Var's boolean value in JavaScript

Question

I am not so expert in JavaScript and I am wonder if there is a way to test the boolean value of a variable in javascript.

In Python I can do this:

>>>list_var = []
>>>bool(list_var)
False  # This is the boolean value of a empty list
>>>

And if I try get an element in JS that does not exist, i.e:

document.getElementById('b-advanced')
[]  // This is what returns

Is there a way to test the expression above as boolean without using an if... statement?

EDIT

I think I need to point something.

This is the full expression I use:

angular.element(document.getElementById('b-advanced'))

getElementById would never return an array. It either returns a DOM element or null. — Felix Kling
– Felix Kling, Commented Jul 14, 2015 at 21:12
Well, I am using that expression inside a angular.element() function. — Gocht
– Gocht, Commented Jul 14, 2015 at 21:13
You are rigth, they don't return the same. postimg.org/image/wix57vk55 — Gocht
– Gocht, Commented Jul 14, 2015 at 21:22

Ed Ballot · Accepted Answer · 2015-07-14 21:20:43Z

2

There is not. The best you can do is list_var.length === 0. For a full test, you'd want to test as follows

// Return true if arg is an array or string containing at least one item
function isTrueness(arg) {
    return !!(arg && arg.length > 0);
}

angular.element(isTrueness(document.getElementById('b-advanced')));

answered Jul 14, 2015 at 21:20

Ed Ballot

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1 Comment

Gocht Over a year ago

this a good approach. I come from python and does not make sense for me that an empty array is equivalent to true.

Richard Kho · Accepted Answer · 2015-07-14 21:07:03Z

1

You can use !! in front which will return its boolean status.

!![] // true
!!null // false

answered Jul 14, 2015 at 21:07

Richard Kho

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5 Comments

Gocht Over a year ago

So, an empty array is equivalent to true?

Richard Kho Over a year ago

Correct! See Conflicting boolean values of an empty Javascript Array for a great explanation of this :)

Gocht Over a year ago

This is really unexpected for me. Thanks for your answer.

Richard Kho Over a year ago

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Gocht Over a year ago

I think this is the simplest way. No functions, no if statements.

Zimmi · Accepted Answer · 2015-07-14 21:06:27Z

1

var x = Boolean(document.getElementById('b-advanced'));

Boolean is a wrapper for boolean values.

answered Jul 14, 2015 at 21:06

Zimmi

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Comments

Sir Neuman · Accepted Answer · 2015-07-14 21:04:21Z

0

you can test to see what type a variable is by using typeof so for example:

var myVar = true;
if (typeof myVar === "boolean") console.log("It is a boolean");

answered Jul 14, 2015 at 21:04

Sir Neuman

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6 Comments

underscore_d Over a year ago

Note that, for some reason, the OP specifically wants to avoid using an if statement. However, I'm not sure this is a possibility. Anyway, if(emptyVar) will equate to false if the variable is empty by the various loose definitions JS employs.

Gocht Over a year ago

Thanks for your answer, but that's not exactly what I am asking for. In JS this: [] is equivalent to false?

Sir Neuman Over a year ago

I see... you're using getElementyById to get that value. Which as pointed out by someone else, would only return either null or a DOM element so i'm not sure in which scenario it would even return an array which is what [] is in JS

Felix Kling Over a year ago

@Gocht: every object is "truthy" in JavaScript, so no.

Sir Neuman Over a year ago

are you maybe trying to get the text or an attribute of the element?

|

Teepeemm · Accepted Answer · 2015-07-14 21:39:10Z

0

In Python, an empty list is false. In Javascript, an empty array is true. You can mimic this by saying myArray.length && myArray. If myArray is empty, then its length is zero and the expression is false. If myArray isn’t empty, then Javascript returns the second operand, just like Python.

answered Jul 14, 2015 at 21:39

Teepeemm

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