如何为通过JavaScript中的对象文字模拟的命名函数参数设置默认值？

Question

I'd like to use named parameters (simulated via object literals) in my JavaScript functions for ease of readability. The code I've written works fine when I explicitly pass my parameters, but I'm having trouble trying to set a default value, so that my function can be called not only via as myFunction({someBoolean:true}) OR as myFunction() which would imply a default of someBoolean:false.

myFunction: function({myArgs}){
    myArgs.someBoolean = myArgs.someBoolean || false;
    if (myArgs.someBoolean) {
        [do stuff]
    }
    [do other stuff]
},

This works fine when I call myFunction({someBoolean:true}), but when I call myFunction(), I get "Uncaught TypeError: Cannot read property 'someBoolean' of undefined."

Thanks in advance!

Answer 1

Note the previous answer wouldn't work if you wanted multiple default arguments and some were provided:

Doing myFunction({otherArg: 'hello'}) wouldn't necessarily set myArgs.someBoolean to false.

Rather you should change it the below change:

myFunction: function(myArgs){ // Switch to regular argument vs {myArgs}
    myArgs = myArgs || {}; // Make sure it provided

    myArgs.someBoolean = myArgs.someBoolean || false;
    myArgs.otherArg = myArgs.otherArg || 'defaultValue'
    if (myArgs.someBoolean) {
        [do stuff]
    }
    [do other stuff]
},

Then this could be called with: something.myFunction({someBoolean: true, otherArg: '...'})

Answer 2

function x({myArgs}){}

is used for object destructuring, such that you can use myArgs as a variable inside the function when an object with the myArgs property is being passed. For example:

function x({myArgs}){
  console.log(myArgs);
}
x({myArgs: 5}); // logs 5

However, you're accessing the boolean like an object property of myArgs ( myArgs.someBoolean ). Then you should use this instead:

function x(myArgs = {someBoolean: false}){
  console.log(myArgs.someBoolean);
}
x({someBoolean: true}); // logs true
x(); // logs false

This now matches your case. If you want to use someBoolean directly, use this:

function x({someBoolean} = {someBoolean: false}){
  console.log(someBoolean);
}
x({someBoolean: true}); // logs true
x(); // logs false

Another option is

function x({someBoolean = false, someOtherBoolean = true} = {}){
  console.log(someBoolean);
}
x({someBoolean: true}); // logs true
x(); // logs false

Reference on MDN .

Answer 3

The proper way would be

 function f({someBoolean = false} = {}){ console.log(someBoolean) } f(); // false f({}); // false f({someBoolean: true}); // true 

That is,

• If no parameter is passed, fallback to {} .

• Destruct the parameter or the fallback, extracting the someBoolean property as a variable. If there is no such property, fallback to false .

Answer 4

myFunction: function({someBoolean = false}){
    if (someBoolean) {
        [do stuff]
    }
    [do other stuff]
},