當我使用一個變量進行比較時，Javascript比這個失敗了

Question

當我將Math.PI的比較右側替換爲我的時候，我有兩個'if less then'塊似乎不適合我變量，this.bottomChainAngleRads。

語境：我動畫鏈兩個齒輪之間，所以迭代的兩個齒輪隱藏/顯示它的鏈接，他們旋轉

代碼早些時候，該變量初始化所用牙齒數學，而不是一個字符串。

this.bottomChainAngleRads = Math.PI + 2 * Math.atan2(...); 

然後我想用它做的東西每過一段時間：

this.step = function() { 
    console.log('this.bottomChainAngleRads = ' + this.bottomChainAngleRads // Just over PI. Usually about 3.4. 
       + ' ' + $.isNumeric(this.bottomChainAngleRads));   // Always true. 

    // Counting the passing through each if block. Expecting a little in each. 
    var frontDisplay = 0, frontHide = 0, rearDisplay = 0, rearHide = 0; 

    $(this.frontGear.div).find('.geartooth').each(function(index, el) { 
    var totalRadians = measureRotation(el); 
    console.log('front totalRadians = ' + totalRadians + ' ' // From 0 to TWO_PI 
       + (totalRadians < this.bottomChainAngleRads)); // Always false. WTF. 
    if (totalRadians < this.bottomChainAngleRads) { // <================ FAILS. NEVER TRUE. 
    // if (totalRadians < Math.PI) { // MOSTLY CORRECT, but expectedly off by minor angle. 
     ++frontDisplay; 
     // .. do stuff 
    } else { 
     ++frontHide; 
     // .. do other stuff 
    } 
    }); 

    $(this.rearGear.div).find('.geartooth').each(function(index, el) { 
    var totalRadians = measureRotation(el); 
    console.log('rear totalRadians = ' + totalRadians + ' '  // From 0 to TWO_PI 
       + (totalRadians < this.bottomChainAngleRads)); // Always false. WTF. 
    if (totalRadians < this.bottomChainAngleRads) { // <================ FAILS. NEVER TRUE. 
    // if (totalRadians < Math.PI) { // MOSTLY CORRECT, but expectedly off by minor angle. 
     ++rearHide; 
     // .. do stuff 
    } else { 
     ++rearDisplay; 
     // .. do other stuff 
    } 
    }); 

    // Below I expected approximately a 50/50 split on each gear. Instead, I get... 
    console.log('front: ' + frontDisplay + ', ' + frontHide  // Nothing, All. 
       + '; rear: ' + rearDisplay + ', ' + rearHide); // All, Nothing 
} 

對不起，冗長的代碼，而是因爲我覺得我已經嘗試了這麼多東西，我想給更大的圖片。

Answer 1

each回調的內部的環境是不是你的對象實例了，this點個人DOM元素（.geartooth），這顯然不具有財產bottomChainAngleRads。

最簡單的解決方法是保存正確的上下文引用。試試這個：

var self = this; 
$(this.rearGear.div).find('.geartooth').each(function(index, el) { 
    var totalRadians = measureRotation(el); 
    if (totalRadians < self.bottomChainAngleRads) { 
    ... 
});