Javascript eval()用法及代碼示例

eval()是全局對象的函數屬性。

• eval()函數的參數為​​字符串。
• 如果字符串表示一個表達式，則eval()評估該表達式。如果參數代表一個或多個JavaScript語句，則eval()將評估這些語句。
• 我們不調用eval()來評估算術表達式； JavaScript自動評估算術表達式。

用法:

eval(string)
參數：
String:
A string representing a JavaScript expression, statement, or 
sequence of statements. The expression can include variables 
and properties of existing objects.

返回值：
The completion value of evaluating the given code is 
returned by using eval(). 
If the completion value is empty, undefined is returned. 

例子：

Input:eval(new String('2 + 2'));
Output:returns a String object containing "2 + 2"

Input:eval(new String('4 + 4'));
Output:returns a String object containing "4 + 4"

下麵的程序將說明eval()的用法：函數更深入：程序1：

<script> 
    // JavaScript to illustrate eval() function 
    function func() { 
  
        // Original string 
        var a = 2; 
        var b = 2; 
  
        // Finding the sum 
        var value = eval(new String(a + b)); 
        document.write(value); 
    } 
// Driver Code 
func(); 
< /script>                    

輸出：

4

程序2:：

<script> 
    // JavaScript to illustrate eval() function 
    function func() { 
  
        // Original string 
        var a = 4; 
        var b = 4; 
  
        // Finding the multiplication 
        var value = eval(new String(a * b)); 
        document.write(value); 
    } 
// Driver code 
func(); 
< /script>                    

輸出：

16