javascript：测试对象属性长度

Question

I have some objects with properties. I wanted to test to see if they had characters in them so I initially wrote this:

if (MyObject.Prop1.length > 0) {....}

However, sometimes the object may not have a certain property so I was getting the error "cannot get length".

I changed it by writing this:

if (MyObject.Prop1 && MyObject.Prop1.length > 0) {....}

I'm using the chrome inspector and when I run the code, I don't get the error anymore. Is this going to work in every browser?

Thanks.

Answer 1

As an alternative:

if ('Prop1' in MyObject && MyObject.Prop1.length > 0) { ... )

Or, to be even more careful:

if (MyObject.hasOwnProperty('Prop1') && MyObject.Prop1.length > 0) { ... }

Of course that might be the wrong thing to do, depending on the nature of "MyObject".

Answer 2

Yes it will work quite fine, although you can save yourself the > 0 and just do

if (MyObject.Prop1 && MyObject.Prop1.length) {....}

since anything other than zero will evaluate to true.

Answer 3

Since undefined , 0 , and "" are all "falsy" in JavaScript, this is equivalent:

if(MyObject.Prop1) {
  // ...
}

Rick Waldron's "Idiomatic JavaScript" is a good reference for simplifying conditional statements without sacrificing correctness. You can test its use yourself:

function testProp(val) {
  if(val) {
    return val.length;
  }

  return "nope!";
}

var myObj = { stringProp   : "foo",
              emptyStrProp : "",
              // undefinedProp is not defined
              zeroProp     : 0,
              twelveProp   : 12,
              otherObjProp : document.createElement('div'),
              arrayProp    : [ 'a', 'b' ]
            };

console.log( testProp( myObj.stringProp    ) ); // => 3
console.log( testProp( myObj.emptyStrProp  ) ); // => "nope!"
console.log( testProp( myObj.undefinedProp ) ); // => "nope!"

// of course if you're expecting values other than strings and undefined
// you'll have to account for them

console.log( testProp( myObj.zeroProp      ) ); // => "nope!"
console.log( testProp( myObj.twelveProp    ) ); // => undefined
console.log( testProp( myObj.otherObjProp  ) ); // => undefined
console.log( testProp( myObj.arrayProp     ) ); // => 2