Javascript-typeof不适用于动态对象

Question

When none of the following child objects have been created

if(typeof this.obj[child1][child2]!=="undefined"){
}

The above does not work however this does

if(typeof this.obj[child1]!=="undefined"){
}

Answer 1

this is because the priority of typeof is lower than that of [] or . , so the [] is executed first and throws an error:

> typeof foo == "undefined"
true
> typeof foo.bar == "undefined"
ReferenceError: foo is not defined

To check long chains of nested properties you can use a function like this:

function hasKeys(obj, keys) {
    for(var i = 0; i < keys.length; i++) {
        if(typeof obj[keys[i]] == "undefined")
            return false;
        obj = obj[keys[i]];
    }
    return true;
}

if(hasKeys(this, ["obj", "child1", "child2"])) ...

or better, handle an exception:

try {
    val = this.obj['foo']['bar']['baz'];
} catch(err) {
    // deal with undefined val
}

Answer 2

You don't really need the typeof ... 'undefined' to determine if this.obj[child1][child2] is not undefined . Instead use:

if (this.obj && this.obj[child1] && this.obj[child1][child2]) {}

As in:

var obj = {};
obj.c1 = {};
alert( obj && obj.c1 && obj.c1.c2 ? 'obj.c1.c2 exists' : 'nope'); 
     //=> "nope"
obj.c1.c2 = 1;
alert( obj && obj.c1 && obj.c1.c2 ? 'obj.c1.c2 exists' : 'nope'); 
     //=> "obj.c1.c2 exists"

You could create a function to determine the existence of a certain path in any Object :

function pathExists(root,path){
  var pathx = path.constructor === Array && path || path.split(/\./)
    , trial
    , pathok = true
  ;

  while (pathx.length && pathok) {
    trial = pathx.shift();
    pathok = (root = root && root[trial] || false, root);
  }
  return pathok;
}
// usage examples
var obj = {c1:1};
pathExists(obj,'c1.c2'); //=> false
pathExists(obj,'c1'); //=> 1
obj.c1 = { c2: {c3: 3} };
pathExists(obj,['c1','c2','c3']); //=> 3
// your case
if ( pathExists(this,[obj,child1,child2]) ) { /*...*/ }

Answer 3

use this for the first case -

if(this.obj[child1] && typeof this.obj[child1][child2]!=="undefined"){
}

Answer 4

This doesn't work because it will throw a JavaScript error. It will throw this.obj[child1] is undefined. You Should always check if it's defined too.

if(this.obj[child1] && typeof this.obj[child1][child2]!=="undefined"){
}

EDIT

If you want to increase your code readability, you could use a function like this:

var test = {child1: {child2: "TEST"}};
alert(is_defined(test, 'child1', 'child2')); // true
alert(is_defined(test, 'child1', 'child2', 'child3')); // false

function is_defined(obj) {
    if (typeof obj === 'undefined') return false;

    for (var i = 1, len = arguments.length; i < len; ++i) {
        if (obj[arguments[i]]) {
            obj = obj[arguments[i]];
        } else if (i == (len - 1) && typeof obj[arguments[i] !== 'undefined']) {
            return true;
        } else {
            return false;
        }
    }

    return true;
}

Answer 5

使用undefined代替“ undefined”或尝试null。