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rated 0 times [  2] [ 2]  / answers: 1 / hits: 47868  / 13 Years ago, fri, august 12, 2011, 12:00:00

I'm trying to calculate a proportional height (while excluding a static height element) from a width that gets passed in via a request (defaults to 560).



However, wF.h evaluates to NaN. If I replace this.w with 560 it works, but not when trying to reference the w property of wF.



var wF = {

       w : 560,

       h : (312 - 42) / (560 / this.w) + 42

};


What gives?



I refuse to use two plain vars in succession, because I'm trying to get nice code out of JS.



Update:



Thanks to everyone who helped explain and solve my problem. I guess i'll just have to get used to that. I'll be setting the object up in stages to get on with the project, even though it still annoys me slightly ;). I found and read a nice article on the topic for anyone who stumbles upon similar issues: http://yehudakatz.com/2011/08/11/understanding-javascript-function-invocation-and-this/


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 Answers
95 
// Your code

var wF = {

       w : 560,

       h : (312 - 42) / (560 / this.w) + 42

};




this isn't what you think it is



Javascript has no block scope, only function scope: this inside the definition for wF does not refer to wF.



(And so this.w, whatever this is, is likely undefined. Dividing by undefined yields NaN.)



So then you might try:



// Let's not use `this`

var wF = {

       w : 560,

       h : (312 - 42) / (560 / wF.w) + 42

};




You haven't finished defining the object yet



However, you're still defining the object where you attempt to use wF.w: it's not ready for that yet.





Solution



So, yes, you will have to use two variables... or set up the object in stages:



// We can't even use `wF`; split up the property definitions

var wF = {};

wF.w = 560;

wF.h = (312 - 42) / (560 / wF.w) + 42;

[#90643] Thursday, August 11, 2011, 13 Years  [reply] [flag answer]
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