JavaScript Patterns 2.7 Avoiding Implied Typecasting

Dealing with == and ===

false == 0 or "" == 0 return true.

always use the === and !==

operators that check both the values and the type of the expressions you compare:

var zero = 0;
if (zero === false) {
    // not executing because zero is 0, not false
}

// antipattern
if (zero == false) {
    // this block is executed...
}   

Avoiding eval()

// antipattern

var property = "name";
alert(eval("obj." + property));

// preferred

var property = "name";
alert(obj[property]);

Security implications (e.g. JSON response from an Ajax request)

1. For browsers that don‘t support JSON.parse() natively, you can use a library from JSON.org.

2. passing strings to setInterval(), setTimeout(), and the Function() constructor is, for the most part, similar to using eval()and therefore should be avoided.

// antipatterns

setTimeout("myFunc()", 1000);
setTimeout("myFunc(1, 2, 3)", 1000);

// preferred

setTimeout(myFunc, 1000);
setTimeout(function () {
    myFunc(1, 2, 3);
}, 1000);   

3. Using the new Function() constructor is similar to eval() and should be approached with care.

1. 1. If you absolutely must use eval(), you can consider using new Function() instead.
      Because the code evaluated in new Function() will be running in a local function scope, so any variables defined with var in the code being evaluated will not become globals automatically.
   2. Or wrap the eval() call into an immediate function.
      

      console.log(typeof un); // "undefined"
      
      console.log(typeof deux); // "undefined"
      
      console.log(typeof trois); // "undefined"
      
      var jsstring = "var un = 1; console.log(un);";
      
      eval(jsstring); // logs "1"
      
      jsstring = "var deux = 2; console.log(deux);";
      
      new Function(jsstring)(); // logs "2"
      
      jsstring = "var trois = 3; console.log(trois);";
      
      (function () {
      
          eval(jsstring);
      
      }()); // logs "3"
      
      console.log(typeof un); // "number"
      
      console.log(typeof deux); // "undefined"
      
      console.log(typeof trois); // "undefined" 

      
   3. No matter where you execute Function, it sees only the global scope. So it can do less local variable pollution.
      

       (function () {
      
          var local = 1;
      
          eval("local = 3; console.log(local)"); // logs 3
      
          console.log(local); // logs 3
      
      }());
      
      (function () {
      
          var local = 1;
      
          Function("console.log(typeof local);")(); // logs undefined
      
      }()); 

      

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JavaScript Patterns 2.7 Avoiding Implied Typecasting

原文：http://www.cnblogs.com/haokaibo/p/Avoiding-Implied-Typecasting.html