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Jose Hermosilla Rodrigo
Asked: 2020-01-03 08:05:35 +0800 CST 2020-01-03 08:05:35 +0800 CST

Different syntax for throw

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I have a question that has arisen using the JavaScript throw keyword .

According to the documentation the syntax is:

throw expression;

Now, I've seen several examples using throw as if it were a function call, i.e. the expression in parentheses:

throw(new Error('Esto es un error'))

Furthermore, if we pass more than one argument to it, it will take the last argument:

throw(1, 2, 3, new Error('Esto es un error'), 'El último')

This behavior seems to be undocumented, the question would be, why doesn't it throw some syntax error?

If the syntax is correct, what's the point of being able to pass it so many arguments?

If I'm totally wrong, what am I missing here?

javascript

1 Answers

  1. Best Answer

    Throw is not a function, it is a statement so when you do this

    throw(new Error('Esto es un error'))

    you are executing the throw statement that is accompanied by an expression and not a list of parameters therefore the first level parentheses are unnecessary and do not indicate the execution of a function but rather serve as a grouping operator here , therefore you can omit them.

    throw new Error('Esto es un error')

    don't get confused with the syntax of a function, that to execute it you must use the parentheses and send a list of parameters

    function hola(nombre) {
  console.log('Hola, ' + nombre);
}

hola('Pepe'); // Esto ejecuta la función
hola 'Pepe'; // Esto es un error de sintaxis

    The parameters you send to the function are expressions

    hola(('Pepe')); // Esto es igual a hola('Pepe');

    here the first level of parentheses executes the function [hello], the second level of parentheses serve as a grouping operator that resolves to the same value 'Pepe'.

    Javascript functions can receive N number of parameters, if when executing you send more parameters than necessary then the function will take the first ones until it matches the number of parameters it receives and discards the excess.

    hola('Pepe', 'Lucho', 28); // la función [hola] solo toma 'Pepe' e ignora el resto

    but this is different than a comma separated list of expressions like for example

    1, 'Hola', hola(('Pepe')), 'Lucho'; // Esto son 4 expresiones separadas por comas
function hola(nombre){console.log(nombre)}

    The comma can be used to separate expressions that you write on a single line so it is equivalent to this.

    1; 'Hola'; hola(('Pepe')); 'Lucho';

    The throw clause can receive an Error object or not, you can send any value to throw like strings, numbers or other objects.

    throw new Error('Error 404 not found');
throw 'Error 404 not found';
throw 404;

    So in your example this line

    throw(1, 2, 3, new Error('Esto es un error'), 'El último')

    It is equivalent to

    throw 1, 2, 3, new Error('Esto es un error'), 'El último'

    Since it is an expression and not a list of parameters, this expression resolves to the last value, in your case it is the string 'Last'.

    throw (1), (2), (3), (new Error('Esto es un error')), ('El último') // Uncaught El último
throw ((1), (2), (3), (new Error('Esto es un error')), ('El último')) // Uncaught El último

    and finally the semicolon delimits the statement therefore these are not equivalent

    throw 1, 2, 3, new Error('Esto es un error'), 'El último' // Uncaught El último

    throw 1; 2; 3; new Error('Esto es un error'); 'El último' // Uncaught 1

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