How to conditionally add properties to a javascript object literal

Question

I am trying to do the following to satisfy the requirements of a code builder (Sencha Cmd to be specific).

This is the essence I what I need to do. The critical factor is that the function body MUST end with a return of an object literal. I cant return a variable due to restrictions in the builder. So, how to add a property 'b' at the point of the pseudo code below if the parameter 'includeB' is true, but NOT add a property AT ALL if it is false. ie b==undefined or b==null is not allowed.

Perhaps it is not possible.

function create(includeB) {
        // Can have code here but the final thing MUST be a return of the literal.
        // ...
    return {
        a : 1
        // pseudo code:
        // if (includeB==true) then create a property called b 
        // and assign a value of 2 to it. 
        // Must be done right here within this object literal
    }
}

var obj = create(false);
// obj must have property 'a' ONLY

var obj = create(true);
// obj must have properties 'a' and 'b'

Thanks for reading and considering,

Murray

Answer 1

If you can use ES6, use the spread properties.

function create(includeB) {
    return {
        a : 1,
        ...(includeB ? { b: 2 } : {}),
    };
}

Answer 2

You've pretty much shown a use case for a constructor function instead of using an object literal:

function CustomObject(includeB) {
    this.a = 1;
    if (includeB) {
        this.b = 2;
    }
}

//has `a` only
var obj1 = new CustomObject(false);

//has `a` and `b`
var obj2 = new CustomObject(true);

---

After re-reading your question it appears that you've got limited access in modifying the function. If I'm understanding your question correctly you can only change a limited portion of the script:

function create(includeB) {
    // modifications may be done here

    // the rest may not change
    return {
        a : 1
    }
}

var obj = create(false);
// obj must have property 'a' ONLY

var obj = create(true);
// obj must have properties 'a' and 'b'

If that's the case, then you could simply skip the later part of the function:

function create(includeB) {
    if (includeB) {
        return {
            a: 1,
            b: 2
        };
    }
    return {
        a: 1
    };
}

Answer 3

You cannot put boolean logic inside a javascript literal definition. So, if your builder requires the the returned object can ONLY be defined as a javascript literal, then you cannot define properties conditionally that way.

---

If you can create an object inside your function, modify that object using logic and then return that object, then that's pretty easy.

function create(includeB) {
    var x = {
        a: 1
    };
    if (includeB) {
        x.b = 2;
    }
    return x;
}

---

Your other option would be to wrap the create function and do it outside the create function.

function myCreate(includeB) {
    var x = create(includeB)
    if (includeB) {
        x.b = 2;
    }
    return x;
}

---

Or, you could even wrap the create function transparently so callers still use create(), but it's behavior has been altered.

var oldCreate = create;
create = function(includeB) {
    var x = oldCreate(includeB);
    if (includeB) {
        x.b = 2;
    }
    return x;
}

Answer 4

I recently had to do this, and found you could use a self-calling function within an object's definition (if using ES6). This is similar to the accepted answer, but might be useful for others who need to do this without first defining a constructor function.

For example:

let obj = (() => {
  let props = { a: 1 };
  if ( 1 ) props.b = 2;
  return props;
})();

makes the object: { a: 1, b: 2 }

It's handy for more complicated objects, keeping the construction continuous:

let obj = {
  a: 1,
  b: (() => {
    let props = { b1: 1 };
    if ( 1 ) props.b2 = 2;
    return props;
    })(),
  c: 3
}

makes the object:

{
  a: 1,
  b: {
    b1: 1,
    b2: 2
  },
  c: 3
}

Answer 5

You could define it later:

var hasA = create(); // has hasA.a

var hasBoth = create();
hasBoth.b = 2; //now has both

Alternatively, using your argument in create:

function create (includeB) {
  var obj = { 
    a : 1
  };
  if (includeB) {
     obj.b = 2;
  }
  return obj;
}

Answer 6

How about this:

function create(includeB) {
    return includeB && { a:1, b:2 } || { a:1 };
}

When includeB is true, the create function will return {a:1, b:2}. If includeB is false, it will return whatever is after the or - in this case, the {a:1} object.

create(true) returns { a:1, b:2 }.

create(false) returns { a:1 }

Answer 7

Below should work. I hope this help.

function create(includeB){

var object = {
    a: 1
};

if (includeB)
    object.b = 2;

return object;

}

Answer 8

If you would like to use a declaration to satisfy the same requirement once without too much bloat, you can also simply do the following:

var created = function(includeB) {
    var returnObj = { a : 1 };

    if(includeB) { returnObj.b = 2; }

    return returnObj;
}}(); //automatically runs and assigns returnObj to created