JavaScript, elegant way to check nested object pro

问题:

This question already has an answer here:

• Test for existence of nested JavaScript object key 48 answers

a "problem" which i have every now and then is that i have an object e.g. user = {} and through the course of using the app this gets populated. Let's say somwhere, after an AJAX call or something i do this:

user.loc = {
    lat: 50,
    long: 9
}

At another place i want to check if user.loc.lat exists.

if(user.loc.lat) {
    // do something
}

If it does not exists, this will cause an error. If user.loc.lat is undefined, user.loc of course is undefined as well.

"Cannot read property 'lat' of null" - Dev Tools error

That means I need to check it like this:

if(user.loc) {
    if(user.loc.lat) {
        // do something
    }
}

or

if(user.loc && user.loc.lat) {
    // do something
}

This isn't really pretty and the bigger my objects are the worse it gets - obviously (imagine 10 levels of nesting). It kind bums me that if(user.loc.lat) isn't just returning false if user.loc is undefined as well.

What's the ideal way to check situations like this?

回答1:

You can use an utility function like this:

get = function(obj, key) {
    return key.split(".").reduce(function(o, x) {
        return (typeof o == "undefined" || o === null) ? o : o[x];
    }, obj);
}

Usage:

 get(user, 'loc.lat')     // 50
 get(user, 'loc.foo.bar') // undefined

Or, to check only if a property exists, without getting its value:

has = function(obj, key) {
    return key.split(".").every(function(x) {
        if(typeof obj != "object" || obj === null || ! x in obj)
            return false;
        obj = obj[x];
        return true;
    });
}

if(has(user, 'loc.lat')) ...

回答2:

Well, javascript has try-catch. Depending on what you actually need to do (i.e. what your else statement would look like if it's undefined), that may be what you want.

example:

try {
   user.loc.lat.doSomething();
} catch(error) {
   //report
}

回答3:

You can combine the checks using lazy and:

if(user.loc && user.loc.lat) { ...

Or, you use CoffeeScript and write

user.loc?.lat ...

which would run the checks for loc property and safeguard against empty objects.

回答4:

Try this if(user && user.loc && user.loc.lat) {...}

You can check value of null and undefined using typeof

If .loc has value false than you can try

if(user && user.loc && typeof(user.loc)!=="undefined"){...}

If you have a huge nested object than have a look at

Source.

function checkNested(obj /*, level1, level2, ... levelN*/) {
  var args = Array.prototype.slice.call(arguments),
      obj = args.shift();

  for (var i = 0; i < args.length; i++) {
    if (!obj.hasOwnProperty(args[i])) {
      return false;
    }
    obj = obj[args[i]];
  }
  return true;
}

var test = {level1:{level2:{level3:'level3'}} };

checkNested(test, 'level1', 'level2', 'level3'); // true
checkNested(test, 'level1', 'level2', 'foo'); // false

Update: Try lodash.get