Weird behavior with expressions JavaScript

Question

Someone can explain to me why this happen? And please, if someone know this behavior name, please edit the title.

With this code:

 const arr = ['RIPA'], varB = "RIPB";
let _params;
_params && Array.isArray(_params) ? arr.push(..._params) : 
arr.push(_params);

_params && console.log("I will never appear");
varB && console.log("I will appear");

arr.push(varB);
console.log('array',arr);
console.log("Type of the _params --> ", typeof _params);

Output:

array [ 'RIPA', undefined, 'RIPB' ]
Type of the _params -->  undefined

jsBIN: https://jsbin.com/bawepasivo/edit?js,console
repl.it: https://repl.it/GaHX

If the _params is undefined , how possible is of executing the second expression, if the && expression returns the first false and the last trusty value.

Answer 1

 false  && false                  ? never executed       : _params is undefined
_params && Array.isArray(_params) ? arr.push(..._params) : arr.push(_params);

other way:

if (_params && Array.isArray(_params)) { // (false && false) === false
    arr.push(..._params); // it will be never executed
} else {
    arr.push(_params); // _params is undefined
}

Answer 2

Your expression is executed like this:

(_params && Array.isArray(_params)) ? arr.push(..._params) : arr.push(_params);

But you meant probably this:

_params && (Array.isArray(_params) ? arr.push(..._params) : arr.push(_params));

You just have to add parentheses.

Answer 3

let _params; // undefined

_params && Array.isArray(_params) ? is false , so the called code is arr.push(_params); , resulting in arr.push(undefined);