming languages. A 4-point bullet list that still requires knowing about typical caller behavior is hardly "simple".

"Predictable" then.

Re: “This” in JavaScript

#75

Post by subpixel »

xiaoma wrote: post #31
couchand wrote: post #16

The author critically misunderstands the way that `this` gets set, and in doing so has developed a significantly overcomplicated mental model. This is exemplified in the statement: You can use this in any function on an object to refer to other properties on that object. This is not the same as an instance created with new. ... Note, there was no use of new , no Object.create and no function called to create obj. Thi…

Even more simply, I'd just say: 1) The keyword "this" refers to whatever is left of the dot at call-time. 2) If there's nothing to the left of the dot, then "this" is the root scope (e.g. Window). 3) A few functions change the behavior of "this"— bind , call and apply 4) The keyword "new" binds this to the object just created So if you're using the value of "this" in someFunction... thing.someFunction(); // this ===…

Thanks for that, it's succinct and useful.

Re: “This” in JavaScript

#76

Post by eric_h »

I've used this gist: https://gist.github.com/erichummel/2595398 a few times in interviews to see just how much of a javascript whiz candidates were.

The basic gist of the interview question is "what is output to the console when this code is run?"

It's obviously intentionally obfuscated, so even if the candidate got it wrong (most did), subsequently talking through the solution told me a lot about how good a candidate was at static analysis and talking through a problem.

Re: “This” in JavaScript

#77

Post by DonHopkins »

marrs wrote: post #66
DonHopkins wrote: post #49

Earlier quoted context omitted.

And the tragic thing about "this" being a magic keyword that looks up a dynamic value in the runtime, instead of a normal lexically scoped variable like it appears, is that 90% of the time "this" is exactly the thing you want a closure to close over, so you have to do ridiculous stuff like "var self = this;". Even knowing this rule very well and being totally hard core OCO (Obsessive Compulsive ORDER, not Disorder) a…

I'd say I use `this` outside of lexical context at least as much as I do within, and I rely on this's behaviour a lot when developing a DSL. It enables some rather elegant patterns, actually.

Patterns which could have been just as easily implemented using another more explicit technique for dynamic binding (like simply passing the dynamic parameter to the function as a normal argument) without fucking up the entire language design with invisible implicit magic, and dooming millions of programmers to repeatedly make difficult to detect, subtle mistakes, bugs and security holes.

Please give me one example of using "this" outside of lexical context, that is impossible or tangibly inconvenient to do by some other technique that JavaScript already supports.

Re: “This” in JavaScript

#78

Post by DonHopkins »

ulisesrmzroche wrote: post #71
DonHopkins wrote: post #49

Earlier quoted context omitted.

And the tragic thing about "this" being a magic keyword that looks up a dynamic value in the runtime, instead of a normal lexically scoped variable like it appears, is that 90% of the time "this" is exactly the thing you want a closure to close over, so you have to do ridiculous stuff like "var self = this;". Even knowing this rule very well and being totally hard core OCO (Obsessive Compulsive ORDER, not Disorder) a…

You don't have to do that, you can tell a function what to use by using 'call' or 'apply'. Some people even use bind, but I think it adds too much line noise.

You don't have to do what? Make mistakes? Of course I don't have to make mistakes, but I do all the time, and so do other people.

You have to REMEMBER that you need to explicitly and inconveniently tell the function which "this" to use, by drilling down into Function.prototype.call/apply, even though it LOOKS like it will simply work like any other variable, which it doesn't.

Please consider that programmers are not perfect, and do not have infinite time or unlimited memory or perfectly focused and undistracted attention, and programming languages are user interfaces for imperfect humans, that should minimize the likelihood of errors, not set traps for less experienced programmers to make obvious mistakes, just so that more experienced programmers can gloat over how much smarter they are for having memorized every detail of the language spec. (-; I'm looking at you, automatically inserted semicolon! ;-)

And my point is that I AM an experienced programmer who has memorized every detail of the JavaScript language spec and uses it all the time, yet I STILL make "this" mistakes AGAIN and AGAIN. While on the other hand, I don't think I've ever made a "self" mistake with Python.

Re: “This” in JavaScript

#79

Post by DonHopkins »

acjohnson55 wrote: post #68
DonHopkins wrote: post #49

Earlier quoted context omitted.

And the tragic thing about "this" being a magic keyword that looks up a dynamic value in the runtime, instead of a normal lexically scoped variable like it appears, is that 90% of the time "this" is exactly the thing you want a closure to close over, so you have to do ridiculous stuff like "var self = this;". Even knowing this rule very well and being totally hard core OCO (Obsessive Compulsive ORDER, not Disorder) a…

I agree. The fact that you have to write library code defensively to still work depending on how your method's call, or that you have to use an awkward bind to preserve the `this` context of a method is so annoying and error-prone to me.

Even when you remember to perform the awkward bind / Function.prototype.call / Function.prototype.apply / var self = this; you still may already be inside of another context where this is not correctly defined. So then your code LOOKS even more like it will work thanks to the cargo cult magic ceremonies you've performed perfectly, but in the wrong place. It's like going into the bathroom like you're supposed to, but then taking a shit on the floor right next to the toilet, then flushing.

Re: “This” in JavaScript

#80

Post by indubitably »

eric_h wrote: post #76

I've used this gist: https://gist.github.com/erichummel/2595398 a few times in interviews to see just how much of a javascript whiz candidates were. The basic gist of the interview question is "what is output to the console when this code is run?" It's obviously intentionally obfuscated, so even if the candidate got it wrong (most did), subsequently talking through the solution told me a lot about how good a candidat…

Does `setTimeout` have a default value for the time?
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