Develop Reference

OR in ternary operator, using &&

Using the traditional if statement I can do this:
if(a===0 || b===0) {console.log('aloha amigo')};
But when I try to do something the same thing with a ternary operator, like this:
a===0 || b===0 && console.log('aloha amigo')
I just get errors about unexpected ||.
According to this answer: Precedence: Logical or vs. Ternary operator, we can do it using
condition1 || condition2 ? do if true : do if false
(Sorry I'm not sure how to call the ? : symbols in this case), but I'm not sure how to get it running using && (It means only run the code if returned true).
I created a codepen to test it easily. Here's the whole code:
var a = 0;
var b = 1;
a===0 || b===0 ? console.log('Works here') : console.log('And here');
a===0 || b===0 && console.log('Doesn\'t work here');
a===0 && console.log('The && works for a single test');
Here's the link

Just take parenthesis to prevent operator precedence of && over ||
(a === 0 || b === 0) && console.log('aloha amigo')
Without parenthesis, you get (now with to show the precedence) a different result.
a === 0 || (b === 0 && console.log('aloha amigo'))
^^^^^^^ first evaluation
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ second evaluation

JavaScript && Operator used for returns

I am working understanding a JavaScript library and I came across this statement:
const assetsManifest = process.env.webpackAssets && JSON.parse(process.env.webpackAssets)
Then later on in the library, it uses the assetsMannifest like an object e.g.
assetsManifest['/vendor.js']
I thought the && operator was only used to return boolean values in logical checks. Can someone explain to me what is going on here?
Many thanks,
Clement

This operator doesn't always return true or false. It doesn't work like in some other programming languages. In JavaScript && operator returns the first value if it's falsy or the second one if not.
Examples:
null && 5 returns null because null is falsy.
"yeah yeah" && 0 returns 0 because every string is truthy.
Not so obvious :-)
Further reading:
Why don't logical operators (&& and ||) always return a boolean result?

&& returns first value converting to false or last value converting to true. It's because no need to calculate full logical condition with && if first value is falsy
console.log(55 && 66);
console.log(0 && 77);
console.log(88 && 0);
Also you can use && or || as if operator:
if (itsSunny) takeSunglasses();
equals to
itsSunny && takeSunglasses();

in that context it is checking if process.env.webpackAssets is a truthy value. If it is it will evaluate and return the next part. in this case JSON.parse(process.env.webpackAssets)
The logic is essentially
if (process.env.webpackAssets) {
return JSON.parse(process.env.webpackAssets)
}
else {
return process.env.webpackAssets // (null | undefined | 0 | '' | false)
}

Both && and || are evaluting there arguments in lazy mode and return the last value, after witch the result is known.
123 && (0 || '' && 78) || 7 && 8 || [] && {} || 90 || 77 && 13
###_^^ both results are possible 123 && ???
#_^^ first part is falsy, resume 0 || ??
#####_^^ can't be true, return ''
^^_########## 0 || '' return ''
^^_################ return ''
#######################_^^ resume
#_^^ resume
^^_# return 8
^^_###### return 8
^^_########################## drop
And the result is 8.

Javascript alert coming up when not expected

I am trying to ensure that all fields of a form are not empty. When there are empty fields this alert comes up as expected, however when all fields are full the alert still comes up. Am I missing something?
var sn = document.myForm.address.length;
var sna = document.myForm.street.length;
var su = document.myForm.city.length;
var st = document.myForm.state.length;
var usn = document.myForm.username.length;
if (sn || sna || su || st || usn == null) {
alert("All fields are required. Please ensure you leave no fields blank.");
return false;
} else {
}

Since you initialized all your variables, your if statement is evaluating true like this:
if (true || true || true || true || true || false)
Only one true makes the entire if condition above evaluate to true because all the || operators are OR operators.
Consider further, if you simply declare but do not initialize a variable for example var sn; //declared as opposed to var sn = document.myForm.address.length; //initialized then its condition evaluates to false because if(sn) is declared but not initialized = false`.
Moreover, to check the value inside each variable rather than whether or not they are initialized, you must do this:
if (sn == null || sna == null || su == null || st == null || usn == null)
or possibly since you're assigned a length you want this
if (sn > 0 || sna > 0 || su > 0 etc...

