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Is there a null-coalescing (Elvis) operator or safe navigation operator in javascript?

Posted by: admin November 29, 2017 Leave a comment

Questions:

I’ll explain by example:

Elvis Operator (?: )

The “Elvis operator” is a shortening
of Java’s ternary operator. One
instance of where this is handy is for
returning a ‘sensible default’ value
if an expression resolves to false or
null. A simple example might look like
this:

def gender = user.male ? "male" : "female"  //traditional ternary operator usage

def displayName = user.name ?: "Anonymous"  //more compact Elvis operator

Safe Navigation Operator (?.)

The Safe Navigation operator is used
to avoid a NullPointerException.
Typically when you have a reference to
an object you might need to verify
that it is not null before accessing
methods or properties of the object.
To avoid this, the safe navigation
operator will simply return null
instead of throwing an exception, like
so:

def user = User.find( "admin" )           //this might be null if 'admin' does not exist
def streetName = user?.address?.street    //streetName will be null if user or user.address is null - no NPE thrown
Answers:

You can use the logical ‘OR’ operator in place of the Elvis operator:

For example displayname = user.name || "Anonymous" .

But Javascript currently doesn’t have the other functionality. I’d recommend looking at CoffeeScript if you want an alternative syntax. It has some shorthand that is similar to what you are looking for.

For example The Existential Operator

zip = lottery.drawWinner?().address?.zipcode

Function shortcuts

()->  // equivalent to function(){}

Sexy function calling

func 'arg1','arg2' // equivalent to func('arg1','arg2')

There is also multiline comments and classes. Obviously you have to compile this to javascript or insert into the page as <script type='text/coffeescript>' but it adds a lot of functionality 🙂 . Using <script type='text/coffeescript'> is really only intended for development and not production.

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Javascript’s logical OR operator is short-circuiting and can replace your “Elvis” operator:

var displayName = user.name || "Anonymous";

However, to my knowledge there’s no equivalent to your ?. operator.

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I think the following is equivalent to the safe navigation operator, although a bit longer:

var streetName = user && user.address && user.address.street;

streetName will then be either the street name or null/undefined.

If you want it to default to something else you can combine with the above shortcut or to give:

var streetName = (user && user.address && user.address.street) || "Unknown Street";

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I’ve occasionally found the following idiom useful:

a?.b.?c

can be rewritten as:

((a||{}).b||{}).c

This takes advantage of the fact that getting unknown attributes on an object returns undefined, rather than throwing an exception as it does on null or undefined, so we replace null and undefined with an empty object before navigating.

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i think lodash _.get() can help here, as in _.get(user, 'name'), and more complex tasks like _.get(o, 'a[0].b.c', 'default-value')

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For the former, you can use ||. The Javascript “logical or” operator, rather than simply returning canned true and false values, follows the rule of returning its left argument if it is true, and otherwise evaluating and returning its right argument. When you’re only interested in the truth value it works out the same, but it also means that foo || bar || baz returns the leftmost one of foo, bar, or baz that contains a true value.

You won’t find one that can distinguish false from null, though, and 0 and empty string are false values, so avoid using the value || default construct where value can legitimately be 0 or "".

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This is more commonly known as a null-coalescing operator. Javascript does not have one.

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I have a solution for that, tailor it to your own needs, an excerpt from one of my libs:

    elvisStructureSeparator: '.',

    // An Elvis operator replacement. See:
    // http://coffeescript.org/ --> The Existential Operator
    // http://fantom.org/doc/docLang/Expressions.html#safeInvoke
    //
    // The fn parameter has a SPECIAL SYNTAX. E.g.
    // some.structure['with a selector like this'].value transforms to
    // 'some.structure.with a selector like this.value' as an fn parameter.
    //
    // Configurable with tulebox.elvisStructureSeparator.
    //
    // Usage examples: 
    // tulebox.elvis(scope, 'arbitrary.path.to.a.function', fnParamA, fnParamB, fnParamC);
    // tulebox.elvis(this, 'currentNode.favicon.filename');
    elvis: function (scope, fn) {
        tulebox.dbg('tulebox.elvis(' + scope + ', ' + fn + ', args...)');

        var implicitMsg = '....implicit value: undefined ';

        if (arguments.length < 2) {
            tulebox.dbg(implicitMsg + '(1)');
            return undefined;
        }

        // prepare args
        var args = [].slice.call(arguments, 2);
        if (scope === null || fn === null || scope === undefined || fn === undefined 
            || typeof fn !== 'string') {
            tulebox.dbg(implicitMsg + '(2)');
            return undefined;   
        }

        // check levels
        var levels = fn.split(tulebox.elvisStructureSeparator);
        if (levels.length < 1) {
            tulebox.dbg(implicitMsg + '(3)');
            return undefined;
        }

        var lastLevel = scope;

        for (var i = 0; i < levels.length; i++) {
            if (lastLevel[levels[i]] === undefined) {
                tulebox.dbg(implicitMsg + '(4)');
                return undefined;
            }
            lastLevel = lastLevel[levels[i]];
        }

        // real return value
        if (typeof lastLevel === 'function') {
            var ret = lastLevel.apply(scope, args);
            tulebox.dbg('....function value: ' + ret);
            return ret;
        } else {
            tulebox.dbg('....direct value: ' + lastLevel);
            return lastLevel;
        }
    },

works like a charm. Enjoy the less pain!

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Answers:

Here’s a simple elvis operator equivalent:

function elvis(object, path) {
    return path ? path.split('.').reduce(function (nestedObject, key) {
        return nestedObject && nestedObject[key];
    }, object) : object;
}

> var o = { a: { b: 2 }, c: 3 };
> elvis(o)

{ a: { b: 2 }, c: 3 }

> elvis(o, 'a');

{ b: 2 }

> elvis(o, 'a.b');

2

> elvis(o, 'x');

undefined

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You can achieve roughly the same effect by saying:

var displayName = user.name || "Anonymous";

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You could roll your own:

function resolve(objectToGetValueFrom, stringOfDotSeparatedParameters) {
    var returnObject = objectToGetValueFrom,
        parameters = stringOfDotSeparatedParameters.split('.'),
        i,
        parameter;

    for (i = 0; i < parameters.length; i++) {
        parameter = parameters[i];

        returnObject = returnObject[parameter];

        if (returnObject === undefined) {
            return undefined;
        }
    }
    return returnObject;
};

And the use it like this:

var result = resolve(obj, 'a.b.c.d'); 

* result is undefined any one of a, b, c or d is undefined

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This was an interesting solution for the safe navigation operator using some mixin..

http://jsfiddle.net/avernet/npcmv/

  // Assume you have the following data structure
  var companies = {
      orbeon: {
          cfo: "Erik",
          cto: "Alex"
      }
  };

  // Extend Underscore.js
  _.mixin({ 
      // Safe navigation
      attr: function(obj, name) { return obj == null ? obj : obj[name]; },
      // So we can chain console.log
      log: function(obj) { console.log(obj); }
  });

  // Shortcut, 'cause I'm lazy
  var C = _(companies).chain();

  // Simple case: returns Erik
  C.attr("orbeon").attr("cfo").log();
  // Simple case too, no CEO in Orbeon, returns undefined
  C.attr("orbeon").attr("ceo").log();
  // IBM unknown, but doesn't lead to an error, returns undefined
  C.attr("ibm").attr("ceo").log();

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Personally i use

function e(e,expr){try{return eval(expr);}catch(e){return null;}};

and for example safe get:

var a = e(obj,'e.x.y.z.searchedField');