Say I have a function to pose an object from other objects, and I am passing arguments to the function - initially an object literal, and then the objects I want to pose to extent the object:

poseFunc({}, obj1, obj2, obj3);

The number of args passed is optional, how do I then pass the args to Object.assign() starting at the 2nd arg. So the function would resemble the following:

function poseObj(objs) {
    return Object.assign(arguments[1], arguments[2], arguments[3]... etc);
}

Thanks in advance :)

Say I have a function to pose an object from other objects, and I am passing arguments to the function - initially an object literal, and then the objects I want to pose to extent the object:

poseFunc({}, obj1, obj2, obj3);

The number of args passed is optional, how do I then pass the args to Object.assign() starting at the 2nd arg. So the function would resemble the following:

function poseObj(objs) {
    return Object.assign(arguments[1], arguments[2], arguments[3]... etc);
}

Thanks in advance :)

• javascript

• You have already passed argument from 2nd as arguments is an array of arguments passed to function; – itzmukeshy7 Commented Apr 16, 2016 at 10:11
• 1 @itzmukeshy7: It's not an array, just array-like (which is relevant to the question). But I think the example was just to show the concept of what they wanted to do. The issue being that the number of arguments is variable. – T.J. Crowder Commented Apr 16, 2016 at 10:12
• @le-moi arguments is an special array having all passed arguments in it; – itzmukeshy7 Commented Apr 16, 2016 at 10:13
• 1 @le-moi Why do you want to skip the first argument? In your example, it would be important not to, assuming poseObj and poseFunc are the same function. – T.J. Crowder Commented Apr 16, 2016 at 10:29
• If you're using ES2015, you might want to switch your accepted answer to Thomas'. Rest args are the way to go in ES2015. – T.J. Crowder Commented Apr 16, 2016 at 11:45

2 Answers 2

If you're using ES2015 rather than just a shim, you can use spread notation, Array.from, and slice:

function poseObj(objs) {
    return Object.assign(...Array.from(arguments).slice(1));
}

function poseObj(objs) {
    return Object.assign(...Array.prototype.slice.call(arguments, 1));
}

...see Thomas' answer using rest args, as that's the right way to do this in ES2015.

If you're not using ES2015, you can do the same thing with just a shimmed Object.assign via apply:

function poseObj(objs) {
    return Object.assign.apply(Object, Array.prototype.slice.call(arguments, 1));
}

There, we're using Function#apply instead of the spread operator (since ES5 and earlier don't have the spread operator). Function#apply calls the function you call it on using the first argument as this during the call, and using the array (or array-like thing) you give it as a second argument as the arguments for the call.

So say you have:

obj.foo(1, 2, 3);

The equivalent using Function#apply is:

obj.foo.apply(obj, [1, 2, 3]);

The first argument, obj, tells apply what to use as this during the call. The second argument is an array of arguments to use.

But if you use the spread operator, there's no need, it spreads out its array-like operand into discrete arguments.

ES2015:

function poseObj(objs, ...rest){
  return Object.assign(...rest);
}

rest will be a true array of all of the arguments starting from the second one.

And the Babel-output:

function poseObj(objs) {
  for (var _len = arguments.length, rest = Array(_len > 1 ? _len - 1 : 0), _key = 1; _key < _len; _key++) {
    rest[_key - 1] = arguments[_key];
  }

  return Object.assign.apply(Object, rest);
}

This seems to be a utility-function that might be used all over the place. As soon as you pass arguments to an other function like Array.prototype.slice() or Array.from(), poseObj won't be optimized by the JS-piler anymore. So, don't do that.

Everything else has already been said on T.J. Crowders Answer and the ments; just showing the better implementation.