Adding the OR operator to a quizz form makes it accept any answers

I'm having a weird problem when trying to create a question form that is validated with Javascript:
If I write my validation like this:
if (typedValue === "myAnswer" && clearedLevels === 1){doStuff}
Everything works. But I want to create several correct answers, so I write:
if (typedValue === "myAnswer"||"secondAnswer" && clearedLevels === 1){doStuff}
..and all of a sudden anything written to the input form is accepted as the answer.

A correct way of writing it is :
if ((typedValue === "myAnswer" || typedValue === "secondAnswer") && clearedLevels === 1) { doStuff() }
You cannot combine the condition (x === y || x === z) as x === y || z and expect the same results.

Any non-empty string in Javascript is true (yes, even the string "false"). Since "secondAnswer isn't empty, it's evaluated as true, and ORed with any other condition will result in true.
You are missing a comparison of typedValue to this literal, and presumably, brackets around the typedValue comparisons, since && has higher precedence than ||:
if ((typedValue === "myAnswer" || typedValue === "secondAnswer") &&
clearedLevels === 1) {
// doStuff
}

extending Akash Pradhan answer you could write
if (typedValue == "myAnswer" || typedValue == "secondAnswer" && clearedLevels == 1) { doStuff() }
but since the && has precedence over the || operator it would evaluate
if (typedValue == "myAnswer" || (typedValue == "secondAnswer" && clearedLevels == 1)) { doStuff() }

Why is this '=' syntax incorrect in this js snippet?

var hungry = true;
var foodHere = true;
var eat = function() {
if (hungry && foodHere === true) {
return(true);
} else {
return(false);
}`
};
The first line is the correct syntax. For a long time I was just saying hungry && foodHere = true... and I couldn't figure out (and still don't understand) why that is wrong. I understand the difference between = (assignment) and === (equal to). I assigned the variables to be true initially, so aren't I asking in the if statement if that's what they're set to? Why am I setting the variables = to in the var definition, but then when checking them I'm using the === value?

= is only used to assign variables. === or == are used to compare. For a good example, we must look into comparison operators.
Syntax
The syntax of comparison operators is fairly simple, use them to evaluate expressions. The comparasin operators are:
=== //strict equality
== //Parsed or partial equality
> //Greater Than
< //Less than
>= //Greater than or equal to
<= //Less than or equal to
To properly use these, you must know the proper syntax. For example, I can't do something like:
if(true == 1 === true) //do something
as that would invalidate the code, and slow it down by much using ==, which brings me to my next section.
Equality
The two equality operators in JavaScript are == and ===. They do two very different things.
===
The strict equality (===) tests whether two values are exactly equivalent, both in type and value.
==
The Parsed equality (==) tests whether two values are equalivent in value, but parses to try and connect different types.
Inequality
There are 2 main inequality value in JavaScript (!==) they are pretty self explainatory based on the equalities (===, and ==)
here's a chart explaining the three.
1 0 true false null undefined ""
1 === !== == !== !== !== !==
0 !== === !== == == == !==
true == !== === !== !== !== !==
false !== == !== === == == ==
null !== == !== == == == ==
undefined !== == !== == == === !==
"" !== == !== == == !== ===
Adding onto what #jcollum said, = defines a variable value, and if(something === true) simplifies into if(something). Similarly, if(something === false) simplifies into if(!something).
You also need to do comparisons separately. if(7 & 6 < 10) returns false because it is the simplified version of if(7 === true && 6 < 10).

It turns:
hungry && foodHere === true
into
hungry && true
or just
hungry
Using the assignment operator instead of the comparison operator is stopping your logic from working correctly.