javascript the 'with' statement-优快云博客

本文链接：https://blog.youkuaiyun.com/iteye_13882/article/details/82396883

The with() {} statement is meant to provide convenience.

and there are some golden laws about the with() {} statement.

it only use/assign existing properties
it does not affect what is 'this'
the scoped variable inside with statement take precedence over others with the same name
it slows things down.,

let's first see some simple example that shows something about the with statement.

/**************************************
*@Summary
* this shows the simple use of with statement
*
* the key rule about the use of the statement is that it 
*   1. does not affect what is 'this' context. 
*   2. it does not help create properties that is not in the 'with' scoped variable. You can only use and assign existing properties in the with() {} statement
*   3. within the scope of the statement, the properties introduced by with(){} take precedence over the other variables.
*   4. and it slows things down (not sure where comes the extra overhead)
* @Usage:
*   
* @TODO:
* test it 
***************************************/

var use = "other";
var katana = {
  isSharp: true,
  use: function () {
    this.isSharp = !this.isSharp;
  }
};

//with (katana) {
//  assert(true, "you can still call outside mehtods.");

//  isSharp = false;

//  use();
//  // this illustrate the fact that local with variable take precedence
//  assert(use !== "other", "Use is a function, from the kantana object");

//  assert(this !== katana, "This isn't changed - it keeps its original value");
//}

// commentted out, to test it , copy the following code to the test html and do the test
//assert(typeof isSharp === "undefined", "Outside the with, the properties don't exist.");
//assert(katana.isSharp, "Verify that the method was used correctly in the with.");

and below is its test cases.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
    <script src="../unit.js" type="text/javascript"></script>
    <script src="simplewith.js" type="text/javascript"></script>
    <script type="text/javascript">
      window.onload = function () {
        test("with(){} doest not change what 'this' is. and it simplifies things", function () {
          with (katana) {
            assert(true, "you can still call outside mehtods.");

            isSharp = false;

            use();
            // this illustrate the fact that local with variable take precedence
            assert(use !== "other", "Use is a function, from the kantana object");

            assert(this !== katana, "This isn't changed - it keeps its original value");
          }

          assert(typeof issharp === "undefined", "outside the with, the properties don't exist.");
          assert(katana.isSharp, "verify that the method was used correctly in the with.");
        });

        test("you cannot access non-existing properties inside with(){} statement", function () {
          with (katana) {
            isSharp = false;
            cut = function () {
              isSharp = false;
            };
          }
          assert(!katana.cut, "The new cut property wasn't introduced here.");
          assert(this.cut, "It was made as a member variable to the anonymous function that contains the test instead.");
        });
      };
    
    </script>

    <style type="text/css">
    #results li.pass {color: Green}
    #results li.fail { color: Red}
    </style>

</head>
<body>
<ul id="results"
</body>
</html>

also, with the 'with' statement, you can chang the way how the code is written to simulate some object-oriented javascript code.

/**************************************
*@Summary
* this shows how you can use the with statement to facilitate writting some OO like code. 
*
* the key rule about why the use of the statement has the advantage is that 
*   1.it introduce a local scope
*   2. you can have something like private and public, which is missing in non-with statement
*   3. you can access the private and public through names alike inside the with statement, but outside the visibility is different.
* @Usage:
*   
* @TODO:
* test it 
***************************************/

function Ninja(){with(this){
  // Private Information
  var cloaked = false;
  // Public property
  this.swings = 0;
  // Private Method
  function addSwing(){
    return ++swings;
  }
  // Public Methods
  this.swingSword = function(){
    cloak( false );
    return addSwing();
  };
  this.cloak = function (value) {
    return value != null ?
    cloaked = value :
    cloaked;
  };
} 
}

// below is the test code.
var ninja = new Ninja();
assert(!ninja.cloak(), "We start out uncloaked.");
assert(ninja.swings == 0, "And with no sword swings.");
assert(ninja.cloak(true), "Verify that the cloaking worked correctly.");
assert(ninja.swingSword() == 1, "Swing the sword, once.");
assert(!ninja.cloak(), "The ninja became uncloaked with the sword swing.");