的JavaScript /打字稿對象空檢查

Question

是否有性能上的差異，而在下面2種方式做null檢查 -

if (!someObject) { 
    // dosomething 
} 

VS

if (someObject != null) { 
    // dosomething 
} 

Answer 1

!someObject檢查所有falsy值。

Not (empty string, undefined, null, 0, false) - will all pass the condition 

其中作爲第一個條件只檢查null。

if (someObject !== null) { 
    console.log('falsey'); 
} 

someObject = null;  // no message in console 
someObject = '';  // falsey 
someObject = undefined; // falsey 
someObject = 0;   // falsey 
someObject = false;  // falsey 

Falsey檢查

if (!someObject) { 
    console.log('falsey'); 
} 

someObject = null;  // no message in console 
someObject = '';  // no message in console 
someObject = undefined; // no message in console 
someObject = 0;   // no message in console 
someObject = false;  // no message in console 

Answer 2

@Sushanth - ！你是不是100％正確，

變量檢查 '假'，但確確實實，請閱讀https://developer.mozilla.org/pl/docs/Web/JavaScript/Referencje/Obiekty/null

!null // true 

所以如果

!null // true 
//then 
null // false 

;）

請做一個簡單的測試：

var a_null = null; //false 
var b_null = !null //true 

console.log(typeof(a_null)); //object, 'null' is an object. 
console.log(typeof(b_null)); //boolean, now it is not an object. 

if (a_null) var test_a = 1; // 'a_null' is false, so 'test_a' is 'Undefined'. 
if (!a_null) var test_b = 2; // '!a_null' is true, so 'test_b = 2'. 

if (b_null) var test_c = 3; // 'b_null' is true, so 'test_c = 3'. 
if (!b_null) var test_d = 4; // '!b_null' is false, so 'test_d' is 'Undefined'. 

console.log('TEST_A =',test_a,', TEST_B =',test_b,', TEST_C=',test_c,', TEST_D=',test_d); //TEST_A = Undefined, TEST_B = 2, TEST_C = 3 TEST_D = Undefined 

最良好的祝願。