如果对象存在则调用方法的Javascript简写

Question

I have a variable and if that variable is a object I would like to call a method on that object, if not I want to do nothing.

I'm wondering if there is any reason why I shouldn't do it like this.

var foo = null;

  ////////////////////////////////////////////////
  // some code that could change foo to a object
  ////////////////////////////////////////////////

 foo && foo.bar();

Answer 1

With ES6, you can combine optional chaining operator with call :

foo?.bar?.call()

If you want to pass arguments, keep in mind that the first one in call assigns this in function call

 var foo = { bar: function(x) { return x; } }; first.value = foo?.bar?.call(0,42); // 42 second.value = foo?.baz?.call(0,42); // undefined, no error

 <input id="first"> <input id="second">

Answer 2

The quick answer is yes, foo && foo.bar() won't throw an exception if foo is null , and if foo is non-null, bar() will be evaluated, and it's value will be the value of the expression.

Longer answer is that any value can be interpreted as a boolean, in the sense that every value is either truthy or falsey, and that the boolean operators do short-circuit evaluation -- left to right, if we see a false && or a true || , there's no reason to carry on evaluating.

One last fact is that the value of boolean expression is the value of the expression where the short-circuit happened.

Answer 3

You need to assign an object to foo and a property with a function.

 var foo; foo = {}; foo.bar = function () { console.log('inside bar'); }; foo && foo.bar && foo.bar();

Answer 4

((typeof foo === 'object') && foo.bar())

or

(!(typeof foo == 'object') || foo.bar())

If foo is of type object then execute.

See my answer here javascript using or operator to iterate over list of strings aswell.

This is a "common" issue for unintended behavior, if some uses assignments inside such a sequence and forgets that evaluation stops in the ||sequence after first true , or stops after first false in the && .