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Wrapper Objects Back


  • we can use properties to an object, a value or a method
var s = "Aloha";
s.substring(s.indexOf("A"), s.length);    //s.length (value)
                                        //s.indexOf() (method)
  • there are no wrapper objects for the null and undefined values, which will cause a TypeError when attempt to access.
  • strings, numbers and boolean values behave like objects when you try to read the value of a property from them. But if you attempt to set the value of a property, that attempt is silently ignored.
var s = "test";
s.len = 4;        //s.len undefined
var t = s.len;    //t = undefined
  • all string methods that appear to return a modified string are, infact, returning a new string value, cause string is immutable.
var s = "Aloha"
s.toUpperCase();    //return a new string, but dosen't change s
s                    // => "Aloha"
  • Object is mutable
var o = {x: 1};  
o.x = 2;        //change the value of x
o.y = 3;        //set a new property y
  • Array is also mutable
var a = [1, 2, 3];
a[0] = 0;    //Change the value of the first element
a[3] = 4;    //Add new element into the array


  • ===: 嚴格相等
  • ==: 相等
var a = 3;
var b = "3";
a == b            // => true: only when the values are the same
a === b            // => false: cause the type is not the same
  • distinct Array and Object are never equal, but equal when two reference points to the same thing.
var a = [], b = [];
a == b            // => false
a === b            // => false

var a = {x: 1}, b = {x: 1};
a == b            // => false
a === b            // => false

var a = [];
var b = a;
a == b            // => true
a === b            // => true
  • when wanting to compare two arrays by checking their values, we can write a function.
function equalArray(a, b){
    if(a.length != b.length)             //first check length
        return false;
    for(var i = 0; i < a.length; i++)        //second check member
        if(a[i] !== b[i])
            return false;
    return true;
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