的Javascript獲取對象的屬性和子屬性

Question

我試圖訪問一個對象的屬性是這樣的：

var scope1 = {a: {value: 25}}; 
var scope2 = undefined; 

var v1 = scope1.b.value || 0; // TypeError: Cannot read property 'value' of undefined 
var v2 = scope2.b.value || 0; // TypeError: Cannot read property 'value' of undefined 

apparentl的||操作員不給我我想要的。我知道我可以做以下代替

var vv2 = (scope1 && scope1.b ? scope1.b.value : 0); 

但是這使得代碼確實冗長的...所以任何人都知道，如果有更簡單的方法做我想做的事？謝謝。

Answer 1

通常的答案是&&操作：

var v1 = scope1.b && scope1.b.value || 0; 

...因爲喜歡||，&&是curiously powerful：&&評估左側操作數，如果它的falsey，採用的是falsey值作爲其結果;如果左側操作數值爲真，則&&將計算右側操作數並將該值作爲結果。

所以如果scope1.b是undefined：

// We start with 
scope1.b && scope1.b.value || 0 
// which is 
undefined && scope1.b.value || 0 
// which is 
undefined || 0 
// which is 
0 

...但如果scope.b就像是一個對象引用一個truthy值：

// We start with 
scope1.b && scope1.b.value || 0 
// which is 
scope1.b.value || 0 
// which is 
scope1.b.value // *IF* scope1.b.value is truthy, or 
0    // If it isn't 

（回想一下， 「falsey」 值是undefined ，null,0,"",NaN，當然還有false;「truthy」值a重新所有其他）

例：

var scope1; 
 

 
scope1 = {a: "foo"}; 
 
snippet.log(scope1.b && scope1.b.value || 0); // 0 
 

 
scope1 = {a: "foo", b: {}}; 
 
snippet.log(scope1.b && scope1.b.value || 0); // 0 
 

 
scope1 = {a: "foo", b: {value: 42}}; 
 
snippet.log(scope1.b && scope1.b.value || 0); // 42

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 --> 
 
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

---

我不推薦它，但你可以寫一個函數爲這個來處理任意深度：

function resolve(obj, path, defValue) { 
 
    var rv = path.split(".").reduce(function(o, p) { 
 
    return o && o[p]; 
 
    }, obj); 
 
    return rv || defValue; 
 
} 
 

 
var scope1; 
 

 
scope1 = {a: "foo"}; 
 
snippet.log(resolve(scope1, "b.value", 0)); // 0 
 

 
scope1 = {a: "foo", b: {}}; 
 
snippet.log(resolve(scope1, "b.value", 0)); // 0 
 

 
scope1 = {a: "foo", b: {value: 42}}; 
 
snippet.log(resolve(scope1, "b.value", 0)); // 42 
 

 
scope1 = {a: "foo", b: {}}; 
 
snippet.log(resolve(scope1, "b.baz.boz.value", 0)); // 0 
 

 
scope1 = {a: "foo", b: {baz: {boz: {value: 67}}}}; 
 
snippet.log(resolve(scope1, "b.baz.boz.value", 0)); // 67

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 --> 
 
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

即實現不支持括號符號，只是點符號，但你可以自由地寫resolve(obj, "foo.0.bar.42", 0)如果你有你需要的數字指標，因爲它不符合標準的JavaScript解析規則。 （或者將其擴展到支持[]就不會那麼難）。