Check function parameters for existence and type in JavaScript

Question

Supposed I have a function defined such as:

var x = function (options, callback) { /* ... */ }

options needs to have properties foo and bar, where foo shall be of type number, and bar of type string.

So, basically, I could check this using the following code:

var x = function (options, callback) {
  if (!options) { throw new Error('options is missing.'); }
  if (!options.foo) { throw new Error('foo is missing.'); }
  if (!options.bar) { throw new Error('bar is missing.'); }
  if (!callback) { throw new Error('callback is missing.'); }
  // ...
}

But this only checks for existence, not yet for correct types. Of course I can add further checks, but this soon becomes lengthy, and not so well readable. Once we start talking about optional parameters, it gets a mess, with parameter-shifting and so on …

What is the best way to deal with this (supposed that you want to check it)?

UPDATE

To clarify my question: I know that there is the typeof operator, and I also know how to deal with optional parameters. But I have to do all these checks manually, and this - for sure - is not the best we can come up with.

The goal of my question was: Is there a ready-made function / library / whatever that you can tell that you are expecting five parameters of specific types, some mandatory, some optional, and the function / library / whatever does the checks and the mapping for you, so that all this comes down to a one-liner?

Basically, something such as:

var x = function (options, callback) {
  verifyArgs({
    options: {
      foo: { type: 'number', mandatory: true },
      bar: { type: 'string', mandatory: true }
    },
    callback: { type: 'function', mandatory: false }
  });
  // ...
};

You could use a loop and have an object tell the types: validateObj = { "foo":"number","bar":"text" } — mplungjan
– mplungjan, Commented Aug 26, 2013 at 7:58
Is there a pre-made function that does that? Of course it's possible to create it for yourself, but if there's already a widely used solution, I'd prefer sticking to it. — Golo Roden
– Golo Roden, Commented Aug 26, 2013 at 8:00
Please see the updated part of my question, a simple typeof is not what I want (and I am fully aware of this solution, but it soon becomes a mess, when you have lots of parameters you want to check). — Golo Roden
– Golo Roden, Commented Aug 26, 2013 at 8:03
@GoloRoden With vanilla JavaScript, you're on your own for this. You could give TypeScript a try, which handles many of the type checks. — Jonathan Lonowski
– Jonathan Lonowski, Commented Aug 26, 2013 at 8:04

Martijn · Accepted Answer · 2013-08-26 07:57:49Z

1

javascript has typeof

console.log( typeof '123' ); // string
console.log( typeof 123 ); // number
console.log( typeof undefinedVar); // undefined


var x = function (options, callback) {
  if (typeof options =='undefined' { throw new Error('options is missing.'); }
  if (typeof options.foo !='number') { throw new Error('foo is missing.'); }
  if (typeof options.bar !='string') { throw new Error('bar is missing.'); }
  if (typeof callback!= 'function') { throw new Error('callback is missing.'); }
  // ...
}

answered Aug 26, 2013 at 7:57

Martijn

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Comments

xdazz · Accepted Answer · 2013-08-26 07:58:49Z

1

You could do like this:

var x = function (options, callback) {
    var options = options || {};
    if (typeof options.foo != 'number') {
        throw new Error('foo need to be a number.');
    }
    if (typeof options.bar != 'string') {
        throw new Error('bar need to be a string.');
    }
    // ...
}

answered Aug 26, 2013 at 7:58

xdazz

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Comments

fusio · Accepted Answer · 2013-08-26 07:58:54Z

0

Use typeof

// Numbers
typeof 37 === 'number';
typeof 3.14 === 'number';
typeof Math.LN2 === 'number';
typeof Infinity === 'number';
typeof NaN === 'number'; // Despite being "Not-A-Number"
typeof Number(1) === 'number'; // but never use this form!

...

answered Aug 26, 2013 at 7:58

fusio

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2 Comments

cHao Over a year ago

typeof new Number(3.14) === 'object'.

Felix Kling Over a year ago

"Despite being "Not-A-Number"" Just to make sure: This is not an error in JS (like typeof null), this is in compliance with the IEEE 754 standard. NaN is a special number value.

CoursesWeb · Accepted Answer · 2013-08-26 08:02:45Z

0

See these examples for checking the type, string or integer:

if(typeof(somevar) === 'string') alert('string');
if(typeof(somevar)==="number" && Math.round(somevar) == somevar) alert('is int');

answered Aug 26, 2013 at 8:02

CoursesWeb

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2 Comments

mplungjan Over a year ago

typeof somevar is the preferred syntax. It is an operator and not a function

Martijn Over a year ago

Also, the Match.round(), why? Maybe 12.4 should be correct too (though typeof might need a change to float). This is an action which doesn't add allot and does slow down

HIRA THAKUR · Accepted Answer · 2013-08-26 08:09:15Z

0

if (myObj.hasOwnProperty(myProp)) { //to check whether mentioned property exist
    if (typeof myObj[myProp] == "string") { //or integer
        // your code to be executed!!!
    } else {
        //ERROR!!!
    }
}

edited Aug 26, 2013 at 8:09

answered Aug 26, 2013 at 8:00

HIRA THAKUR

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Comments

urzeit · Accepted Answer · 2013-08-26 08:27:47Z

0

While this is not exactly the idea behind javascript's type model, I usually used a for-loop to iterate over the data members of my to-check-object:

/* (.startsWith has to be implementes somehow) */
for (key in my_object_with_data) {
    if (!key.startsWith(typeof my_object_with_data[key]+"_") {
        /* The member "key" has wrong type. */
    }
}

The members of the object have to be named number_a for example to check it's type for being a number.

Additional validation can be done via some generic check methods, that can be applied inside the for loop according to the typeof value.

edited Aug 26, 2013 at 8:27

answered Aug 26, 2013 at 8:20

urzeit

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Comments

Golo Roden · Accepted Answer · 2013-08-26 08:53:23Z

0

Okay, I found a library that does what I want by myself, it's called ArgueJS:

function range(){ 
  arguments = __({start: [Number, 0], stop: Number, step: [Number, 1]})

  for(var i = arguments.start; i < arguments.stop; i += arguments.step)
    console.log(i);
}

answered Aug 26, 2013 at 8:53

Golo Roden

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