using bool without if and var expressions in javascript

Question

What string console && (typeof console.log === "function") && console.log('contains called'); is doing in this function:

(function ($) {
$.validator.addMethod('contains', function (value, element, param) {
    console && (typeof console.log === "function") && console.log('contains called');
    if (this.optional(element)) { return true; } // let required rule deal with this.

    var pattern = new RegExp('' + param, "gi");
    return value && ('' + value).match(pattern);
}, "Part of the word is invalid");

$.validator.unobtrusive.adapters.add('contains', function (options) {
    var element = options.element,
            message = options.message;
    options.rules['contains'] = $(element).attr('data-val-contains-word');
    if (options.message) {
        options.messages['contains'] = options.message;
    }
});
})(jQuery);

I would understand if that string was like: var var1 = console && (typeof console.log === "function") && console.log('contains called');

or like: if(console && (typeof console.log === "function") && console.log('contains called'))

It's not a buggy code. It's working.

Guffa · Accepted Answer · 2013-02-09 10:32:13Z

1

It's just a less readable way of writing:

if (console && (typeof console.log === "function")) {
  console.log('contains called');
}

It (mis)uses the fact that you can write an expression that doesn't do anything with the result, and that a statement is also an expression, so it can be written inside an expression to do something.

answered Feb 9, 2013 at 10:32

Guffa

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TweeZz · Accepted Answer · 2013-02-09 10:20:25Z

1

It just writes something to the browsers console.. But only if that console is available and if it has a 'log' function. The first 2 conditions can prevent the third from running, thus not causing a js error in case console.log(..) does not exist.

answered Feb 9, 2013 at 10:20

TweeZz

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Johannes Egger · Accepted Answer · 2013-02-09 10:23:03Z

1

The runtime checks the first condition. If there isn't a console, it stops checking the other conditions because false && ... is in every case false. If there is a console the runtime checks if console.log is a function and if so it logs something to the console. This is called short-curcuit-evaluation. Otherwise the runtime would always try to execute each condition which would lead to runtime errors if e.g. there were no console object.

answered Feb 9, 2013 at 10:23

Johannes Egger

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Frederik.L · Accepted Answer · 2013-02-09 10:36:17Z

0

It's doing the same as

if (console && (typeof console.log === "function")) {
    console.log('contains called');
}

The interpreter will stop as soon as something is false so console.log won't be called if console doesn't have a log function. It won't check if this function exist neither if console doesn't even exist.

answered Feb 9, 2013 at 10:36

Frederik.L

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