Function parameters not previously defined

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Function parameters not previously defined

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#1 by (1 votes)

1

I've seen some javascript functions with parameters that are not pre-defined, but still work. Doubt came up with the function below. The myFunction() function calls the pessoas.sort() method, which in turn receives an auto-invoked function as argument. This has the parameter a and b . Why does it even work those not having been previously defined? Does the JS engine interpret them alone? How can this be done on other occasions and with other parameters?

var pessoas = [
{country:"Brazil", name:"Lucas"},
{country:"EUA", name:"Jhon"},
{country:"Japan", name:"Sushi"}]

function myFunction() {
    pessoas.sort(function(a, b){return a.name - b.name});
    show();
}

function show() {
  document.getElementById("demo").innerHTML =
  pessoas[0].country + " " + pessoas[0].name + "<br>" +
  pessoas[1].county + " " + pessoas[1].name + "<br>" +
  pessoas[2].country + " " + pessoas[2].name;
}

<div id="demo"></div>
<button onclick="myFunction()">Mostrar</button>

javascript

asked by anonymous 01.02.2017 / 04:37

1 answer

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score 1 · Accepted Answer

Having optional parameters is common in many native methods, what is "strange here" is the organization callback itself being optional. This is documented , and if it is not passed this callback says to MDN:

If omitted, the array is sorted according to the Unicode code potentiation of each of the characters, depending on the conversion of each element to string.